Modern Concept Science and Technology 9

12. Total internal reflection : The phenomenon of light returning into the same medium when it passes from an

optically denser to an optically rarer medium with an angle of incidence greater

than the critical angle (i > C) is called the total internal reflection.

13. Mirage : Mirage is an optical illusion in which there is an appearance of water mainly on

roads, deserts, etc. during the peak summer.

14. Light pipe : A light pipe is a flexible fiber that confines a beam of light by the process of total

internal reflection.

15. Dispersion : The phenomenon due to which white light splits into seven colors (VIBGYOR),

when passed through a prism, is called dispersion.

16. Cause of the dispersion : The difference in speed of seven different wavelength colors inside a prism is the

cause of the dispersion of light.

17. Electromagnetic waves : The waves that do not need a material medium for their propagation and can travel

even through a vacuum are called electromagnetic waves.

18. Electromagnetic spectrum : The orderly classification of electromagnetic waves according to their wavelength

or frequency is called the electromagnetic spectrum.

Introduction Memory Tips

We see a variety of objects in the world around us. This is Light travels along a straight line.
because the light travels from the luminous sources to the This property is called the rectilinear
object and then comes from them to our eyes. During the propagation. The straight line along
day, the sunlight that falls on objects gets reflected which which the light travels is called a ray.
enabling us to see them. Thus, light is a form of energy

that produces the sensation of vision. In this unit, we will study the phenomena of bending

and splitting of light. We will also discuss the total internal reflection and electromagnetic

spectrum.

Optically Denser Medium and Rarer Medium

Optical medium is a material medium through which the Memory Tips
electromagnetic waves propagate. Light does not travel
with the same speed in all media. It travels the fastest in It is not the mass density of a medium
a vacuum, with its highest speed of 3×108m/s. In air, the that determines an optically denser
reduction in speed of light is negligible, compared to that and an optically rarer medium. For
in a vacuum. It is reduced considerably in glass or water. example, steam has less density
The value of speed of light in water is 2.25 × 108m/s and that than dry air but the steam is an
in glass is 2 × 108m/s. The medium in which light travels optically denser medium and dry air
faster is called an optically rarer medium and the medium is an optically rarer medium.

in which light travels relatively slower is called an optically denser medium.

FACT WITH REASON

Water is optically rarer medium as compared to glass, why?

When light passes through water to glass, the value of speed of light in water is 2.25 × 108m/s and that in glass
is 2 × 108m/s. Light travels faster in water than in glass. So water is an optically rarer medium than glass.

Modern Concept Science - 9 93

Refraction of light N

According to the ray model of light, it travels through space along I

straight lines. Light does not travel in the same direction in all

media. When light enters obliquely from one transparent medium Air i

to another, the direction of propagation of light in the second O
medium changes. This phenomenon is called refraction of light.
Thus, the phenomenon of bending of light as it passes obliquely Glass r Refracted ray

from one optical medium to another is called the refraction of light.

Cause of Refraction R

Refraction of light

Light travels more slowly when it enters into an optically denser

medium from an optically rarer medium. On the other hand, when light travels from an

optically denser to an optically rarer medium, it speeds up. The change in speed of the light

wave upon crossing the boundary of two optical media is the cause of refraction.

Refraction through Glass Slab N
General Terms Used in Refraction of Light

i) Incident Ray : A ray which strikes the surface
of separation of two optical media is known as
the incident ray. In the given figure, AO is the
incident ray.

ii) Refracted Ray : The ray which travels in

the second optical medium, with a change

in direction, is called the refracted ray. In the N'

given figure, OB is the refracted ray. Refraction through glass slab

iii) Normal : A perpendicular line drawn at the point of incidence is called the normal. In

the given figure, NN' is a normal line.

iv) Angle of Incidence : The angle made by an incident ray with the normal, at the point
of incidence, is known as the angle of incidence. In the given figure 'i' is the angle of

incidence.

v) Angle of Refraction : The angle made by refracted ray with the normal, at the point
of incidence, is known as the angle of refraction. In the given figure, 'r' is the angle of
refraction.

Rules for the Bending of Rays at the Interface of Two Optical Media

i) When a ray of light travels from an optically rarer medium (air) to an optically denser
medium (glass), it bends towards the normal at the interface of the two media.

FACT WITH REASON

For a ray of light travelling from an optically rarer to an optically denser medium, Normal
the angle of incidence is always greater than the angle of refraction, why? Rarer (air) i

A ray of light travelling from a rarer medium to a denser medium slows down and r
bends towards the normal at the point of incidence. So, for this ray of light, the angle Denser (glass)
of incidence is always greater than the angle of refraction.

94 Light

ii) When a ray of light travels from an optically denser medium (glass) to an optically rarer
medium (air), it bends away from the normal at the interface of the two media.

FACT WITH REASON

For a ray of light travelling from an optically denser to an optically rarer medium,
the angle of incidence is always less than the angle of refraction, why?
A ray of light travelling from a denser medium to a rarer medium goes faster and
bends away from the normal at the point of incidence. So, for this ray of light, the
angle of incidence is always less than the angle of refraction.

iii) When a ray of light strikes the interface of two media at an angle of 90°, it does not bend

from its path.

Memory Tips

Rarer air If a ray of light travels from air

to water along a certain path, it

retraces the path, when light travels

Denser (glass) from water to air. The path of light is
reversible.

Light passing through normal

ACTIVITY 1

1. Fix a sheet of white paper on a drawing board using drawing pins. X P1
xy P2 M
2. Place a rectangular glass slab over the sheet in the middle. Draw the
i
outline of the slab with a pencil and name the outline as ABCD. A B
O
3. Draw a line XO as an incident ray on the face AB. r

4. Take four identical pins. Fix two pins, say P1 and P2, vertically on the line M1
N

XO. D O2 P3 C
5. Look for the images of the pins P1 and P2 through the opposite edge. Fix e P4

two other pins, say P3 and P4, such that these pins and the images of P1 N1
and P2 lie on a straight line. Y

6. Remove the pins and the slab. Join the positions of the tip of the pins P3 and P4 and produce it up to
the edge DC with the help of a ruler. Let P3P4 meets DC at O2.
7. It is found that the light ray has changed its direction at points O on the surface AB and at a point O2
on the surface DC. When light enters into the glass slab from air, it bends towards normal and when

light emerges out of the glass slab, it bends away from the normal.

FACT WITH REASON

When a ray of light passes from a denser medium to a rarer medium, it bends away from the normal, why?
A ray of light travelling from a rarer medium to a denser medium slows down, causing it to bend towards
the normal. But when it travels from a denser medium to a rarer medium, it goes faster, causing it bend
away from the normal.

Modern Concept Science - 9 95

Laws of Refraction

i) The incident ray, the normal, and the refracted ray all lie in the same plane at the point

of incident.

ii) For a given pair of media, the ratio of the sine of the Memory Tips

angle of incidence to the sine of the angle of refraction Snell's law is not applicable when

is a constant. This law is also known as Snell’s law of the angle of incidence is equal to

refraction. If 'i' is the angle of incidence and 'r' is the zero, i.e., when the ray is incident
along the normal.
angle of refraction, then,

sin i = constant (µ)
sin r

This constant value is called the refractive index of the second medium with respect to

the first.

Refractive Index

The degree of the change in direction that takes place in a given pair of media is expressed
in terms of the refractive index. It refers to the degree to which a medium manages to slow
down the wave that is transmitted through it. Thus the higher the refractive index of a
material medium, the slower a wave passes through it. The refractive index indicates how
much slower the electromagnetic wave passes through the medium with respect to its speed
in vacuum (3 × 108 m/s).

There is a relationship between the refractive index of a material and the speed of light in

that material. The refractive index of a material medium can be defined as the ratio of the

speed of light in vacuum (c) to the speed of light in the medium (v).

µ= Speed of light in vacuum = c
Speed of light in the medium v

The refractive index of a medium, considered with respect to vacuum, is called the absolute

refractive index of the medium. The absolute refractive index of a medium is simply called its

refractive index. The refractive index of several media is given in the table.

Medium Absolute Refractive Index Memory Tips
Vacuum 1
Ice In comparing two media, the one
Water 1.31 with the larger refractive index is an
Ethyl Alcohol 1.33 optically denser medium than the
Kerosene 1.36 other. The other medium of lower
Glass 1.44 refractive index is optically rarer.
Crown Glass 1.50
Diamond 1.52
2.42

96 Light

Solved Numerical 5.1

The refractive index of diamond is 2.4. What is the speed of light in diamond if its speed in

a vacuum is 3 × 108 m/s? Memory Tips

Solution: Given, An optically denser medium may
Speed of light in vacuum (c) = 3 × 108 m/s not possess a greater mass density.

Refractive index = 2.4 For example, kerosene, which has a

From the formula, higher refractive index than that of
water, is optically denser than water
µ= Speed of light in vacuum (c) but mass density of kerosene is less
Speed of light in the medium (v) than that of water and it floats on the
surface of water.
or, 2.4 = 3 × 108
Speed of light in the medium
× 108
or, Speed of light in diamond = 3 2.4 = 1.25 × 108 m/s

Real Depth and Apparent Depth

Light ray bends away from the normal when it passes from water to air. This bending of light
gives us a false impression of depth. When light rays pass from water to air, the light rays
from the point at the bottom of a pond bend away from the air-water interface. The observer
outside sees the point to be at a higher position above the actual position. The real position of
the bottom or a point inside water is called a real depth. Whereas, the position of the bottom
or a point inside water which is observed from outside is called an apparent depth. Apparent
depth is due to the refraction of light that occurs at the water-air interface.

The real depth and apparent depth are related to the refractive index of water by the given equation.

µ = Real depth
Apparent depth

Solved Numerical 5.2

An observer looks at a water tank. According to him, half of the tank is filled with water.
If the height of the tank is 180 cm, find the real height of the water in the tank. (Refractive
index of water = 1.33)

Solution: Given,

Height of the tank = 180 cm Memory Tips

Apparent depth = 180 = 90 cm When you look at the depth of a
2 swimming pool, the bending of light
can give you a false impression of
Refractive index of the water = 1.33 depth.

From the formula,

µ = Real depth
Apparent depth

or, 1.33 = real depth
90

or, real depth = 90 × 1.33 = 119.7 cm

The real height of the water in the tank is 119.7 cm.

Modern Concept Science - 9 97

FACT WITH REASON

A spear hit by a fisherman does not hurt the fish inside water, why?
When the rays of light refracted from the fish inside water pass from the water to the air, they bend away
from the normal. Hence the fish appears at a position above its actual position. A person hitting at the
apparent position of the fish inside water misses the target. So a spear hit by a fisherman where the fish
appears to him does not hurt the fish inside the water.

Effects of Refraction
i) A stick immersed obliquely in water appears to be bent.
ii) A coin in a cup, just below the eye sight, becomes visible when the cup is filled with

water
iii) A spear hit by a fisherman does not hurt the fish inside water
iv) When a thick glass slab is placed over a book page, the letters appear raised while

viewing through the glass slab
v) Twinkling of Stars in the sky
vi) The sun is visible to us about two minutes before the sunrise (i.e. advanced sunrise) and

about two minutes after the sunset (delayed sunset)
vii) The sun near the horizon appears flattened at the sunset and sunrise

FACT WITH REASON Apparent bend
Apparent
A stick immersed obliquely in water appears to be bent. Why? position
When rays of light from a part of the stick inside water pass from water to air, they bend of stick
away from the normal. Hence, the stick appears to be bent at the air-water interface.
Water

ACTIVITY 2

1. Take a beaker and fill ¾ th of it with water. Apparent

2. Place a pencil obliquely in the beaker and observe it from a side of the beaker. position

Real position

3. Does the pencil appear bent at the air-water interface?

ACTIVITY 3 Water
Coin (real
1. Place a coin at the bottom of a glass tumbler filled with water. position)
2. With your eye to a side above water, make an attempt to touch the coin with the

help of a pencil. Did the pencil touch the surface of the coin exactly?
Apparent
position of
the coin

98 Light

ACTIVITY 4

1. Place a cup on a table in your classroom and put a coin in it.
2. Move away slowly from the cup and stop to move when the coin just disappears from your sight.
3. Ask a friend to pour water gently into the cup without disturbing the coin.
4. Keep looking for the coin from your position. Does the coin become visible again from your position?

FACT WITH REASON

A coin in a cup, just below the eye sight, becomes visible when the cup is filled with water, why?
The direction of light reaching our eyes from the coin inside water through water (optically denser medium)
to air (optically rarer medium) bends away from the air-water interface. Due to this, the coin seems just
above of its actual position inside water in the cup. So a coin in a cup, just below the eye sight, becomes
visible when the cup is filled with water.

ACTIVITY 5

1. Place a glass slab over a page of a book on your table.
2. Look at the letters under the slab from the sides. Do you see the letters raised?

FACT WITH REASON

When a thick glass slab is placed over a book page, the letters appear raised while viewing through the
glass slab, why?
The direction of light reaching our eyes from the letters under the glass slab through glass (optically denser
medium) to air (optically rarer medium) bends away from the air-glass interface. Due to this, the letters
seem just above of their actual position under the slab. So when a thick glass slab is placed over a book
page, the letters appear raised while viewing through the glass slab.

Twinkling of Stars
Twinkling of stars is due to the atmospheric
refraction. Light from distant stars undergoes a
continuous refraction through layers of atmosphere.
During the refraction in the atmosphere, the refracted
rays bend towards the normal and the stars appear
slightly higher than where their actual position
is. The apparent position of stars fluctuates continuously due to the change in the earth's
atmosphere. Stars appear brighter some times and fainter at the other times. So twinkling of
stars is due to the fluctuation of the apparent positions of the stars during their refraction.

FACT WITH REASON

Stars twinkle but planets do not, why?
Compare to the planets, stars are very far. Light from the distant stars undergoes a continuous refraction
through layers of atmosphere. It also fluctuates the apparent position of the stars and twinkle. But, such
continuous refraction does not occur in planets. Hence planets do not twinkle.

Modern Concept Science - 9 99

The sun is visible to us about two minutes before the sunrise (i.e. advanced sunrise)

The actual sunrise means the actual crossing of the Apparent sun

horizon by the sun. Light from the sun travels through

layers of the atmosphere. When the sun is just below Horizon

the horizon, its rays enter the earth's atmosphere

from rarer to the denser medium and get refracted. Observer

But the observer on the earth thinks that the light Earth

is coming straight from the sun. Refracted rays that Rising sun
reach us make the sun appear on the horizon. This is

the apparent sunrise. But the actual sunrise occurs when the sun reaches horizon a little later.

Sun set occurs about two minutes after the sunset (delayed sunset)

The apparent sunset occurs slightly later than the
actual sun set. The light from the sun is already below
the horizon. It gets refracted towards the earth. While
travelling through rarer to denser medium of the
atmosphere, the sunlight from apparent position of the
sun reaches the observer on the earth. This enables us
to see the apparent sunset for a while after the sun has
already set. So, we can see the sun about 2 minutes before
the actual sunrise, and about 2 minutes after the actual sunset because of atmospheric refraction.

The sun near the horizon appears flattened at the sunset and sunrise

When the light rays from the sun enter the earth's atmosphere, they get refracted towards the
earth. Due to the atmospheric refraction, the rays from the top and bottom portions of the sun
on horizon are refracted by different degrees. This causes the apparent flattening of the sun.
But the rays from the sides of the sun on a horizontal plane are generally refracted by the same
amount. So the sun still appears circular along the sides.  

Critical Angle

When light travels from a denser medium to a rarer medium (e.g. glass to air as shown in
figure), it bends away from the normal. At this condition, the angle of refraction (r) becomes
greater than the angle of incidence (i). If the angle of incidence in denser medium is increased
then the corresponding angle of refraction also increases. At some stage, the angle of refraction
becomes 900. At this condition, the angle of incident is also called critical angle. Thus, the angle
of incidence in a denser medium for which the corresponding angle of refraction in the rarer
medium is 90°­ is called the critical angle. It is denoted by C. In the given figure (ii) angle of
incidence is equal to critical angle (i.e. i = C).

Rarer r Rarer r = 900 Rarer

Denser i Denser ic Denser ir

Critical angle and total internal reflection

100 Light

FACT WITH REASON

Critical angle of glass is 42°. What does it mean?
Critical angle of glass is 42°. It means that, when light passes from glass to air, at the incident angle 42° the
corresponding angle of refraction in air will be 90°.

Critical angles for some substances with respect to air

Substances Critical angle Substances Critical angle
43°
Ice 50° Turpentine 43°
42°
Water 49° Glycerine 24°

Alcohol 48° Glass

Paraffin 44° Diamond

Total Internal Reflection (TIR) Rarer ir
Denser
When the angle of incidence is further increased beyond the critical
angle, the ray of light reflects back to the same medium instead Total internal reflection
of refraction. This phenomenon is called total internal reflection
of light. thus, the phenomenon of light returning into the same
medium when it passes from an optically denser to an optically
rarer medium with an angle of incidence greater than the critical
angle (i > C) is called the total internal reflection.

ACTIVITY 6

To demonstrate the total internal reflection of light
1. Immerse an empty test tube in a beaker containing water as shown in the given figure.
2. Gradually tilt the test tube and look at it from outside the beaker.
3. At a certain angle of incidence on the water-air interface, the test tube shines brightly.
Explain: The test tube shines due to the total internal reflection. The small thickness of the
glass wall of the test tube in between the air-water interface is neglected in this case.

Conditions for Total Internal Reflection
In order to have total internal reflection, two conditions must be fulfilled. They are:

i) Light must travel from a denser medium to a rarer medium.
ii) The angle of incidence in the denser medium must be greater than the critical angle.
Effects of Total Internal Reflection
Total Reflecting Prism
A total reflecting prism is a right-angled isosceles prism. It rotates light through 90° or 180°
based on the total internal reflection. It is used to obtain a correct image of objects.

Modern Concept Science - 9 101

P

R 450 Inverted P
image
450 Q 450 900
Erect
450 450 image
450 450
450 450 R
450 900 450 Q 450 450

Q R R

Effects of total internal reflection

Fibre Optics and Light Pipe

Fibre optics and light pipes are the devices used to transmit light in any desired path. They
work because of the total internal reflection. When a ray of light enters a fibre, it suffers a
series of total internal reflections and finally emerges out from the other end. During this
process, the light energy is not absorbed and it gets transmitted over a long distance.

i) Use of fibre optics in communication: Optical fibre is used extensively as infrastructure
for long distance telephony and internet. For this purpose, communication signals like
sound are converted into electrical energy and finally into light energy. The light energy
confines itself inside the material by total internal reflection and the signal is transmitted
over a long distance.

ii) Use of light pipe for endoscopy or fibrescopy: A light pipe is a flexible fibre that
confines a beam of light by total internal reflection. It is used to examine internal organ
of human body by physicians.

Memory Tips

Light pipe Endoscope is a device used by
physicians to examine the internal
organs of the human body.

Mirage

A mirage is generated by layers of air at different Denser layer of air
temperatures. This effect can commonly be seen on Tree

asphalt roads and deserts during the summer. It is an Rarer layer of air

optical illusion in which there is an appearance of water

mainly on roads, deserts, etc. during the peak summer. Virtual image
Mirage
Formation of a Mirage
During peak summer days, the black-topped road and

the sand on desert absorb solar radiations and make the air above them hot very quickly.

This makes the lower layers of air above the road hotter than the upper layers. Cold air is

denser than hot air. At the boundary of the denser upper layer and rarer lower layer, light

from the blue sky and surroundings bends more and more away from the normal. After

several refractions, the angle of incidence becomes greater than the critical angle, which is the

condition for the total internal reflection. Hence, light is reflected. The viewer no longer sees

the road or desert floor. But the light reflected from the blue sky and surroundings, gives him

to an illusion of water.

102 Light

FACT WITH REASON

Why does mirage occur?
Mirage occurs due to total internal reflection of light on the asphalt roads and deserts during the summer.

Sparkling of Diamond

Diamond has a special cut of the faces along with

its high refractive index of 2.42. Its critical angle is

only of 240, which allows total internal reflection

over a wide range of angles. Therefore, a lot of

incident light undergoes several total internal Sparkling of diamond

reflections inside the diamond before exiting through it and causes the “sparkling” effect.

This effect makes diamonds so appealing. A well-cut diamond sparkles more than a poorly

cut diamond.

FACT WITH REASON

An air bubble inside water shines. Why?

Water is an optically denser medium and air is a rarer medium. When light rays passing through water fall
on air bubble inside water at an angle greater than the critical angle then they suffer total internalreflection.
These reflected rays enter an observer's eyes. Due to this, the air bubble inside water shines.

Differences between Reflection and Total Internal Reflection

S.N. Reflection S.N. Total Internal Reflection

1. A smooth surface like mirror is 1. An optically denser and an optically
required for reflection of light. rarer media are required for total
internal reflection.

2. It takes place at any angle of incidence. 2. It occurs only when the angle of
incidence in denser medium is greater
than its critical angle.

3. Some percentage of light is absorbed 3. The entire incident light reflects back
by the reflecting surface. totally without absorption at the
interface of two media.

Dispersion

A prism is a piece of transparent material like glass, White light beam VIBGYOR
which is bounded by two triangular surfaces and
three rectangular surfaces. The rectangular surfaces Glass prism
are called the refracting faces and the angle between
two refracting faces is called angle of prism. Dispersion of light through glass prism

The white light consists of seven colors in which the
violet color has the least wavelength and red color
has the maximum wavelength. Dispersion causes

Modern Concept Science - 9 103

the spreading of all the colors. When white light falls at the first face of an equilateral prism,
the red color bends least and the violet color bends maximum. Other colors also bend in their
respective order. Thus, when white light falls on the refracting face of a prism it splits up into
seven colors and gets dispersed. While passing out from the opposite face of the prism, these
seven colors suffer further refraction and separate out further. Thus, the phenomenon due to
which white light splits into seven colors (VIBGYOR), when passed through a prism, is called
dispersion.

ACTIVITY 7

1. Take a prism and keep it on the table. Ensure that the room is considerably dark for the light to be
obvious.

2. When light travels through the prism, it splits the white light into 7 colors. Observe the colors and
check whether you find them in the order of Violet, Indigo, Blue, Green, Yellow, Orange, and Red
(VIBGYOR) or not.

Cause of Dispersion of White Light by a Prism
When light touches the surface of the prism, refraction takes place at the boundary of the
prism i.e. boundary between air and glass. Then the white light is separated into its component
colors, viz. red, orange, yellow, green, blue, indigo, and violet.

The seven colors in white light constitute seven different wavelengths. In a glass prism, the
speed of the shortest wavelength (the violet color) is slower than the longest wavelength
(the red color). The shortest wavelength (the violet color) color bends more than the longest
wavelength (the red color). Due to this, the violet color comes at the bottom and the red color
comes at top on the screen. Therefore, the difference in speed of seven different wavelength
colors inside a prism is the cause of the dispersion of light.

FACT WITH REASON

How is Rainbow formed?
The colors of the rainbow are formed by the dispersion of light in small water droplets. A ray of white light
from the sun is refracted as it enters a spherical raindrop and dispersion occurs. The dispersed light rays un-
dergo total internal reflection. These reflected rays refracted again as they leave the water drop. The emer-
gent rays that come out are now travelling in different directions, depending on their frequencies. So, they
appear to come from different parts of the sky.

Recombination of White Light: Newton's Experiments

a) Two-prism experiment
In this experiment, Newton took two prisms P1 and P2 of the same material and having
the same refracting angle. He allowed a fine beam of white light to fall on the prism P1.
He obtained a spectrum VIBGYOR on the screen. He removed the screen and placed
another prism P2 in an inverted position as shown in the given figure. It was observed
that the light coming out from the prism P2 is white again. The prism P1 that disperses
a white light into its constituent colors is called a dispersing prism. The second
prism P2 that recombines the seven constituent colors to form a white light is called a
recombination prism.

104 Light

AD E
White spot

White light beam VIBGYOR Coloured

Glass prism Beam of C B light F
white light

Two-prism experiment

b) Newton's color disc

It is a circular metallic disc divided into seven sectors. These sectors Newton's color disc
are painted with the colors of the solar spectrum. The disc is made
capable of rotating about an axis passing through its center. When
the disc is rotated at high speed with the help of the handle, the
colored sectors are no longer visible. The whole disc appears dull
white in color. It proves that white color of sunlight has seven colors.

Electromagnetic Waves and Electromagnetic Spectrum
Electromagnetic Waves

A wave is a fine path or disturbance that carries energy from one place to another. Waves
are of two types, viz. longitudinal wave and transverse wave. Light energy propagates in the
form of a transverse wave. It is an electromagnetic wave. The radio wave that comes from a
radio station is also an electromagnetic wave. Light wave does not need material medium to
propagate. It can travel in vacuum too. Thus, the waves that do not need a material medium
for their propagation and can travel even through a vacuum are called electromagnetic waves.
The gamma ray, X-ray, ultraviolet ray, infrared radiation, microwaves and radio waves are all
electromagnetic waves.

Properties of Electromagnetic Waves
i) The electromagnetic waves are transverse waves.

ii) They travel with the speed of light but differ in their frequency and wavelength.

iii) These waves have oscillating electric and magnetic fields.

iv) They are not affected by electric and magnetic fields.

v) They obey the laws of reflection and refraction.
Electromagnetic Spectrum

The orderly classification of electromagnetic waves according to their wavelength or
frequency is called the electromagnetic spectrum. It is the range of all types of electromagnetic
waves. Electromagnetic waves are classified in order to their wavelength or frequency in an
electromagnetic spectrum. We see only a small region of visible light in the electromagnetic
spectrum. Wavelengths and corresponding frequencies of electromagnetic waves that make
up the electromagnetic spectrum are given in the table below.

Modern Concept Science - 9 105

Cell phone
(3KHz–3000Hz)

extremely low
frequency (ELF)

very low
frequency

(VLF)
radio waves
infrared
radiation
visible light
ultravoilet
radiation

x-rays
gamma rays
microwaves

Frequency

non-ionizing radiation ionizing radiation

Electromagnetic spectrum

S.N. Electromagnetic Waves Wavelength (in meter) Frequency (in hertz)

1. Gamma rays 10-13 to 10-11 1019 above
2. X- rays 10-11 to 10-9 1017 to 1019

3. Ultraviolet rays 5×10-9 to 5×10-7 1015 to 1017

4. Visible rays 4×10-7 to 8×10-7 1014 to 1015
5. Infrared rays 8×10-7 to 8×10-5 1011 to 1014
6. Microwaves 10-5 to 10-3 109 to 1011

Radio waves

7. (short wavelength: satellite, radar, 10-3 to 103 103 to 109
TV and long wavelength: in radio

broadcasting)

Gamma Rays

Gamma rays have wavelengths ranging from 10-13m to 10-11m. These rays are produced by

radioactive elements. Gamma rays have the highest frequency as well as highest penetrating

power. Memory Tips

Uses of Gamma Rays The spectrum within the range of

i) Gamma rays are used in treatment of cancer. 400nm to 750nm that strikes the

ii) Gamma rays are able to kill harmful bacteria. So they retina is called the visible spectrum
and the region of spectrum that does
are used for sterilizing medical equipment.
not strike the retina is called the
iii) They are used to produce photoelectric effect. invisible spectrum.

106 Light

X- rays
X-rays have wavelengths ranging from 10-11m to 10-9m. These rays are produced when a high-
speed electron is blocked by a target. X-rays of longer wavelengths are called soft X-rays. They
can pass through flesh but not bones because bones absorb X-rays more than flesh.

Uses of X- rays
i) X-rays are used in radiography.

ii) With 'CAT scan', detective, the agencies detect gold, silver, revolver, etc. sealed in bags.

FACT WITH REASON

X-rays are used in radiography, why?
Soft X-rays can penetrate flesh but they are absorbed more by bones. So they are used in radiography to take
out photographs of fractured bones.

Ultraviolet Rays

Electromagnetic waves whose wavelength is just shorter than the violet light are called
ultraviolet rays. The wavelength of ultraviolet rays ranges from 5×10-9m to 5×10-7 m. These
rays are produced from the sun, mercury vapor in fluorescent lamps, etc.

Uses of Ultraviolet Rays

i) The ultraviolet rays absorbed by a human body produce vitamin D. It helps the growth
of bones and teeth.

ii) They are used to detect original gems from fake ones.

iii) Ultraviolet rays kill germs and micro-organisms. Hence, they are used to sterilize
surgical instruments and drinking water.

Visible Rays

The wavelength of visible light is from 4×10-7m to 8×10-7m and the corresponding frequency
is 7.5 ×1014 Hz to 3.75 ×1014 Hz. Our eyes detect visible light. Fireflies, light bulbs, and stars all
emit visible light.

Uses of Visible Rays
i) Visible light is used for seeing objects.

ii) It is used to take photographs.
Infrared Rays

Electromagnetic waves whose wavelength is just longer than the red light are called infrared

rays. The wavelength of infrared rays varies from 8×10-7m to 8×10-5m. Sun, lamps, burning

gases, etc. are the sources of infrared rays. Memory Tips

Uses of Infrared Rays Infrared rays have long wavelengths

i) Due to the heating effect of infrared rays, they are and they can travel long distances

used to relieve pains from muscles and swollen joints. in fog without much scattering. So

ii) Infrared rays are used in infrared photography. infrared rays are used in infrared
photography.

Modern Concept Science - 9 107

iii) Night vision devices can detect infrared rays emitted by hot objects. Such devices are
used to see objects in dark. For example, night vision goggles pick up the infrared light
emitted by our skin and objects with heat.

iv) Any disease in crops changes their heat radiation. So infrared rays are used to detect
diseases in crops.

v) Infrared radiation is commonly used in wireless remote-controls for televisions and for

short distance wireless data transfer. Memory Tips
Microwaves
The shorter the wavelength, the
The wavelength of microwaves ranges from 10-5m to 10-3m. higher the frequency. Higher
These waves are produced by electromagnetic oscillators of frequencies can deliver more energy,
high frequency in an electric circuit. which makes the electromagnetic
waves more dangerous.
Uses of Microwaves

i) Microwaves are used in satellite communications.

ii) Microwaves are used in microwave ovens to cook food.

Radio Waves

The wavelength of radio waves ranges from 10-3m to 103m. Radio waves are produced by
electromagnetic oscillators of low frequencies. Radio waves of short wavelength are used in
communication including satellite, RADAR and TV. Longer wavelengths are used in radio
broadcasting.

Uses of Radio Waves

Radio waves are used mainly for communications. For examples: radio, television, telephone,
RADAR, etc.

Electromagnetic Waves and Their Sources, Nature and Use

S.N. Electromagnetic Wave Source Nature Use
1. Gamma rays
2. X- rays Radioactive Harmful Radiography, treating cancer,
measuring thickness
3. Ultraviolet rays materials,
4. Visible rays
5. Infrared rays Nuclear reactions

X-ray tube, stars Harmful Taking X-ray radiography,
treating cancer tumors, etc.
'CAT scan' devices are used by
agencies to detect gold, silver,
revolver, etc. sealed in bags.

The sun, mercury Harmful Sterilizing food, detect invisible
vapor lamp, writing, etc.
electric arc, etc.

The sun, hot Non- Seeing, photography,
objects
dangerous photosynthesis

Very hot objects Non- Heating, night-vision devices,

dangerous treat muscular strain

108 Light

6. Microwaves Electronic Harmless Used in microwave ovens for
7. Radio waves devices such as Harmless cooking, warming food, radar,
microwave oven etc.

Electrons Communication of radio,
vibraters, television, telephone, RADAR,
radio and TV etc.
transmitters, etc.

ANSWER WRITING SKILL

1. What is the velocity of light in air, glass and water media?

Ans: Velocity of light in air 3 × 108 m/s, Glass 2 × 108 m/s, Water 2.25 × 108 m/s.

2. Light is passing from denser medium to rarer medium, in which direction it bends?

Ans: If light is passing from denser medium to rarer medium, then light bends away from the normal ray.

3. What do you mean by lateral shift?

Ans: Lateral shift is the perpendicular distance between the original path of incident ray and the emergent
ray. It is also called lateral displacement.

4. The critical angle of diamond is 24 ˚. What does it mean?

Ans: The critical angle of diamond is 24 ˚. It means, when light passes from diamond to air, if the angle of
incidence in diamond is 24 ˚, the corresponding angle of refraction in air will be 90˚.

5. What happens if angle of incidence in denser medium is more than critical angle?

Ans: If angle of incidence in denser medium is more than critical angle, then the light reflects back totally into
the same denser medium instead of refraction.

6. What is light pipe? On which principle does it work?

Ans: Light pipe is a flexible tube made of glass or plastic fibres. It can transfer light from one opening to another
opening of the tube. Light pipe is made using the principle of total internal reflection of light.

7. Rays of shorter wave length are dangerous to our body, why?

Ans: Rays of shorter wave length have higher frequency and more energy. They have lot of penetrating power.
They can kill body cells or cause cancer. So, such rays are harmful to our body.

8. Differentiate between refraction of light and dispersion of light.

Ans: Differences between refraction of light and dispersion of light are:

SN Refraction of light SN Dispersion of light

1 Bending of light as it passes from one 1 Splitting of a white ray of light into

medium to another medium is called constituent seven colors as it passes from a

refraction of light. prism is called dispersion of light.

2. Glass slab can refract light. 2. Glass prism can disperse light.

Modern Concept Science - 9 109

9. Differentiate between ultra-violet ray and infra-red.

Ans: The differences between ultra-violet rays and infra-red rays are:

SN Ultra-violet SN Infra-red

1 Ultra-violet does not produce heating 1 Infra-red produces heating effect.
effect.

2. Ultraviolet ray is produced from very hot 2. Infra-red is produced by hot object like fire.
objects like sun, sparks of welding, etc.

10. Diamond sparkles more than the glass that cut in the same shape. Explain.

Ans: Critical angle of diamond is 24˚ and that of glass is 42˚. Due to smaller critical angle of diamond, there
is high range of total internal reflection. As a result, there are multiple total internal reflections inside
diamond. So, diamond sparkles more than glass.

11. Study the given diagram and answer the following questions.
a) What is the phenomenon shown in the diagram?

Ans: Dispersion of light is shown in the given diagram.
b) Which ray of light has more speed in the prism? Which has the lowest speed?

Ans: “A” has the greatest speed in prism and B has the lowest.
c) Name the ray of light A and B.

Ans: A – Red ray of light
B – Violet ray of light
d) Why does a ray B bend more than ray A?

Ans: Ray 'B' is violet ray of light. Violet ray has the shortest wavelength among the visible light. So,
it has slowest speed in prism. It has more angle of deviation, bends more and remains at the
bottom of the spectrum. But 'A' is red ray with longest wavelength. It has the highest speed and
lowest angle of deviation. So red colour bends less and stays at the top of spectrum.

12. The speed of light in a medium 'M' is 0.75 × 108m/s and that in vacuum is 3 × 108m/s. Find the refractive
index of 'M'.

Solution:

Speed of light in vacuum (c) = 3×108m/s

Speed of light in a medium ( v) = 0.75 × 108m/s

From the formula,

µ= speed of light in vacuum (c) = 3 × 108 =4
speed of light in the medium (v) 0.75 × 108

Therefore, the refractive index of 'M' is 4.

110 Light

STEPS EXERCISE

STEP 1

1. Define

a) Rarer medium b) Denser medium
c) Refraction of light d) Cause of refraction
e) Refractive index f) Absolute refractive index
g) Real depth h) Apparent depth
i) Critical angle j) Total internal reflection
k) Mirage l) Light pipe
m) Dispersion n) Cause of the dispersion
o) Electromagnetic waves p) Electromagnetic spectrum

2. Very Short Answer Questions

a) How much is the speed of light in vacuum, water and glass?

b) What do you understand by 'rectilinear propagation of light'?
c) State the principle of reversibility of light.
d) What is the angle of refraction when a ray of light is incident normally on the

water surface?
e) State Snell's law of refraction of light.
f) What is fiber optics?
g) Write down the use of fibre optics.
h) Write down the use of light pipe.
i) Name any two waves of solar radiation that are not visible to us.
j) Write the major sources of the following γ-rays, X-rays, UV-rays, Infrared rays

and Radio waves

STEP 2

3. Short Answer Questions

a) State the laws of refraction of light.
b) How are the angle of incidence and angle of refraction related to each other when

a ray of light travels from:
i) a rarer to a denser medium. ii) a denser to a rarer medium.
c) What does the refractive index of a medium indicate? Explain with an example.
d) Under what conditions does Total Internal Reflection (T.I.R.) take place?
e) Explain any two effects of total internal reflection.
f) On which factor does the refractive index depend?
g) What is a mirage? Explain its formation.
h) How can you show that the white light consists of seven colors.
i) Write a short note on :
i) visible light ii) infra-red rays

Modern Concept Science - 9 111

j) Write two uses of the following radiations:

i) γ - ray ii) x-rays iii) infrared rays

iv) UV-rays v) microwaves vi) radio waves

k) Write properties of electromagnetic waves.

4. Differentiate Between

a) Optically rarer medium and optically denser medium
b) Real depth and apparent depth
c) Reflection of light and total internal reflection
d) γ- rays and X- rays

5. Give reason

a) A ray of light travelling from a rarer medium to a denser medium bends towards
the normal.

b) A pond appears to be shallower than it really is when viewed obliquely.
c) The apparent depth of the water in a pond is less than the real depth.
d) Stars twinkle on a clear night.
e) Diamond sparkles.
f) γ- ray and X-rays are very harmful for living beings.
g) Radio waves are not harmful for human beings.

6. Answer the questions with the help of the given figure

a) A ray of light is incident normally on various surfaces of glass. Complete the path
of the light ray.

Air Air

420 450

Glass Glass 900 450 (d)

(a) (b) (c)

STEP 3

7. Numerical Problems

a) The speed of light in a medium 'M' is 0.75 × 108 m/s and that in vacuum is 3 ×

108m/s. Find the refractive index of 'M'. [ Ans: 4]

b) Light enters from air into diamond, which has a refractive index of 2.42. Calculate

the speed of light in diamond. [Ans: 1.24 × 108 m/s]

c) The speed of light in vacuum is 3 × 108 m/s. If the refractive index of water is 1.33,

what is the speed of light in water? [Ans: 2.25 × 108m/s]

STEP 4

8. Discuss the role of a medium in bending of light when light propagates through it.

9. Explain any two effects of refraction of light experienced by you.

10. Explain the importance of electromagnetic waves in our daily life.

112 Light

UNIT Estimated teaching periods Theory Practical
8 2
6
Sound

Syllabus issued by CDC Heinrich Hertz

 Sound and nature of sound waves
 Production and propagation of longitudinal waves
 Characteristics of sound wave (frequency, time period, wavelength, amplitude, wave velocity)
 Relation between velocity of sound and medium
 Speed of sound in air
 Spectrum of sound wave (infrasound, audible sound and ultrasound)
 Application of ultrasound
 Reflection of sound (echo and reverberation)
 Application of reflection of sound
 Refraction of sound
 Intensity, pitch, speed and noise

LEARNING OBJECTIVES

At the end of this unit, students will be able to:
 describe the nature of sound wave.
 find the sources and frequency of infra, audible and ultrasounds.
 describe the reflection and refraction of sound with appropriate figure and explain their effect in daily life.
 find the speed of sound, intensity and pitch.
 describe the effects of sound in the environment.
 find the effects of noise pollution and ways to control it.

Key terms and terminologies of the unit

1. Sound : Sound is a form of energy which produces the sensation of hearing.
2. Wave :
A wave is the periodic disturbance in a medium that carries energy from one place
to another.

3. Mechanical wave : The wave that requires a material medium for its propagation is called a mechanical
wave.

4. Electromagnetic wave : The wave that does not require any material medium for its propagation is called
an electromagnetic wave.

5. Transverse wave : The wave in which the direction of the vibration of the particles of the medium is
perpendicular to the direction of the propagation of the wave is called a transverse
wave.

6. Crest : In a transverse wave, the upward displacement of a vibrating particle from the
mean position is called a crest.

Modern Concept Science - 9 113

7. Trough : In a transverse wave, the downward displacement of a vibrating particle from the

mean position is called a trough.

8. Amplitude : In a transverse wave, the maximum displacement of a vibrating particle from the

mean position (either upward or downward) is called an amplitude.

9. Wavelength : In a transverse wave, the distance between two consecutive crests or troughs is

called the wavelength.

10. Frequency : The number of waves produced per second is called frequency.

11. Longitudinal wave : The wave in which the direction of the vibration of particles of the medium is parallel

to the direction of the wave propagation is called a longitudinal wave.

12. Compressions : In a longitudinal wave, compressions are the regions where density, as well as

pressure, is high.

13. Rarefactions : In a longitudinal wave, rarefactions are the regions of low pressure and low density

where particles are spread apart.

14. Medium for sound : The matter or substance through which sound is transmitted is called medium for

sound.

15. Speed of sound : The distance travelled per unit time by a sound wave propagating through a medium

is called speed of sound.

16. Pitch of the sound : The shrillness or sharpness of the sound is called pitch of the sound.

17. Loudness : Loudness is the characteristic of a sound that distinguishes a feeble sound from a

loud sound of the same frequency.

18. Tone : The sound of a single frequency is called tone.

19. Note : Sound of a mixture of several frequencies is called a note.

20. Infrasound: The sound waves with frequencies below 20Hz are called infrasound or subsonic
wave.

21. Audible sound : The sound wave that a human ear can hear is called an audible sound. The

frequency of an audible sound is 20Hz to 20,000Hz (20kHz).

22. Ultrasound : The sound above the frequency of 20,000Hz is called an ultrasound.

23. Reflection of sound : The bouncing back of sound when it strikes a hard surface is called reflection of

sound.

24. Echo : The repetition of sound because of the reflection of the sound waves is called an

echo.

25. Echolocation : Echolocation is the process of locating objects in the surroundings with the help of

a reflected sound.

26. Timber of sound: The quality of sound is called timber of sound.

27. SONAR: The full form of SONAR is Sound Navigation And Ranging. This is a method to

measure the depth of sea bed or locate scraps, wrecks, and submarines of enemies,

etc. in the water by producing ultrasound.

28. Reverberation : The phenomenon of prolongation of sound due to a series of reflections is called

reverberation.

29. Refraction of sound: Bending of sound waves at the interface of the two media is called refraction of

sound wave.

30. Sound pollution : The disturbance produced in the environment by undesirable, loud and harsh

sounds from various sources is called sound pollution or noise.

114 Sound

Introduction

We hear many sounds every day. These sounds are produced from humans, birds, bells,
machines, vehicles, televisions, radios, etc. After production, these sounds propagate through
the material medium and reach to our ears. Thus, sound is a form of energy which produces
the sensation of hearing. It is an important part of existence for many living beings. Animals use
sound to hunt and to warn of the approach of predators. Sounds are of various types. The sound
of a siren makes people more attentive, while the sound of music has a tremendous effect of
relaxing the mind. In this chapter we are going to learn how sound is produced and how it gets
transmitted through a medium in the form of a wave. We will also learn about the characteristics
of sound, sound spectrum, reflection of sound, refraction of sound and noise pollution.

FACT WITH REASON

Light wave is an electromagnetic wave, but sound wave is a mechanical wave. Why?
Light wave consists of both the electric and magnetic fields. It doesn't need a material medium for its
propagation. It can travel through vacuum. But, sound wave needs material medium for its propagation.
So, light wave is an electromagnetic wave, but sound wave is a mechanical wave.

Production of Sound

Sound is produced from different sources due to their vibrations. The vibrating body causes
the medium like water, air, etc. around it to vibrate. Such vibrations in a medium are called
waves. They carry energy from one place to another.

ACTIVITY 1

1. Take a drum and beat it. Do you hear the sound of the drum?
2. Touch the membrane of the drum. What do you feel?
3. When the sound stops, touch the membrane of the drum again. Do you feel

vibrations on the membrane of the drum?
It can be concluded that the vibrating membrane of the drum produces sound.

Waves Ripples produced in a pond

What do you see on a still water surface when a stone is dropped
in it? Do you see the ripples on the surface of water? When waves
propagate through water, the molecules of water vibrate up and
down at their respective positions. The ripples carry energy from
stone to all parts without the actual movement of the water molecules
from one point to another. A wave is the periodic disturbance in a
medium that carries energy from one place to another.

FACT WITH REASON

Waves transport energy but not matter from one region to another, why?

When a stone is dropped in water, its kinetic energy displaces the water molecules around it. Each molecule
of water displaces the next molecule near it. This process continues until all the energy gets transferred. So
waves transport energy but not matter from one region to another.

Modern Concept Science - 9 115

Types of Waves
a) Classification Based on the Requirement of a Medium for the Wave Propagation

Mechanical wave

The wave that requires a material medium for its propagation is called a mechanical
wave. This wave is also called an elastic wave because it depends on the elastic nature
of the medium. For example, sound waves, waves set on a stretched spring, etc.

FACT WITH REASON

Sound wave is a mechanical wave, why?
Sound wave carries energy by sharing vibration of molecules in a medium like air, water, steel, etc.
Sound needs a material medium for its propagation. It cannot travel through vacuum. So sound wave is a
mechanical wave.

Electromagnetic wave

Electromagnetic waves consist of oscillating electric and magnetic fields. The wave that does
not require any material medium for its propagation is called an electromagnetic wave.
Such waves can travel through vacuum, too. For example, light waves, radio waves, etc.

b) Classification Based on the Direction of the Vibration of the Particles of the Medium

Transverse wave Direction of vibration of particles

The periodic motion set in the rope, with a wave P

of crests and troughs, is called a transverse

wave. Thus, the wave in which the direction of

the vibration of the particles of the medium is A B
perpendicular to the direction of the propagation Direction of propagation of wave

of the wave is called a transverse wave. For

example, ripples on water surface, light wave, a Q

wave pattern set in a stretched string, etc. Vibration of medium particle in a transverse wave

ACTIVITY 2

1. Take a rope and attach it to a fixed place. Direction of wave motion
2. Shake the rope up and down. Direction of vibration
3. Observe the up-and-down motion of the rope creating of rope particles

high points and low points.

Terms Used in Transverse Wave

i) Crest: In a transverse wave, the upward displacement of a vibrating particle from
the mean position is called a crest.

ii) Trough: In a transverse wave, the downward displacement of a vibrating particle
from the mean position is called a trough.

116 Sound

iii) Amplitude: In a transverse wave, the maximum displacement of a vibrating
particle from the mean position (either upward or downward) is called an amp-
litude. It is measured in meter or centimeter.

iv) Wavelength: In a transverse wave, the distance between two consecutive crests or
troughs is called the wavelength. The wavelength is represented by λ (Greek let-
ter lambda). Its SI Unit is meter (m).

v) Frequency: The number of waves produced per second is called frequency. The

frequency is represented by 'f'. Its SI Unit is hertz (Hz). A wave with one complete

cycle in one second has a frequency of 1Hz. The Memory Tips

bigger units of hertz are: Hertz was honored with the unit of
1 kilohertz (kHz) = 103 Hertz (Hz) frequency after the name Heinrich

1 Megahertz (MHz) = 106 Hertz (Hz) Hertz.

Displacement Wavelength Crest Crest
Crest

Trough Trough Direction of propagation

Wavelength

Graphical representation of a transverse wave

FACT WITH REASON

A wave has a frequency 100Hz. What does it mean?
A wave has a frequency 100Hz, it means that the wave makes 100 complete cycles in the one second.

vi) Time period : The amount of time required for Memory Tips
a wave to complete one full cycle is called the
time period. It is represented by 'T'. Its SI Unit Relation between frequency (f) and
time period (T) is T = 1/f
is second.

Longitudinal wave

Stretch a 'slinky' spring along the floor and give it a quick wriggle to and fro along the

length of the spring. What do you observe? If you mark a dot on the slinky, you will

observe that the dot on the slinky will move back and forth, parallel to the direction

of the propagation of the disturbance. Thus, the wave in which the direction of the

vibration of particles of the medium is parallel to the direction of the wave propagation

is called a longitudinal wave. For example, sound wave, a wave pattern set in a stretched

spring, etc.

Particles vibrating along the direction of motion of wave

C R C R C Direction of propagation
Particles of medium vibrating in a longitudinal wave

Modern Concept Science - 9 117

Terms Used in Longitudinal Wave

i) Compressions : In a longitudinal wave, compressions are the regions where den-
sity, as well as pressure, is high. They are represented by the letter C.

ii) Rarefactions : In a longitudinal wave, rarefactions are the regions of low pressure
where particles are spread apart. They are represented by the letter R.

FACT WITH REASON

The sound waves are longitudinal waves. Why?
Propagation of sound in air consists of areas of high pressure called compressions and areas of low pressure
called rarefactions. So, the sound waves are longitudinal waves.

iii) Amplitude : The maximum displacement of vibrating particle from its mean posi-
tion is called amplitude. Its SI unit is metre (m).

iv) Wavelength: The distance between two consecutive compressions (C) or two
consecutive rarefactions (R) is called the wavelength.

v) Frequency: The number of complete cycles made in one second is called frequency.
It is measured in hertz (Hz).

Tuning Gderenastietyr dLeensssity Gderenastietyr dLeensssity Gderenastietyr dLeensssity
fork

C = Compression
R = Rarefaction

Longitudinal sound wave propagation in air

Differences between Transverse Waves and Longitudinal Waves

Transverse Waves Longitudinal Waves

1. The wave in which the direction 1. The wave in which the direction of the
of the vibration of the particles of vibration of particles of the medium
the medium is perpendicular to the is parallel to the direction of the wave
direction of the propagation of the propagation is called a longitudinal
wave is called a transverse wave. wave.

2. These waves can be generated in 2. These waves can be generated in solids,
solids and liquids. liquid and gases.

3. A transverse wave consists of a crest 3. A longitudinal wave consists of a
and a trough. compression and a rarefaction.

4. It travels with high speed. 4. It travels with low speed.

5. It can travel through vacuum. 5. It cannot travel through vacuum.

118 Sound

Propagation of Sound

The matter or substance through which sound is transmitted is called a medium. It can be
solid, liquid or gas. Sound moves through a medium from the point of generation to the
listener in the form of longitudinal waves.
A vibrating object vibrates the particles of the medium around it. The particles do not travel
all the way from the vibrating object to the ear. A particle of the medium in contact with the
vibrating object is first displaced from its equilibrium position. It then exerts a force on the
adjacent particle to displace it and returns to its original position. This process continues in the
medium till the sound reaches our ear.

FACT WITH REASON

The sound of explosion is not heard on the moon, why?
Moon has no atmosphere. The sound never propagates through a vacuum. If some explosion takes place
on the moon, the sound of explosion will not be propagated. So the sound of explosion is not heard on the
moon.

Speed of Sound

The distance travelled per unit time by a sound wave propagating through a medium is called
speed of sound. Sound propagates with different speeds in different media. Its speed is 343
m/s when travelling through the air at 20oC.
Wave Equation: Relationship among Frequency, Wavelength and Velocity
The speed of sound wave is equal to the product of wavelength (λ) and frequency (f).
Mathematically,
v=f×λ

FACT WITH REASON

Sound waves of all frequencies propagate with same speed through a medium, why?
When frequency of a wave increases, its wavelength decreases and vice versa. The product v = f × λ remains
the same. So, sound waves of all frequencies propagate with same speed through a medium.

Solved Numerical 6.1

When the C4 key on a piano keyboard is pressed, a string inside the piano is struck by a
hammer and begins vibrating back and forth at approximately 260 cycles per second.
a) What is the frequency in Hertz of the sound wave?
b) Assuming the sound wave moves with a velocity of 343m/s, what is the wavelength

of the wave?
Solution: Given,
Velocity of sound waves (v) = 343 m/s
Number of cycles per second in vibrating string = 260
Frequency of vibrating string (f) = 260 Hz

Modern Concept Science - 9 119

According to the wave equation,

Wavelength of the sound wave,

λ = v = 343 = 1.32
f 260

Therefore, the wavelength of the sound wave is 1.32 m. Memory Tips

Speed of Sound in Different Media Of the three mediums (gas, liquid,
and solid) sound waves travel
The speed of sound varies greatly in different media. It is the the slowest through gases, faster
property of a medium that determines the speed of sound through liquids, and fastest through
in it. The more compact the medium, the faster the speed of solids. vgas < vliquid < vsolid
sound. Temperature also affects the speed of sound. 

a) In solids : Solids are packed together tighter than liquids and gases, i.e., solids are
denser than liquids and gases. The distances between molecules in solids are very small.
They can collide very quickly. It takes less time for a molecule of the solid to colloid into
its neighbor. Hence, sound travels the fastest in solids.

b) In liquids : Liquids are denser than gases, but less dense than solids. The distances
between molecules in liquids are shorter than in gases, but longer than in solids. So it
takes a bit more time to share vibrations in liquids than in solids. Hence, sound travels
faster in liquids than in gases but slower than that in solids.

c) In gases : Gases are the least dense. The molecules in gases are very far apart compared
with solids and liquids. It takes more time for molecules in sharing vibrations in gases
than in solids and liquids. Hence sound travels the slowest in gases.

S.N. Medium Temperature Speed of Sound (in m/s)
0°C 332
1. Air 25°C 346
25°C 316
2. Gases Oxygen 25°C 1284
25°C 1207
3. Hydrogen 25°C 1450
25°C 1498
4. Ethanol 25°C 1531
25°C 5120
5. Liquid Mercury 25°C 3980
6. Distilled water 25°C 5960

7. Sea water

8. Aluminium

9. Solid Glass(flint)

10. Steel

FACT WITH REASON

When a firework explodes, the light is perceived before the sound, why?

Sound travels more slowly than light. So, when a firework explodes, the light energy is perceived before the
sound energy.

120 Sound

Factors Affecting Speed of Sound in Air Memory Tips

a) Density : At a constant pressure, speed of sound in v=f×λ

air is inversely proportional to the square root of the or, 1 = λ or, T = λ
f v v
density of air.
Hence, time period is equal to the
1
Speed (v) ∝ √ density ratio of wavelength to wave speed.

The speed of sound becomes greater in air when its density decreases.

b) Temperature : The speed of sound is directly proportional to square root of temperature,

Speed (v) ∝ √ temperature (T)

It means, the speed of sound increases with increase in temperature.

FACT WITH REASON

We hear more clearly on a hot day than on a cold day, why?

The speed of sound increases with the increase in temperature. For example, the speed of sound waves in
air at 0°C is 332 m/s and the speed of sound at 20°C is 343 m/s. So, we hear more clearly on a hot day than
on a cold day.

c) Humidity : When moisture is present in the air, the density of air decreases. It is because
the density of water vapors is less than that of dry air. As the density of air decreases due to
moisture, the speed of sound increases. Therefore, an increase in humidity in air increases
the speed of sound and decrease in humidity in air decreases the speed of sound.

Density of moist air < density of dry air

Therefore, Speed of sound in moist air > speed of sound in dry air.

FACT WITH REASON

On a rainy day sound travels faster than on a dry day, why?
An increase in humidity in air increases the speed of sound and decrease in humidity in air decreases the
speed of sound. So, on a rainy day sound travels faster than on a dry day.

d) Direction of Wind : The speed of sound increases in the direction of wind. i.e. Speed of
sound = speed of sound in still air + speed of wind

If the wind blows in the opposite direction in which the sound travels, the speed of
sound decreases.

Characteristics of Sound

A sound can be characterized by its frequency (pitch), amplitude (loudness) and quality.
Pitch

Do you distinguish the shrillness of the voice in boys and girls? The voice of boys is less shrill
than the voice of girls. A more shrill sound is said to be of a high pitch and the less shrill
sound is said to be of a low pitch. The shrillness or sharpness of the sound is called pitch of
the sound. It is the effect produced in the ear due to the sound of some particular frequency.

Modern Concept Science - 9 121

Pitch is not a measurable quantity but it can be given a name on the musical scale. For example,
the middle C note has a frequency of 262 Hz.

Relationship between Pitch and Frequency of a Sound
Pitch depends on the frequency of a vibrating body. Pitch and frequency of a sound are directly
proportional. The higher the frequency of a sound, the high is the pitch and vice versa.

A body vibrating with high frequency produces a sound of high pitch and the body vibrating
with low frequency produces a sound of low pitch. For example, a mosquito’s wings vibrate
rapidly producing a sound of high pitch.

FACT WITH REASON

There is a variation in the pitch of sound in boys and girls, why?
The tighter vocal cords in girls vibrate more rapidly and the sound produced is of a high pitch. This causes
the girls to produce sound in higher pitches than that of the boys.

Factors Affecting Pitch of Sound
i) Thickness of Vibrating String
The pitch of sound increases with the decrease in thickness of the vibrating wire. It is for
the same reason that stringed musical instruments like guitar are provided with strings of
different thickness, so as to produce notes of different pitches.

FACT WITH REASON

Small thickness of vibrating string produces a high pitched sound, why?
Small thickness of vibrating string vibrates rapidly. That is why; it has high frequency and high pitch of sound.

ii) Length of the Vibrating String
Small length of vibrating string produces a high pitched sound and vice versa. This is the
reason why musicians plunk the same string at different points so as to have a sound of
different frequencies or pitches.

FACT WITH REASON

Small length of vibrating string produces a high pitched sound, why?
Small length of vibrating string vibrates rapidly. That is why; it has high frequency and high pitch of sound.

Loudness or Intensity of Sound
Loudness is the characteristic of a sound that distinguishes a feeble sound from a loud sound
of the same frequency. Its SI unit is W/m2. It is the human perception of how much energy a
sound wave carries. The loudness of sound is commonly measured in the unit decibels (dB).

A small change in the dB value of a sound represents a large change in its loudness. Every 0.1
increase in the value of dB, will increase the loudness by 10. For example, the sound of 20 dB
is 100 times louder than the sound of 10 dB.

Human ears can percept sounds from 10 dB to 180 dB. A sound of loudness in between 50 dB
to 60 dB is considered as normal for hearing.

122 Sound

Sounds Sound level in dB
1. Threshold of hearing 0
2. Swishing of leaves
3. Whisper 0-20
4. Normal conversation 0-20
5. Moving vehicles 40-60
6. Factory noise 60-80
7. Loud music in disco 80-90
8. Jet aircraft 130
9. Noise of railway station 140
10. Sound of printing press 85-110
11. Sound of motor car 70-80
12. Heavy street traffic 110-120
13. Listening limit of hurt 60-70
14. Sound produced due to mechanical failure 120-140
140-160
Factors Affecting Loudness of Sound

Amplitude (a)

Amplitude depends on the energy the source puts into the wave. So the loudness of a sound
is determined by the amplitude of the sound wave. A larger amplitude means a louder sound,
and a smaller amplitude means a softer sound.

Thus, the loudness of a sound is directly proportional to the square of the amplitude of the
sound.

Loudness of sound ∝ (amplitude)2

FACT WITH REASON

In the given figure, which sound has higher pitch and which one is louder. Why? A
B
In the given figure the frequency of sound A is higher than the frequency of sound B and C. So C
sound A has higher pitch than the sound B and C. The amplitude of sound C is longer than that of
sounds A and B. So, sound C has more intensity and more louder.

Distance from the Source
Loudness of sound decreases with the increase in distance of the observer from the source.
This is the reason why in a classroom, if a student at the back benches cannot hear the teacher,
then the student is asked to sit on a front bench.

FACT WITH REASON

When we are close to the loud speaker, we hear a loud sound. Why?
Loudness of sound decreases with the increase in distance of the observer from the source. When we are
close to the loud speaker, we hear a loud sound as compared to far distance.

Modern Concept Science - 9 123

Area of Sounding Surface

A larger surface of a sounding object sets more molecules in vibration than a smaller sounding
surface. Thus, a larger sounding surface produces a louder sound than that from a smaller
sounding surface. For example, a large drum will produce a louder sound than a smaller
drum when they are struck equally.

FACT WITH REASON

The sound produced from the big bell can be heard for a long distance than the small bell. Why?
Loudness of sound is directly proportional to the surface area of the sounding body. Thus, a larger bell
produces a louder sound than that from a small bell.

Density of the medium

The loudness of sound is directly proportional to the density of the medium, i.e. the greater
the density of the medium, the louder is the sound.

Differences between Loudness and Pitch

Loudness Pitch

1. Loudness is the characteristic of a sound 1. The shrillness or sharpness of the sound

that distinguishes a feeble sound from is called pitch of the sound.

a loud sound of the same frequency.

2. Loudness of sound is determined by 2. Pitch of sound is determined by the

the amount of sound energy carried by frequency of the sound wave.

the sound.

3. Loudness does not change with change 3. Pitch of a sound is directly proportional

in frequency. with its frequency.

Quality

The quality of sound is also called timber of sound. It is one of the characteristics of sound that
enables us to differentiate between two different types of sound. Even as the vocal cords are
of the same length and thickness in the human beings, their sounds do not resemble. It is due
to the modification of sound produced depending upon the shape of mouth cavity, shape of
teeth, shape of lips, etc. Thus, the quality of sound is peculiar to an individual.

FACT WITH REASON

The sound of different persons can be recognized, how?

Sounds produced from different people have different timber. Because of the difference in timber of sound,
the sound of different persons can be recognized.

Tone and Note of Sound

The sound of a single frequency is called tone and the sound of a mixture of several frequencies
is called a note. A note is pleasant to listen.

124 Sound

Music and Noise

Music consists of sound waves that are produced by regular and periodic vibrations. It is
pleasant to hear because of the good quality called timber. Noise is produced when vibrations
are irregular and non-periodic. A noise is unpleasant because of bad quality.

Amplitude
Amplitude

Time Time

Graphical representation of a musical sound Graphical representation of a noise

Musical Sound Noise

1. It has a pleasing effect on the ears. 1. It does not have a pleasing effect on the
ears.

2. It is produced by regular periodic 2. It is produced by irregular vibrations in

vibrations in a material. a material.

3. The amplitude of vibration and its 3. The amplitude of vibration and its

frequency do not change suddenly. frequency change suddenly.

Spectrum of Sound Waves

The orderly classification of sound waves according to their wavelength or frequency is called
the sound spectrum. Infrasound, audible sound and ultra sound are the three types of sound
on the basis of frequency.

Infrasound (Infrasonic) or Subsonic

The sound waves with frequencies below 20Hz are called infrasound or subsonic wave.
Infrasound is produced because of very slow vibrations. We cannot hear infrasound but
many animals; such as whales, rhinoceros, elephants, etc. can produce and hear infrasound.
Animals use infrasound for their communication. Elephants communicate using sound waves
at 10 Hz. Rhinoceroses communicate with one another by using infrasonic sound of frequency
as low as 5 Hz. Earthquake waves are subsonic waves. We do not hear these waves but some
animals do. As they hear infrasonic sounds produced by the earthquake, they show abnormal
behavior before we feel the earthquake.

Audible Sound

The sound wave that a human ear can hear is called an audible sound. The frequency of an
audible sound is 20 Hz to 20,000 Hz (20 kHz). Sounds produced from musical instruments,
our vocal cord, bell, etc. are all audible sounds.

Human ear is not equally sensitive to all frequencies. Most people cannot hear sounds with
frequencies below 20 Hz or above 16,000 Hz. Older people are less sensitive to frequencies
above 10,000 Hz. By age 70, most people cannot hear sounds with frequencies above 8000 Hz.
However, children under 5 years of age can hear the sound up to 25,000 Hz.

Modern Concept Science - 9 125

10–20,000 Hz 20–20,000 Hz 15–50,000 Hz 60–65,000 Hz 40–12,000 Hz

Ultrasound

Sound above the frequency of 20,000 Hz is called an ultrasound.
Humans cannot hear the ultrasound. However, many animals
such as dog, cat, bat, monkey, deer, etc. can detect sound beyond
the audible range of humans. Bats and dolphin can produce and
hear ultrasound. Dogs and cats cannot produce ultrasound, but
they can hear it.

Uses of Ultrasound
Due to high frequency of ultrasound, it is associated with more energy and can penetrate upto
a large extent. This characteristic of ultrasound makes it very useful for many purposes. Some
of its uses are given below.

i) Doctors can identify the diseases of inner body and locate the position of the tumour
with the help of an ultrasound.

ii) An ultrasound scan of the womb helps to obtain the image of a human foetus in the
womb.

iii) A concentrated beam of ultrasound can be used to break up kidney stone without a
surgery.

iv) Bats produce ultrasounds and detect the reflected sound waves coming from obstacles
on the way. This helps bats to identify the position of preys or obstacles on the way.

v) Dolphins use ultrasound to catch their prey.

vi) Ultrasonic waves are used in fathometer to calculate the depth of water in sea.

vii) To clean machinery parts which are beyond reach without disassembling of their parts.

viii) It is used to detect the deformities in metal blocks.

ix) It is used in detection of any blockade in a pipe line.

FACT WITH REASON

How can a bat identify location of preys and obstacles?
Bats produce ultrasounds and detect the reflected sound waves coming from obstacles on the way. This
helps bats to identify the position of preys or obstacles on the way.

126 Sound

Differences between Infrasound and Ultrasound

Infrasound Ultrasound

1. The sound waves of frequency below 20 1. The sound waves of frequency above

Hz is called infrasound. 20,000 Hz (20kHz) is called ultrasound.

2. Seismic waves are infrasound. 2. Bats and dolphins produce ultrasound.

Reflection of Sound

Sound wave gets reflected at the surface of a solid or liquid and follows the same laws of
reflection as those of light. Thus, the bouncing back of sound when it strikes a hard surface is
called reflection of sound.

FACT WITH REASON

The ceilings of concert halls are curved, why?
The concert halls are big, so audiences at the back rows of the hall may not hear the sound of speaker clearly.
To overcome this problem, the ceiling of the concert halls is made into a concave shape. Concave ceilings
help the sound wave to get reflected and sent to a farther distance. It enables the audience to receive a clear
sound even while sitting in the back rows of the hall. So the ceilings of concert halls are made curved.

Echo

Echo is the sound that we hear after its reflection. One can hear the echo by shouting loudly
at a place surrounded by big buildings or by hills. After shouting loudly, the same sound
reaches to our ears after getting reflected from the surface of the walls. Thus, the repetition
of sound because of the reflection of the sound waves is called an echo. Echoes can be heard
from almost any large surface such as a large wall, building or cliff. A person can hear two
sounds distinctly in the interval of 0.1 second. This time duration to listen to two distinct
sounds clearly is called the persistence of hearing. So an echo can only be heard if the same
sound comes to one’s ear after an interval of 0.1 second. At the room temperature (20°C),
sound moves through air at sea level at a speed of 343 m/s. The minimum distance to listen to
the echo can be calculated as:

2d = v × t

or, d= 343 × 0.1 = 17.15 m
2

FACT WITH REASON

One can hear sounds repeating while shouting loudly at a place surrounded by big buildings or by hills,
why?

As the light waves, sound waves also get reflected from a hard surface. After shouting loudly, the same
sound reaches to our ears after its reflection from the surface of the walls. So one can hear the repeated
sounds by shouting loudly at a place surrounded by big buildings or by hills.

Modern Concept Science - 9 127

Conditions to Listen to Echo

i) The reflecting surface must be relatively flat, perpendicular to the source of the sound.

ii) Reflection of sound must reach to the brain after an interval of 0.1 second or the reflector
of sound must be at least at 17 m, but not so far away that the echo is too weak.

Echolocation or Sound Navigation and Ranging (SONAR)

Echolocation is the process of locating objects in the
surroundings with the help of a reflected sound. The reflection
property of sound is used in SONAR (sound navigation and
ranging) to measure the depth of sea and to locate the position
of objects in the sea.

Echolocation in Animals

Animals like dolphins, bats, etc. use echolocation to hunt for
food and to find objects in their path.

Calculation of the Depth of the Ocean Bed

A fathometer on a ship generates ultrasound waves. Let the downward distance travelled up
to bed of the ocean by the waves be 'd'. When these waves reflect back from the bed of the
ocean and are received by the fathometer, the upward distance travelled is also 'd'. Hence the
total distance travelled is equal to '2d'. If 'v' be the velocity of waves in water and 't' be the total
time taken for the waves to reach back to the fathometer, then from the definition of speed,

Speed = Distance travelled = 2d
Total time taken t

or, d= v×t
2

This equation gives the depth of the ocean bed.

Solved Numerical 6.2

An ultrasound is sent from a fathometer towards the bottom of the ocean. If the fathometer
receives the reflected ultrasound from the bed of the ocean in 6 seconds and the speed of
the ultrasound in sea water at 25oC is 1531 m/s, calculate the depth of the ocean.

Solution: Given,

Speed of ultrasound in water at 25°C (v) = 1531 m/s

Time taken to receive the reflected sound (t) = 6 seconds

The depth of the ocean (d) =?

From the formula,

d= v × t = 1531 × 6 = 4593 m
2 2
Thus, the depth of the ocean is 4593 m.

Solved Numerical 6.3

If you shout into a well that is 122.5 m deep, as shown in the given figure, how soon after
you will hear the reflected sound from the bottom of the well? (Speed of the sound = 345m/s)

128 Sound

Solution: Given,

Speed of ultrasound (v) = 345 m/s

Depth of the well (d) = 122.5 m

Time taken to receive the reflected sound (t) = ?

From the formula,

v = 2d
t

or, t = 2d = 2 × 122.5 = 0.71 s
v 345

Thus, the sound will be heard after 0.71 seconds.

Solved Numerical 6.4

An automatic focus camera is able to focus on objects by the use of an ultrasonic sound
wave. The camera sends out sound waves that reflect off distant objects and return to the
camera. If a sound wave travelling with a speed of 345 m/s returns to the camera 0.115
seconds after leaving the camera, how far away is the object?

Solution: Given,

Speed of ultrasound (v) = 345 m/s

Time taken to receive the reflected sound (t) = 0.115 seconds

The distance of the object (d) =?

From the formula,

d= v × t = 345 × 0.115 = 19.8 m
2 2
Thus, the object is at a distance of 19.8 m from the camera.

Reverberation

When a source (S) produces sound in a small closed room, the observer (O) hears the sound
that travels not only directly from the source, but also indirectly from a series of reflections
that are taking place at the walls of the room. Due to multiple reflections, the observer can
hear sounds, even after the source has stopped producing it. Thus, the phenomenon of
prolongation of sound due to a series of reflections is called reverberation.

Sound becomes too blurred to be heard in the auditorium because of reverberation. This can
often lead to irritation. To overcome this problem, sound absorbing materials, such as curtains,
plant fibres, compressed fireboard, carpets, etc. are used in the auditorium. These materials
absorb undesired reflected sound and reduce reverberation.

Conditions for Reverberation
i) The distance between the source of sound and the reflector should be less than 17m.
ii) There should not be an absorber of sound in the room or hall.

Methods to Reduce Reverberation

In an auditorium or a big hall, an excessive reverberation is highly undesirable. Following
measures are taken to reduce the reverberation.

Modern Concept Science - 9 129

i) The walls and roofs are covered with sound absorbing materials like rough plaster.

ii) Sound absorbing material is used as a carpet on the floor.

iii) Heavy curtains are used at the entrance, exit and door.

iv) Sound absorbing panels are kept near the stage.

FACT WITH REASON

In an auditorium or a big hall, the walls and roofs are covered with sound absorbing materials, why?
In an auditorium or a big hall, the observer can hear sound even after the source has stopped producing it.
Sound becomes too blurred to be heard. So in an auditorium or a big hall, the walls and roofs are covered
with sound absorbing materials.

Differences between Echo and Reverberation

Echo Reverberation

1. The repetition of sound because of the 1. The phenomenon of prolongation of

reflection of the sound waves is called sound due to a series of reflections is

an echo. called reverberation.

2. The distance between the source of 2. The distance between the source of sound

sound and the reflector should be more and the reflector should be less than 17m.

than 17m.

Applications of reflection of sound Memory Tips

i) Velocity of sound in air can be calculated with the In stethoscope, sound is received
help of an echo. from the chest by multiple reflection
of sound in the tube of stethoscope.
ii) Reflection of sound is used in SONAR to find the
depth of ocean beds.

iii) Bats can move easily in the dark with the help of reflected ultrasound.

iv) Stethoscope is used to hear the sounds of internal organs. It works on the low of
reflection of sound.

v) Sound board is used to send the sound towards audiences in a big hall or auditorium.

Refraction of Sound

Bending of sound waves occur at the interface of the two media. Sound waves also follow the
laws of refraction as in the case of light. The speed of a sound wave changes when it travels
from one medium to another.

Sound becomes more distinct in the night time

In the night time, the ground cools quickly. Also, the air layers
near the ground become colder than those above it. In such
conditions, the sound waves bend away from normal in each
refraction through the air layers. As a result, the sound bends
towards the earth’s surface. Thus, sound becomes more distinct
in the night time.

130 Sound

FACT WITH REASON

Sound becomes more distinct in the night time. Why?
During night, the ground layer of air is thicker than the upper layer. The sound waves bend away from
normal in each refraction through the air layers. As a result, the sound bends towards the earth’s surface.
Thus, sound becomes more distinct in the night time.

Sound becomes fainter in the day time
In the day time the ground gets heated quickly. Also, the air
layers near the ground become hotter than those above. The
speed of sound is more in a hot and light air than that in a cold
dense air. In such conditions, the sound waves bend towards
normal in each refraction through the air layers. As a result, the
sound bends away from the earth’s surface. Thus, sound is not
heard clearly at long distances on the earth in the day time.

FACT WITH REASON

Sound becomes fainter in the day time. Why?
During day, the ground layer of air is thinner than the upper layer. The sound waves bend towards normal
in each refraction through the air layers. As a result, the sound bends away from the earth’s surface. Thus,
sound is not heard clearly at long distances on the earth in the day time.

Sound Pollution or Noise

The disturbance produced in the environment by undesirable, loud and harsh sounds from
various sources is called sound pollution or noise. It is an adverse or unpleasant effect.
Causes of Sound Pollution (Noise)
The increasing dependence of the man on various kinds of machines at home or workplace or
factories is the cause of noise pollution.
Sources of Noise
i) Noise at homes : Television, radio, power music system, washing machines, mixture/

grinder, vacuum cleaner, etc. are the sources of noise at our homes.
ii) Noise in surroundings : Loudspeaker, exploding crackers on various functions and

festivals, vehicle horns, announcements, sound from construction works, etc. are the
sources of noise in our surroundings.
iii) Noise in factories : Various machines used in small and big factories produce noise.
Effects of Sound Pollution
i) Noise reduces concentration, increases stress, causes headache etc.
ii) Noise results loss of work efficiency.
iii) It causes anger, irritation and high blood pressure.

Modern Concept Science - 9 131

iv) Due to sound pollution, victims may even suffer heart attacks.

v) A constant hearing of noise in the level of 130 dB to 140 dB may result in the loss of
hearing or deafness.

Ways to Reduce Sound Pollution
i) Sound pollution can be reduced by using sound absorbing materials like thermocol,

thick curtains, carpets, etc.

ii) Vehicles exceeding allowed noise limits should be penalized.

iii) It can be reduced by restricting the use of loudspeakers in public places.

iv) It can be reduced by using noise control techniques in industries.

v) Airports and noise producing factories and industries must be shifted far away from the
residential areas of the city.

ANSWER WRITING SKILL

1. How is sound produced? Name the organ that produces sound in a human body.

Ans: Sound is produced by the vibration of the object like loud speakers, bell, turning fork, etc. Vocal cord
vibrates to produce sound in human beings.

2. What do you mean by infrasound? Write an use of it.

Ans: Sound having frequency less than 20 Hz is called infrasound. Infrasound is used by Giraffe to
communicate.

3. We do not hear sounds of bats, why?

Ans: Sound produced by bats is an ultrasound. Its frequency is beyond 20 kHz. So, it is not audible to us.

4. What is reflection of sound? Does sound follow law of reflection like light?

Ans: The bouncing of sound wave back into the same medium after striking in a surface of solid or liquid is
called reflection of sound. Yes, sound follows all the laws of reflection like light.

5. What is the minimum distance and minimum time necessary for echo to occur?

Ans: Minimum distance for echo to occur is 17.2 meters. Similarly, minimum time for echo to occur is 1
seconds. 10

6. Differentiate between music and noise.

Ans: The differences between music and noise are:

SN Music SN Noise

1 Music has regular and periodic vibration. 1 Noise has irregular and non-periodic
vibration.

2 Music is pleasant to hear. 2 Noise is unpleasant to hear.

7. How can we tell if a dark room is empty or not?

Ans: We may not see in dark room but we can speak. If sound reverberates or echoes then the room is empty.
If sound does not reflect then it is not empty because there must be something like furnitures, clothes,
or any stuff that absorbs sound.

132 Sound

8. List the factors that affect speed of sound in gases and loudness of sound.

Ans: Factors affecting speed of sound in gases are density of gas, temperature of gas, humidity and direction
of wind. Similarly, factors affecting loudness of sound are amplitude of sound wave, distance from the
source of sound, surface area of vibrating body, density of medium, motion of wind, etc.

9. Frequency of a wave is 50 Hz. Find out the time period.

Solution:

Frequency of the wave (f) = 50Hz

Now, time period of the wave is given by T = 1 = 1 = 0.02 s
Thus, time period of the wave is 0.02s. f 50

10. Speed of sound in air is 332 m/s. Find the wavelength of a sound wave having a frequency of 10 Hz.

Solution:

Speed of sound waves (v) = 332 m/s

Frequency of vibrating string (f) = 10 Hz

According to the wave equation,

Wavelength of the sound wave, λ = v = 332 = 33.2 m
Thus, wavelength is 33.2 m. f 10

11. A person standing near a cliff shouts and hears the echo after 2 seconds. If the velocity of sound in air is
332 m/s, find the distance between him and the cliff.

Solution:

Speed of sound in air (v) = 332 m/s

Time taken to receive the reflected sound (t) = 2 seconds

The distance of cliff from the observer (d) =?

From the formula,

d= v×t = 332 × 2 = 332 m
2 2

Thus, the distance of cliff from the observer is 332 m.

12. A person has a hearing range from 20 Hz to 20 kHz. Find the minimum and the maximum wavelengths
of the sound waves that normal people can hear. Take the speed of sound in air as 332 m/s.

Solution:

When frequency is minimum the corresponding wavelength is the maximum.

The minimum frequency of the audible sound fmin = 20 Hz
Now, according to the wave equation,

Wavelength of the sound wave, λ = v = 332 = 332 = 16.6 m
f f 20

The longest wavelength of the audible sound is 16.6 m

When frequency is maximum the corresponding wavelength is the minimum.

The maximum frequency of the audible sound fmax = 20 kHz = 20,000 Hz
Now, according to the wave equation,

Wavelength of the sound wave,

λ = v = 332 = 332 = 0.017 m
f f 20000

The minimum wavelength of the audible sound is 0.017 m.

Modern Concept Science - 9 133

13. Study the given table and answer the following questions.

i) Explain the relation between speed of sound and density of gas. Medium Speed of sound
Ans: The velocity of sound in gas is inversely proportional to the 332 m/s
square root of density of the gas. If density of medium is high A 1270 m/s
then the velocity of sound will be less and vice versa. B 258 m/s

ii) Which gas has minimum density among A, B and C? Why? C

Ans: Medium (B) has minimum density because this medium has the highest velocity of sound.

iii) Which gas has the highest density among A, B and C?

Ans: Medium (C) has the highest density because it has lowest velocity of sound.

STEPS EXERCISE

STEP 1

1. Define

a) Sound b) Wave
c) Mechanical wave d) Electromagnetic wave
e) Transverse wave f) Crest
g) Trough h) Amplitude
i) Wavelength j) Frequency
k) Longitudinal wave l) Compressions
m) Rarefactions n) Medium for sound
o) Speed of sound p) Pitch of the sound
q) Loudness r) Tone
s) Note t) Infrasound
u) Audible sound v) Ultrasound
w) Reflection of sound x) Echo
y) Echolocation z) Timber of sound
aa) SONAR ab) Reverberation
ac) Refraction of sound ad) Sound pollution

2. Very Short Answer Questions

a) Which organ of our body vibrates to produce sound?
b) Frequency of a wave is 100 Hz, what does it mean?

c) What is a wave equation?

d) Write down the range of audible sound to humans?
e) Write the relationship between the pitch and wavelength of a sound.
f) What is the intensity of sound?
g) Write the SI Unit of the following:

i) amplitude i) wavelength

iii) frequency iv) intensity of sound
h) What is the sound spectrum?

134 Sound

i) Write two examples of animals which use ultrasound.
j) What is meant by musical sound and noise?
k) Write the conditions to listen to an echo.
l) State the conditions to listen to reverberation.

STEP 2

3. Short Answer Questions
a) All vibrating bodies produce sounds. Are we able to hear all the sounds? Explain.
b) How is sound produced?
c) Explain the factors affecting the speed of sound in air.
d) What factors determine the pitch of a sound? Explain is short.

e) What causes the variation in the pitch of sound in boys and girls?

f) How is loudness related to amplitude?
g) How does the distance of a listener from a source of sound affect the loudness of

the sound?
h) If the wavelength of a sound wave is reduced by 2, what happens to the wave

frequency and speed of the wave?
i) Explain which of the following is (i) louder (ii) shriller

i) sounds produced by a drum and a guitar
ii) vibrating mosquito wings and roar of a lion
iii) voice of a girl and voice of a boy
j) What is meant by echolocation? How do the bats use echolocation to detect the
prey's position?

4. Differentiate Between

a) mechanical wave and electromagnetic wave
b) longitudinal wave and transverse wave
c) sound wave and light wave
d) compressions and rarefactions

e) musical sound and noise
f) infrasound and ultrasound.
g) echo and reverberation

5. Give Reason

a) Waves transport energy but not matter from one region to another.
b) Sound waves are longitudinal waves.
c) Light wave is an electromagnetic wave.

d) Sound waves of all frequencies propagate with the same speed through a medium.

e) Sound travels the fastest in solids.
f) Sound travels the slowest in gases.

g) Sound produced by a mosquito is shrill.

Modern Concept Science - 9 135

h) The thinner strings of a guitar give out shriller sounds than thicker strings.
i) The skin of a TABALA is tightened to produce shrill sounds.
j) Sound produced from different people can be recognized.

STEP 3

6. Write three applications of an ultrasound.

7. State three applications of an echo.

8. Write three methods to reduce reverberation.

9. What is sound pollution? How is it caused?

10. What are the harmful effects of sound pollution?

11. What measures can be taken to minimize sound pollution?

12. Answer the following questions based on the given figure.
a) What kinds of waves are given in the figure? Why?
b) Is it mechanical or electromagnetic wave? Why?
c) Which organs in our body produce these waves?

13. Numerical Problems

a) Find the frequency of the wave whose time period is 0.01s. [Ans: 100 Hz]

b) Speed of sound in air is 332 m/s. Find the wavelength of a sound wave having a

frequency of 10 Hz. [Ans: 33.2m]

c) The wavelength of a wave is 22m and its frequency is 15Hz. Find its speed.

[Ans: 330m/s]

d) A wave of frequency 1kHz has wavelength of 100cm. How long will it take to

move through a distance of 2km? [Ans: 2s]

e) A radar signal is reflected by an airplane and received in 2 × 10-5seconds after it
was sent. If the speed of the wave is 3 × 108m/s, how far is the airplane? [Ans: 3km]

f) A dog has a hearing range from 67 Hz to 47 kHz. Find the minimum and the

maximum wavelengths of the sound waves that the dog can hear. Take the speed

of sound in air as 332 m/s. [Ans: 4.96m and 0.0071m]

STEP 4

14. Explain the effect of density, temperature, humidity and direction of wind in speed of
sound when it propagates through air.

15. Explain the effect of refraction of sound during day time and in night time.

16. What is sound pollution? Explain the causes, effects and controlling measures of sound
pollution.

17. Explain the factors affecting intensity of sound.

136 Sound

UNIT Estimated teaching periods Theory Practical
10 2
7
Current Electricity and Magnetism

Syllabus issued by CDC Andre Ampere

 Introduction to current, electromotive force and potential difference
 Ohm's law
 Resistance and factors affecting resistance
 Conductivity and resistivity
 Magnetism, magnetic field and magnetic lines of force
 Terrestrial magnetism and elements of earth's magnetism (Angle of declination and angle of dip)

LEARNING OBJECTIVES

At the end of this unit, students will be able to:
 describe units of current, voltage, power and resistance with their measurement.
 explain Ohm's law and establish the relation among V, R and I.
 compare and measure the conductivity of different substances.
 demonstrate and describe magnetic field and magnetic lines of force.
 describe the elements of earth's magnetism (angle of dip and angle of declination).

Key terms and terminologies of the unit

1. Current electricity : The form of energy produced due to the continuous flow of electrons through a
conductor is called current electricity.

2. Source of electricity : A device that continuously generates electricity is called a source of electricity.
3. Cell :
A cell is a device which converts different forms of energy like chemical energy,

light energy, etc. into electrical energy.

4. Electrochemical cell : The apparatus that is used to produce electric current from spontaneous chemical
reactions is called an electrochemical cell.

5. Primary cell : A non-rechargeable cell in which the chemical reactions that convert chemical

energy into electrical energy cannot be reversed is called a primary cell.

6. Secondary cell : A rechargeable cell in which the chemical reactions that convert chemical energy into

electrical energy can be reversed by passing electricity is called a secondary cell.

7. Photocell : A photocell is a device that converts light energy into electricity.

8. Solar panel : The combination of photocells in a series is called a solar panel.

9. Dynamo : A dynamo or generator is a device that converts mechanical energy into electricity.

10. Hydroelectricity : The electricity generated from generators by using flowing water is known as

hydroelectricity.

Modern Concept Science - 9 137

11. Conductors : Substances that allow the electrons to flow through them are called conductors.

12. Insulators : Substances that do not allow the electrons to flow through them are called

insulators.

13. Charge : The electric property of electrons and protons is called the charge.

14. Electric current : The rate of flow of charge through a conductor is called the electric current.

15. One ampere (A) : When one coulomb of charge flows through a conductor in one second, then the

current flowing through it is said to be 1 ampere (A).

16. Electric circuit : A continuous path made by connecting an electric source, electric load, switch,

etc. with the help of a conducting wire is called an electric circuit.

17. Open circuit : The electric circuit through which current cannot flow due to an interrupted path is

called an open circuit.

18. Closed circuit : The electric  circuit  through which current can flow in an uninterrupted path is

called a closed circuit.

19. Resistance : The property of a conductor by virtue of which it opposes the flow of electrons

through it is called its resistance.

20. Series combination : The end to end combination of resistors is called a series combination of resistors.
21. Parallel combination : The type of combination of resistors in which all positive terminals are connected

at one point and all negative terminals are connected at another point is called a

parallel combination.

22. Electromotive force : Electromotive force (e.m.f.) of a cell is defined as the energy supplied by the cell

to drive a unit charge round the whole circuit.

23. Potential difference : The potential difference between any two points in an electric circuit is defined as

the amount of work done in moving a unit charge from one point to the other point.

24. One volt : If 1 joule of work is done in moving a unit charge from one point to the other point,

then the potential is called 1 volt.

25. Ammeter : An ammeter is a device to measure the electric current flowing in a circuit.

26. Galvanometer : A galvanometer is an electrical instrument used to detect the current in a circuit.

27. Voltmeter : The instrument that measures the potential difference across any two points in a

circuit is called a voltmeter.

28. Ohm's law : According to Ohm's law, "When physical conditions like temperature, pressure,

length of wire etc. remain constant, then the electric current (I) flowing through a

conductor is directly proportional to the potential difference across its ends."

29. One ohm : When a current of 1 ampere flows through a conductor by applying a potential

difference of 1 volt then the resistance of conductor is one ohm.

30. Conductance : The reciprocal of resistance is called electrical conductance.

31. Resistivity : The resistance of a conducting material having unit area of cress section and unit

length is called resistivity.

32. Magnets : Magnets are those substances which can attract magnetic substances like iron,

nickel, cobalt, etc.

138 Current Electricity and Magnetism

33. Magnetism : The properties shown by a magnet is called magnetism.

34. Magnetic compass : A magnetic compass is an instrument having a small magnetic needle, which is

free to rotate on a pivot at the center of a small box.

35. Magnetic field : The space around a magnet within which its effect can be experienced is called

the magnetic field.

36. Magnetic line of force : The path along which a unit north pole moves in a magnetic field is called a

magnetic line of force.

37. Terrestrial magnetism : Terrestrial magnetism is the study of the earth's magnetism and its various

elements.

38. Earth's magnetic field : The area where the earth's magnetic influence is felt is called the earth's magnetic

field.

39. Angle of declination : The angle between the magnetic meridian and the geographical meridian at a

place is known as the declination.

40. Angle of inclination : The angle made by the axis of dip needle with the horizontal line in magnetic

meridian is called angle of inclination or dip.

41. Neutral point : A point around a magnet at which the field due to the magnet is equal in magnitude

but opposite in direction to the earth's magnetic field is called a neutral point.

Introduction

Nowadays, we cannot imagine of our life without electric energy. It is the most useful and
convenient form of energy for mankind. It can be converted to different forms of energy easily.
It can be stored and transmitted over a long distance.

Elements are made up of atoms. Each atom has electrons revolving around its nucleus. In
case of metals like silver, copper, aluminium, etc. electrons flow from one point to another
to conduct electricity. Therefore, the form of energy produced due to the continuous flow of
electrons through a conductor is called current electricity. In this unit, we will discuss about
the sources of current electricity, components of electric circuit, electric potential, resistance
offered by a conductor, Ohm's law, conductivity and resistivity, etc.

Sources of Electric Current

Nowadays, a large number of electrical appliances are used in our daily life. So there is a
high demand of sources of electric current to make the electric devices work. For example,
a dry cell is used as a source of electric current in a remote controller. Thus, a device that
continuously generates electricity is called a source of electricity. Sources of electricity convert
different forms of energy like chemical, mechanical, light, etc. to generate electricity. The
various sources of electric current are:

Cell

A cell is a device which converts different forms of energy like chemical energy, light energy,
etc. into electrical energy. The combination of cells is called a battery.

Modern Concept Science - 9 139

a) Electrochemical Cells
The apparatus that is used to produce electric current from spontaneous chemical
reactions is called an electrochemical cell. The electrochemical cells are of two types:

i) Primary cells : A primary cell gets discharged when the chemicals used in it are
consumed. It cannot be recharged. So, a non-rechargeable cell in which the chemical
reactions that convert chemical energy into electrical energy cannot be reversed is called
a primary cell. Simple Voltaic cell, Leclanche cell, Daniel cell, dry cell, etc. are some
examples of primary cell.

ii) Secondary cells or accumulator : A rechargeable cell in which the chemical reactions
that convert chemical energy into electrical energy can be reversed by passing electricity
is called a secondary cell. Lead-acid cells, Nickel-Cadmium cells, Nickel-iron cells are
some examples of secondary cells.

FACT WITH REASON

The secondary cells are also called storage cells, why?
Secondary cells store electrical energy in the form of chemical energy during charging and convert the
chemical energy into electrical energy during discharging. So the secondary cells are also called storage
cells.

b) Photocell
A photocell is a device that converts light energy into electricity. To obtain a large
amount of current, a number of photo cells are joined in a series. Such combination of
photo cells in a series is called a solar panel.

Dynamo or Generator
A dynamo or generator is a device that converts mechanical energy into electricity. When
magnet or coil present in a generator is rotated, then electricity is generated through two
terminals of the coil. The faster the magnet or coil spins the more electricity is produced.
Hydroelectricity
The electricity generated from generators by using flowing water is known as hydroelectricity.
Water falling at a high speed is allowed to fall over the turbines connected with the generator.
It rotates the turbines to generate electricity.

Conductors and Insulators

Substances are classified as conductors and insulators on the basis of their nature to allow
electrons to flow through them and to resist the flow of charges.

Substances that allow the electrons to flow through them are called conductors. All metals,
solutions of acids in water, solutions of alkalis in water and solutions of soluble salts in water
are conductors. Metallic conductors have a large number of free electrons.

Substances that do not allow the electrons to flow through them are called insulators. For
example, rubber, plastic, wood, glass, etc. Insulators do not have free electrons.

140 Current Electricity and Magnetism

Charge

The electric property of electrons and protons is called the charge. The SI Unit of charge
is coulomb (C). Charge of an electron is 1.6 ×10-19 C. One coulomb charge is equivalent to
6.24×1018 electrons.

Current

The rate of flow of charge through a conductor is called the electric current. Current is denoted
by the letter 'I'. It is a scalar quantity.

If Q is the charge that is flowing through a conductor in time t, then current 'I' is given by

I = Q
t
The SI Unit of current is ampere. It is denoted by 'A'.

Multiple Units of Electric Current Sub-multiple Units of Electric Current

1 kiloampere (kA) = 103A 1 milliampere (mA) = 10–3A

1 megaampere (MA) = 106A 1 microampere (μA) = 10–6A

FACT WITH REASON

Current is a scalar quantity, why?

Electric current has a magnitude and a direction in a closed circuit. It may seem that current is a vector.
A vector always obeys the laws of addition of vectors. But current doesn't obey it and ordinary laws of
algebra are used to add currents. So current is a scalar quantity.

One ampere (1 A) current

We know that,

1 ampere (A) = 1 coulomb (C)
1 second (s)

When one coulomb charge flows through a conductor in one second, then the current flowing

through it is said to be 1 ampere (A).

Electric Circuit

An electrical circuit is a complete path through which electrons flow. Electric source, electric
load and a conducting wire are the three main components of the electric circuit. Electric load
is a device that consumes electricity. For example, electric fan, bulb, TV, heater, etc. Thus, a
continuous path made by connecting an electric source, electric load, switch, etc. with the help
of a conducting wire is called an electric circuit. Open circuits and closed circuits are the two
types of circuits.

Open Circuit

An open circuit does not have a complete path for current to flow. The electric circuit through
which current cannot flow due to an interrupted path is called an open circuit. Circuit becomes
open when the switch is turned off. Electric load does not work in an open circuit.

Modern Concept Science - 9 141

Battery Battery Brecairkciunitthe

Open Closed
Switch Switch

Electric load Electric load
(a) (b)

Open electric circuit

Closed Circuit Battery

A closed circuit has a complete path for the current to flow. Electric load Closed Switch
The electric  circuit  through which current can flow in an
uninterrupted path is called a closed circuit. A circuit becomes Close electric circuit
closed when the switch is turned on. Electric load works in a
closed circuit.

Direction of Current in a Closed Circuit

In an electric circuit, there is a flow of electrons. However, electrons were not known at the
time when the phenomenon of electricity was first observed. So, the electric current was
considered to be the flow of positive charges and the direction of flow of positive charges was
taken to be the direction of electric current. Conventionally, in an electric circuit the direction
of electric current is taken as going from the positive terminal to the negative terminal, which
is opposite to the direction of the flow of electrons.

Differences between Conventional Current and Electric Current

Conventional Current Electric Current

1. The conventional current implies the 1. Electric current implies the movement

movement of positive charges. of electrons.

2. Its direction is from positive terminal of 2. Its direction is from negative terminal

the source to its negative terminal. of the source to its positive terminal.

Symbols used in an electric circuit

In order to draw a circuit diagram, various electric components are represented by suitable
symbols. Some of the most common circuit symbols are given below.

Electrical Symbol Function
Components

Connecting wire To connect components in an electric circuit

Jointed wire To conduct electricity

Wire crossing Overlap of wires without joint
One cell Source of electricity
Two cells Source of electricity

142 Current Electricity and Magnetism