Science and Environment 8

7uNIt Estimated teaching periods : Th Pr
4 1

Image

lIgHt

Objectives

After completing the study of this unit, students will be able to:
• introduce mirror and its types (plane and spherical) and demonstrate

reflection of light from spherical mirrors.
• introduce real image and virtual image and demonstrate them.
• draw and demonstrate the images formed by spherical mirrors.
• explain the uses of spherical mirrors.

Course of Study
• Introduction to light, ray and beam of light
• Mirror and its types
• Characteristics of images formed by plane mirror
• Real and virtual image
• Images formed by concave and convex mirror

Points to be Focused/Questions to be Discussed
• What is light?
• What is mirror? What are its types?
• What are spherical mirrors?
• What are concave and convex mirrors?
• What is meant by real image and virtual image?
• What are uses of concave and convex mirrors?

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7.1 Introduction

Light is a form of energy which produces the sensation of vision. We can see different
things around us due to the presence of light. So, light makes the things visible. If we
keep an object in a dark room, we cannot see it. Various sources of light help us to see
things around us. Generally, extremely hot body emits light. When the filament of a bulb
is heated, it produces light. Light travels at a speed of 3 × 108 m/s in vacuum.

7.2 Ray and Beam of Light

Light travels in a straight path. So, it is easy to represent a ray of light by a straight line
with an arrow. The arrow in the line gives the direction of light. The narrow path of the
light which is represented by a straight line with an arrow is called a ray of light.

Fig. 7.1 A ray of light

A collection of rays of light in a certain pattern is called a beam of light. There are three
types of beam of light:

a. Parallel beam of light
b. Convergent beam of light
c. Divergent beam of light

a. Parallel beam of light: A beam of light in which the rays are parallel to each other
is called parallel beam of light. The light emitting from a source at a distant place is
represented by the parallel beam.

Fig. 7.2 Parallel beam of light

b. Convergent beam of light: A beam of light in which all rays meet at a point is
called the convergent beam of light. The light reflecting from a concave mirror is
the convergent beam of light. Similarly, convex lens also produces convergent beam
when it refracts light.

concave /ˈkɒnkeɪv/ Fig. 7.3 Convergent beam of light
- curving in

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c. Divergent beam of light: A beam of light is called divergent if the rays of light scatter
or diverge from a single point. The light reflecting from a convex mirror is divergent
beam of light. Divergent beam is obtained in a short distance from a source of light.

Fig. 7.4 Divergent beam of light

7.3 Reflection of Light

When light strikes on a surface, a part of it is absorbed by the surface. Some part of the
light is reflected by it and some part may be transmitted through it (if the surface is
transparent). The ray of light which returns to the same medium after striking a surface is
a reflected ray which makes the things visible. The process of returning light to the same
medium after striking a surface is called reflection of light.

7.4 Terminology

a. Incident ray: The ray of light which strikes a N

surface is called incident ray. In the figure, IO is I R

an incident ray. ir
b. Normal: The perpendicular to the surface drawn

at the point of incidence is called normal. In the P O Q

figure, ON is normal. Fig. 7.5 Reflection of light
c. Reflected ray: The ray of light which returns to on a plane mirror

the same medium after striking a surface is called

reflected ray. In the figure, OR is a reflected ray.

d. Angle of incidence: The angle made by an incident ray with normal is called angle of
incidence. In the figure, angle ION is an angle of incidence. It is represented by ∠i.

e. Angle of reflection: The angle made by a reflected ray with normal is called angle of
reflection. In the figure, angle RON is an angle of reflection. It is represented by ∠ r.

7.5 Laws of Reflection of Light

1. The incident ray, the reflected ray and normal, all lie on the same plane at the point
of incidence (fig. 7.5).

2. The angle of incidence is always equal to the angle of reflection, i.e. ∠i = ∠r.

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7.6 Types of Reflection of Light

There are two types of reflection of light, viz. regular reflection and irregular reflection.

(i) Regular Reflection

When a parallel beam of light, coming from a source, strikes a surface and reflects in
parallel way, such type of reflection is called regular reflection of light. An example
of regular reflection of light is the reflection by a plane mirror.

(a) Regular reflection of light (b) Irregular reflection of light

Fig. 7.6

(ii) Irregular Reflection

When a parallel beam of light strikes a surface and reflects in different directions,
such type of reflection is called irregular reflection of light. The reflection of light
from a rough surface is an example of irregular reflection. We are able to see the
object from different directions due to the irregular reflection of light.

7.7 Mirror and Its Types

A smooth surface which forms an image due to the reflection of light is called mirror.
There are two types of mirror, viz. plane mirror and spherical mirror.

7.8 Plane Mirror

The mirror in which reflection of light occurs on a smooth plane is called plane mirror. It
has a polished surface which reflects the rays of light coming from a source.

Polished surface

Reflecting surface

(a) (b)
Fig. 7.7

The image formed by a plane mirror is erect and of the same size to that of the object but
laterally inverted.

erect /ɪˈrekt/ - upright in position

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Lateral inversion: The phenomenon in which the left side of an image is formed on the
right side and vice-versa is called lateral inversion.

Fig. 7.8 Lateral inversion

7.9 Characteristics of the Image Formed by a Plane Mirror

a. Size: See yourself in a dressing mirror. Move back and forth. Look at the images
of the various objects around you. Do you feel that the images of these objects are
changing as compared to the size of the objects? You will find the images formed by
a plane mirror and objects are equal in size. This means that plane mirror forms the
image having equal size as that of the object.

b. Erect (or upright) image: Have you ever seen your image in a plane mirror inverted
with your feet pointing to the ceiling of the room? You see yourself upright. Plane
mirror forms the image upright in position.

c. Image distance and object distance: Stand in front of a mirror and move back and
forth. Observe your image closely. What do you find? When you move closer to the
mirror, the image also seems to move closer. And when you move away from the
mirror, the image also seems to move away. This is because, in a plane mirror, the
image distance is always equal to the object distance.

d. Lateral inversion (left-right inversion): Paper
Write your name on a paper and hold Mirror
it in front of a mirror. What do you
see? You see that your name has been Papyrus
completely reversed. Furthermore, some surypaP
letters also seem peculiar. This happens
because the reflected image undergoes Fig. 7.9 Lateral inversion
left-right inversion which is called lateral
inversion.

inversion /ɪnˈvɜːʃn/ - the act of changing the position of sth

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7.10 Real Image and Virtual Image
The image which can be obtained on the screen is Fact file-1

called real image. It is formed when any two rays Real image is formed when two
of light meet each other at a point. Real image is rays of light meet each other at a
always inverted. The concave mirror forms a real point after reflection.

image.

The image which cannot be obtained on the screen is called virtual image. It is formed at a
point where the rays of light appear to meet when they are produced back. Virtual image
is always erect. The plane mirror forms a virtual image.

Difference between real image and virtual image

Real image Virtual image

1. It is formed at the point where the reflected 1. It is formed at the point where the reflected

rays meet. rays appear to meet after diverging.

2. It is always inverted. 2. It is always erect.

3. It is always formed in front of the mirror. 3. It is always formed behind the mirror.

7.11 Spherical Mirrors

The mirror which is a part of a sphere is called spherical mirror. If the spherical part is
polished from inner surface, it is called convex mirror. Similarly, if the spherical part is
polished from outer surface, it is called concave mirror.

Psoulirsfhaecde Reflecting surface Resfluercftaincge Polished surface

C P C P

(a) Convex mirror (b) Concave mirror

Fig. 7.10

Let us observe a spoon. Its shallow bowl acts as a concave as well as a convex mirror.

Concave surface

Shallow bowl

Convex surface

Fig. 7.11 Spoon as a convex as well as well as a concave mirror

virtual / ˈ v ɜ ː t ʃ ʊ ə l / - almost or very nearly the thing described

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Reasonable fact-1

Why is concave mirror called converging mirror?
Concave mirror is called converging mirror because it converges the light rays falling
on it at a point after reflection.

Reasonable fact-2

Why is convex mirror called a diverging mirror?
Convex mirror is called a diverging mirror because it diverges the light rays incident
on its reflecting surface.

7.12 Terminology

a. Pole of the mirror: The geometrical centre of a CF P
spherical mirror is called pole of the mirror. In
the fig. 7.12, P is the pole of the mirror. All the Fig. 7.12 Real focus of a concave mirror
distances should be measured from the pole of
the mirror.

b. Centre of curvature: The centre of the sphere
of which the mirror is a part is called centre of curvature. It is denoted by C.

c. Radius of curvature: The radius of the sphere of which the mirror is a part is called
radius of curvature. In the fig. 7.12, PC is the radius of curvature.

d. Principal axis: The line passing through the centre of curvature and the pole of the
mirror is called principal axis.

e. Principal focus: The point at which all rays parallel to the principal axis strike a
mirror meet or appear to meet after reflection is called principal focus. In a convex
mirror, reflected rays appear to meet at focus so it is called virtual focus. But in a
concave mirror, reflected rays meet at focus, so the focus is called real focus.

F
P

Fig .7.13 Virtual focus of a convex mirror

aperture /ˈæpətʃə(r)/ - the total reflecting surface of a mirror

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The rays of light parallel to the principal axis are converged at a point by a concave
mirror. So, it is called converging mirror. In case of convex mirror, the rays which
are parallel to the principal axis are diverged from a point. So, it is called diverging
mirror.

f. Aperture: The total reflecting surface of a mirror is called aperture of the mirror.

g. Focal length: The distance between focus and pole of the mirror is called focal length.
Focal length is equal to half of the radius of curvature, i.e.

Focal length (f) = Radius of curvature (R)
2

Activity 1

• Take a concave mirror. Focus the image of any object on a screen or a paper.
Observe the image by changing the object distance. What do you observe?

• Different types of images can be observed at different position.

Activity 2

• Take a concave mirror and produce a clear image of an object present outside
the room on a paper. Measure the distance between the paper and the mirror.
The distance is called focal length of the mirror.

7.13 Rules for Drawing Ray Diagrams in a Concave Mirror

Rule 1: A ray of light parallel to the principal axis P
passes through the principal focus after reflection.

F
C

Fig. 7.14

Rule 2: A ray of light passing through the centre F P
of curvature reflects along the same path and is C
perpendicular to the surface of the mirror.

Fig. 7.15

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Rule 3: A ray of light passing through the principal focus reflects parallel to the principal
axis.

CF P

Fig. 7.16

7.14 Images Formed by a Concave Mirror

Now let us draw the ray diagrams for a concave mirror when object is placed at different
positions.
a. Object at infinity (distant object)
When an object is at infinity, parallel beam of light falls on the mirror and image is

formed at the focus after reflection. The image is real, inverted and highly diminished.

Image

Rays of light C F Concave mirror
from distant object
P

Fig. 7.17 When object is at infinity

b. Object beyond the centre of curvature (2F or C)
When the object is kept beyond the centre of curvature, its image is formed between

C and F. The image is real, inverted and diminished.

B

Object C F P
A'
A
B'
Image

Fig. 7.18 When object is beyond 2F

c. Object at centre of curvature (C)
When the object is kept at the centre of curvature (C), image is also formed at the

same point. The image is real, inverted and of the same size as the object.

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B F P

Object

A
C

Image

B'
Fig. 7.19 When object is at C

d. Object between C and F

When the object is kept between C and F, image is formed beyond C. The image is
real, inverted and magnified.

B

Object P

C

Image A F

Fig. 7.20 When object is between C and F

e. Object at F
When the object is kept at F, image is formed at infinity. The image is real, inverted

and highly magnified.

Object

Imaingfeinaityt C F P

Fig. 7.21 When object is at F

f. Object between F and P
When the object is kept between F and P, image is formed beyond the mirror. The

image is virtual, erect and magnified.

B1

Object B Image
C FA P A1

Fig. 7.22 When object is between F and P

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Summary of the images formed by a concave mirror

S. N. Position of an object Position of the image Nature of the image formed
1. At infinity At F Real, inverted, diminished
2. Beyond C or 2F Between F and C Real, inverted, diminished
3. At C or 2F At C or 2F Real, inverted and of the same
size as the object
4. Between C and F Beyond C or 2F Real, inverted enlarged
5. At F At infinity Real, inverted, enlarged
6. Between F and P Behind the mirror Virtual, erect, enlarged

How to draw mirror, centre of curvature, pole and principal axis?
a. Draw a circular arc AB with a compass taking C as a centre.

b. The centre C is the centre of curvature.

c. Locate the mid-point of arc AB. Let it be 'P' which is called pole of the mirror.

d. Now, join the points P and C. The line passing through P and C is called principal
axis.

e. The mid-point of PC is called focus. Let it be F.

A P Mid point of AB
Mid point of PC

C
Focus (F)

B
Fig. 7.23

f. According to the necessity, draw the object. Now, follow the rules to draw ray
diagrams.

7.15 Rules to Draw Ray Diagram for Convex Mirror

Rule 1: A ray travelling parallel to the principal axis appears to diverge from the principal
focus.

PHYSICS P FC

Fig. 7.24
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Rule 2: A ray travelling through centre of curvature is reflected back along its own path.

P FC

Fig. 7.25

Rule 3 : A ray of light coming from an object which strikes the pole of the mirror at a
certain angle, reflects at the same angle.

450 FC
450 P

Fig. 7.26

7.16 Images Formed by a Convex Mirror

When an object is kept in front of a convex mirror at any point, its image is formed behind
the mirror. The image is virtual, erect and diminished.

B B1 Image
Object
A1 F C
A

Fig. 7.27 Image formed by a convex mirror

7.17 Uses of Mirrors

a. Uses of a plane mirror

(i) For a looking glass at home, shops or hair cutting saloons
(ii) For making periscope, kaleidoscope, etc.
(iii) For the practical purposes in laboratory

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Reasonable fact-3

Concave mirror is used in torch light and search light.
The search lights is used for searching something. Since, it would be better as much as
area it can illuminate. Concave mirror also spreads light and illuminate larger area as
compared to the plane mirror. So, concave mirror is used in torch light and search light.

b. Uses of a concave mirror
(i) For shaving and making cosmetic mirror
(ii) For making astronomical telescope
(iii) For making the reflector of searchlights, torches, etc.
(iv) For making solar cooker to converge solar radiation

Reasonable fact-4

Concave mirror is used in headlight of vehicles.
Concave mirror allows the light rays to be focused as a single beam and gives more
power to the light that makes more efficient for seeing and to be seen by others. So,
concave mirror is used in headlight of vehicles.

c. Uses of a convex mirror
(i) To make street light reflector so that light can be spread in wider regions
(ii) To make the back view mirror in automobiles as it has wide field of view

Reasonable fact-5

Convex mirror is used as side view mirror of vehicles.
Convex mirror gives a diminished, virtual and an erect image of the side or rear with
wider field of view. A convex mirror enables the driver to view much larger area than
would be possible with a plane mirror. So, convex mirror is used as side-view mirror of
vehicles.

7.18 Refraction of Light

When a ray of light is travelling in vacuum or any other single medium, it travels in a
straight path. When the medium changes, the ray of light follows a new direction by
bending between two media. The bending of light in between two media when it travels
from one medium to another is called refraction of light.

Light travels through various media such as water, glass, plastic, etc. The medium through
which light can travel is called optical medium. There are two types of optical medium:

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denser medium and rarer medium. The medium having relatively higher density is
called a denser medium and the medium having relatively lower density is called a rarer
medium. If we compare air and water, the density of water is more than that of air. So,
water is a denser medium and air is a rarer one. Similarly, the medium in which speed
of light is more is called rarer medium and the medium in which speed of light is less is
called denser medium.

Refraction of Light through a Glass Slab

When a ray of light IO strikes a glass slab ABCD, it bends at O and travels along OR. The
ray OR again refracts and emerges out along RE.

I Air (Rarer)
iM

AO B
r Glass slab
(Denser)
N

D r M1 C
Air (Rarer) R

e

N1 Lateral
shift

E

Fig. 7.28 Refraction of light through a glass slab

7.19 Terminology

a. Incident ray: The ray which strikes a transparent medium is called the incident
ray. In the figure, IO is an incident ray.

b. Normal: The perpendicular to the surface drawn at the point of incidence is
called normal. MN and M1N1 are normals.

c. Refracted ray: The ray of light obtained in the second medium after refraction
is called refracted ray. In the figure, OR is a refracted ray.

d. Emergent ray: The ray which comes out from the second medium is called
emergent ray. In the figure, RE is an emergent ray.

e. Angle of incidence (∠i): The angle between the normal and the incident ray is
called angle of incidence. In the figure, ∠IOM is an angle of incidence.

f. Angle of refraction (∠r): The angle between the normal and the refracted ray
is called angle of refraction. In the figure, ∠NOR is an angle of refraction.

g. Emergent angle (∠e): The angle between the normal and the emergent ray is
called the emergent angle. In the figure, ∠ERN1 is an emergent angle.

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h. Lateral shift: The perpendicular distance between incident ray and emergent
ray is called lateral shift or lateral displacement.

7.20 Laws of Refraction of Light

(i) The incident ray, refracted ray and normal, all lie on the same plane at the
point of incidence.

(ii) The ratio of sine of angle of incidence to the sine of angle of refraction, for a

given pair of media, is constant. This constant is called refractive index, i.e.

Refractive index (µ) = Sini
Sin r

This law is popularly called Snell’s law. It can be explained in the following way.

a. When a ray of light travels from a rarer medium to a denser medium, it bends

towards normal.

I M
Air (rarer medium)

O Glass (denser medium)

NR I M Air (rarer medium)
(a) Glass (denser O R
medium)
N
Fig. 7.29 (b)

b. When a ray of light travels from the denser medium to rarer medium, it bends
away from the normal.

c. The ray of light does not bend when it passes through the normal.

Air Normal

Glass

Fig. 7.30

Reasonable fact-6

Light bends when it travels from one medium to another.
When light travels from one medium to another there is change in the speed of light. So,
light bends when it travels from one medium to another.

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Experiment 1

Objective : To verify laws of reflection of light

Materials required : A drawing board, a white sheet of paper, a glass slab, some
thumb pins, some paper pins, a pencil, a protractor

Procedure

(i) Fix a white paper on a drawing board.

(ii) Put a glass slab on the middle of the paper and draw an outline ABCD.

(iii) Draw a line X at a certain angle which meets AB at O. Draw a perpendicular line MN.

(iv) Fix the pin P and Q by observing the line X properly.

(v) Now observe the pin P and Q in a straight position from another side of the glass
slab and fix two pins R and S.

XP B
xy Q M

P1

P2 i

A
O

r
M1

N

D O2 R C

e S
P3

N1 P4
Y
Fig. 7.31

(vi) Remove the glass slab and join R and S with a line Y.

(vii) Now join line X and Y to get refracted ray.

(viii) Measure the angle of refraction, angle of incidence and draw the conclusion of
this activity.

7.21 Some Examples of Refraction of Light

a. When light travels from a denser medium to a rarer medium, it bends away from
the normal. In our daily life we observe a large number of phenomena based on this
principle. Some of them are as follows:

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1. Bending of stick in water

A stick immersed in water appears to be bent and short at the surface of water
when it is viewed obliquely from above. The rays of light coming from the
lower end of the stick bend away from the normal as light passes from water
(denser medium) to air (rarer medium). As a result, a virtual image of the part
of the stick below water is formed as shown in the figure. Thus the immersed
part of the stick appears to be raised and bent on the surface of water.

Apparent bend
Apparent position
of stick

Surface

Water

Fig. 7.32

2. A coin placed at the bottom of a vessel full of water appears to be raised
The rays of light coming from the coin bend away from the normal as light passes

from water (denser medium) to air (rarer medium). As a result, a virtual image of
the coin is formed above the real position due to refraction of light. Therefore, a
coin placed at the bottom of a vessel full of water appears to be raised.

Apparent position Water
of the coin
Coin (real position)

Fig. 7.33

Similarly, more phenomena can be explained on the basis of the principle of
refraction of light, when it travels from a denser medium to a rarer medium.

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(i) A water pond or a pool appears less deep than its actual depth.

(ii) The legs of a person standing in a swimming pool appear shorter.

(iii) During spear fishing, the fisherman aims at the tail of the fish.

(iv) A piece of paper lying at the bottom of the glass slab appears to be raised when
viewed through the glass.

(v) The print appears to be raised up when a glass is placed over it.

SuMMarY

• Light is a form of energy which produces the sensation of vision.
• The light travelling in a medium returns to the same medium when it strikes a

surface. This process is called reflection of light.
• The path of light represented by a line with an arrow is called a ray of light.
• The collection of rays of light is called a beam of light.
• The ray of light coming from the source that falls on a surface is called incident ray.
• The ray reflected from the surface to the same medium is called reflected ray.
• A concave mirror converges the parallel beam of light falling on it. So, it is called a

converging mirror. But a convex mirror diverges the parallel beam of light falling
on it. So, it is called a diverging mirror.
• The image that can be obtained on the screen is called a real image.
• The image that cannot be obtained on the screen is called a virtual image.
• Concave mirror can form both real image and virtual image but convex mirror
always forms a virtual image.
• When light travels from one medium to another, it bends. This phenomenon is
called refraction of light.
• When a ray of light travels from a rarer medium to a denser medium, it bends
towards the normal.
• When a ray of light travels from a denser medium to a rarer medium, it bends away
from the normal.
• A straight stick appears to be bent when it is immersed in water due to the refraction
of light.

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exercise

1. Choose the best answer from the given alternatives.

a. The speed of light in a vacuum is ............................... .

i. 3 × 108 km/s ii. 3 × 108 m/s

iii. 2 × 108 m/s iv. 8 × 103 m/s

b. The perpendicular to the surface drawn at the point of incidence is
called ............................... .

i. angle of incidence ii. normal

iii. refracted ray iv. incident ray

c. The total reflecting surface of a mirror is called ............................... .

i. pole of the mirror ii. aperture of the mirror

iii. focal length of the mirror iv. principal axis of the mirror

d. The image formed by a concave mirror when an object is placed at F is
............................... .

i. real, inverted and diminished

ii. virtual, erect and enlarged

iii. real, inverted and enlarged

iv. virtual, inverted and diminished

e. Which of the following mirror is used in search lights?

i. concave mirror ii. convex mirror

iii. plane mirror iv. spherical mirror

2. Tick (√) the correct statement and cross (×) the incorrect one.
a. A collection of rays of light is called a beam of light.

b. The angle of incidence is always less than the angle of reflection.

c. Virtual image can be obtained on the screen.

d. Concave mirror always forms enlarged image.

e. The ray of light does not bend when it passes through the normal.

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3. Fill in the blanks using appropriate words.

a. The point where the parallel rays of light converge in a concave mirror is
called ............................ .

b. The mid-point of the aperture of a mirror is called ............................ .
c. ............................ image cannot be formed on the screen.
d. When the object is at ............................, concave mirror forms a virtual image.
e. The ratio of sine of angle of ............................ to the sine of angle of

............................ is constant. This constant is called ............................ .
4. Answer the following questions.

a. What is reflection of light? Explain with a figure.
b. Write down the laws of reflection of light.
c. Write any two characteristics of the image formed by a plane mirror.
d. What is meant by lateral inversion ?
e. What is a concave mirror?
f. What is a convex mirror?
g. What is meant by real image and virtual image?

5. What is refraction of light? State Snell's law.

6. Define:

i. Pole of the mirror ii. Focus

iii. Centre of curvature iv. Radius of curvature

v. Principal axis vi Focal length

7. What happens when a ray of light is travelling from a rarer medium to a
denser medium?

8. State the laws of refraction of light with suitable diagrams.

9. Draw a diagram showing the reflection of light on a plane mirror.

10. Give the relation between focal length and the radius of curvature of a spherical
mirror.

11. Write any two uses each of concave mirror and convex mirror.

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12. Draw the ray diagram for a concave mirror when the object is placed

i. at F. ii. at C.

iii. between F and C. iv. between F and P.

Also, write down the properties of the image formed.

13. Give reason.
a. We use a plane mirror to see our face.

b. Concave mirror is called converging mirror.

c. Convex mirror is used to see the back side of the road in a vehicle.

d. A straight stick appears to be bent when it is immersed in water.

e. A pond of water appears shallower than it actually is.

14. Differentiate between:

a. Concave and Convex mirror

b. Real image and Virtual image

c. Denser medium and Rarer medium

d. Reflection and Refraction of light

15. Complete the given ray diagrams and write down the properties of the image
formed.

CF P CF P

(a) (b)

FC

(c) (d)

16. Draw a neat and labelled diagram showing the refraction of light through a glass
slab.

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8uNIt Estimated teaching periods : Th Pr
2 1

Temple bell

SOUND

Objectives

After completing the study of this unit, students will be able to:
• define terminology related to sound, viz. speed, frequency and wavelength.
• introduce echo and reverberation and differentiate between them.
• explain the effects of echo and reverberation.

Course of Study
• Introduction to sound
• Sound wave, wavelength, frequency
• Speed of sound
• Reflection of sound
• Echo, reverberation
• Simple numerical problems based on echo

Points to be Focused/Questions to be Discussed
• What is sound? How is it produced?
• What is longitudinal wave?
• Define sound wave, wavelength and frequency.
• What is meant by compression and rarefaction?
• What is meant by reflection of sound?
• Differentiate between echo and reverberation.

116 Oasis School Science and Environment - 8 PHYSICS

8.1 Introduction

Sound is a form of energy which produces the sensation of hearing. The world would be
unpleasant if there were no sound. Can you imagine a world without sound? Of course
not. If there is no sound, we cannot hear the warning bells, sirens or even automobile
horns. We can hear the sound coming from a distant place through the material medium.
Sound travels through the medium in the form of waves, called sound waves. Actually,
there are two types of waves: longitudinal waves and transverse waves. Sound wave is a
longitudinal wave.

Sound is produced from different sources due to their vibrations. So, we can say, sound is
produced due to the vibration of a body. The vibration of the source of sound creates the
vibration in the surrounding particles which propagates in the form of a longitudinal wave.
The wave in which particles of the medium (the substance through which sound is travelling)
vibrate along the direction of the propagation of sound is called longitudinal wave.

Activity 1

• Take a metal bowl and strike it with a small stick. What happens? Touch the vibrating
bowl. What do you feel?

• Sound is produced in the bowl due to the vibration and when we touch the bowl, the
vibration stops. As a result, sound also stops.

8.2 Sound Wave

Sound is produced by a vibrating body and it travels in all directions from the source.
Sound requires a medium for its propagation. The sound waves coming from a vibrating
body propagate through air and reach our ears. As a result, we hear the sound. Sound can
propagate through solids, liquids and gases. But it cannot propagate through vacuum.

Tuning Greater Less Greater Less Greater Less
fork density density density density density density

C = Compression
R = Rarefaction

Fig. 8.1 Sound wave having compressions and rarefactions

Sound travels in the form of longitudinal waves. When a sound wave passes through air,
the particles of air vibrate back and forth parallel to the direction of propagation of sound
wave. It forms regions of compressions and rarefactions.

PHYSICS Oasis School Science and Environment - 8 117

8.3 Longitudinal Wave

The wave in which the particles of the medium vibrate to – and – fro (back and forth) in
the direction of the wave is called a longitudinal wave. Sound wave, waves formed in a
slinky when pulled and pushed, etc. are some examples of longitudinal waves.

A longitudinal wave consists of regions of compressions and rarefactions. A compression
is that part of a longitudinal wave in which the particles of the medium are closer to one
another and a rarefaction is the part of the wave in which the particles are farther apart. A
longitudinal wave is shown in the given figure.

Rarefaction Rarefaction Pull

Compression Compression Push of a slinky

Fig. 8.2 Longitudinal wave in a slinky showing compressions and rarefactions

Reasonable fact-1

Sound wave is called a longitudinal wave.
In a sound wave, the vibration of molecules and transmission of wave occurs in the
same direction. So, sound wave is called a longitudinal wave.

8.4 Wavelength

The distance between any two consecutive compressions or rarefactions is called
wavelength. It is denoted by λ (lambda). The SI unit of wavelength is metre (m).

Differences between wavelength and frequency

Wavelength Frequency

1. The distance between any two consecu- 1. The number of complete cycles made in
tive compressions or rarefactions is called one second is called frequency.
wavelength.

2. Its SI unit is metre (m). 2. Its SI unit is hertz (Hz).

8.5 Frequency

The number of complete cycles made in one second is called frequency. It is denoted by
'f'. The SI unit of frequency is hertz (Hz). The larger units of frequency are kilohertz (kHz)
and megahertz (MHz).

1 kHz = 1000 Hz = 103 Hz

1MHz = 1000 kHz = 106 Hz

longitudinal wave /ˌlɒŋɡɪˈtjuːdɪn l weɪv/ - a wave that vibrates in the direction that it is moving

frequency /ˈfriːkwənsi/ - the number of complete cycles made in one second

118 Oasis School Science and Environment - 8 PHYSICS

The frequency of a sound wave is 60 Hz means that the sound wave produces 60 complete
cycles or 60 complete waves in one second.

8.6 Speed of Sound

Sound travels with different speed in different media. Solids are more elastic in nature

than gases and liquids, i.e. Egas < Eliquid < Esolid. The velocity of sound is maximum in solids
and minimum in gases. This is because the molecules are packed closer in solids and
liquids than in gases. Since molecules carry the vibrations, they do so more effectively
when they are close together.

One can press his ears against the railway track to find whether a train is approaching or not.
This is because we know that the speed of sound in solids is much more than that in gases.
The sound produced by the moving wheel of the train travels much faster through the steel
track than through the air. So, the sound is heard through the steel track before it is heard
through the air.

The speed of sound in some common materials is given below:

S.N. Medium Examples Speed of sound

1. Gases Air (0 0C) 332 m/s
2. Liquids Air (20 0C) 344 m/s
Hydrogen (0 0C) 1284 m/s
3. Solids
Distilled water (200C) 149 m/s
Alcohol (25 0C) 1210 m/s
Turpentine (25 0C) 1325 m/s
Sea water (25 0C) 1533 m/s

Aluminium (25 0C) 5100 m/s
Copper (25 0C) 3560 m/s
Glass (25 0C) 5500 m/s
Iron (200C) 5130 m/s

The speed of sound (v) can be calculated by the given formula:
speed of sound (v) = frequency(f) × wavelength(λ)

∴ v =f×λ

Worked out Numerical 1

The frequency of a sound wave is 50 Hz and the wavelength is 6.6 m. Calculate the
speed of sound.

Solution:

Given, Frequency (f) = 50 Hz


PHYSICS Oasis School Science and Environment - 8 119

Wave length (λ) = 6.6m
Speed of sound (v) = ?
We know,
Speed of sound (v) = f × λ
= 50 × 6.6
= 330 m/s
∴ The speed of sound is 330 m/s.

Worked out Numerical 2

The speed of sound in air is 330 m/s and the frequency of the sound is 40 Hz. Calculate

the wavelength.

Solution: Speed of sound (v) = 330 m/s

Given,


Frequency (f) = 40 Hz

Wavelength (λ) = ?

We know, f × λ
v =

or, λ = v = 34300 = 8.25 m
f

∴The wavelength (λ) = 8.25 m.

8.7 Reflection of Sound R1sigt hRteSfliedcetiwonalflrom
Diffuser Disperses
When the sound traveling in a medium strikes the Near Wall Reflection
other surface, it returns to the same surface. This
process is called reflection of sound. The bouncing Direct sound

back of sound when it strikes a hard surface is called

reflection of sound. Hard surfaces like walls, metal Absorbent Removes left
sheets, plywood, etc. reflect sound waves. The laws of Side Wall Reflection

reflection of light are obeyed during the reflection of Fig. 8.3 Reflection of sound

sound. Sound can be reflected from any hard surface

whether it is rough or smooth. The reflection of sound is utilized in the working of

megaphone, sound boards and ear trumpet. The reflection of sounds causes echoes.

8.8 Echo

If we stand near a cliff or one corner of a big empty hall and shout the word "HELLO!" we
will hear the word "HELLO!" coming from the cliff or empty hall in the form of reflected
sound. This sound is called echo. It is heard after reflection of the sound from a cliff or
wall or any other hard surface. So, the repetition of sound caused by the reflection of

echo /ˈekəʊ/ - the repetition of sound caused by reflection

120 Oasis School Science and Environment - 8 PHYSICS

sound waves is called echo. The minimum distance from a sound reflecting surface to
hear an echo is 17.2 metres.

Conditions for formation of echo
i. The minimum distance between the source of sound and the reflector should be

at least 17.2 m.
ii. The size of the reflector must be large.
iii. The intensity or loudness of the sound should be sufficient.

Reasonable fact-2

Echo is heard fainter than the original sound.
Original sound loses much energy after getting back from a distant object. So, echo is
heard fainter than the original sound.

Calculation of Minimum Distance to Hear an Echo

Scientists have found that the human ear can hear two sounds separately only if there is a

time interval of 1 th of the a second (or more) between the two sounds. It means that we
10

can hear the original sound and the echo separately only if there is a time gap of at least

1 th of a second between them. Knowing the minimum time-gap required for an echo to
10

be heard and the speed of sound in air, we can calculate the minimum distance from a

sound reflecting surface which is required to hear an echo as follows:

Solution: Speed of sound in air = 344 m/s
Time taken
Distance travelled = 1 S = 0.1 s
10

= ?

We know, Speed = Distance travelled
Time taken
or 344
or, Distance travelled = Distance travelled
0.1

= 344 × 0.1 = 34.4 m

∴ The distance from the sound reflecting surface to hear an echo should be half of 34.4m which

is 34.4 = 17.2 m. From this calculation, we can conclude that the minimum distance from a
2

sound reflecting surface is 17.2m to hear an echo when the sound travels in air. However,

when the sound travels in water or any other solid, the minimum distance for hearing the
echo will be different because the speed of sound in water and other solids is different.

reverberation /rɪˌvɜːbəˈreɪʃn/ - the prolongation of the original sound

PHYSICS Oasis School Science and Environment - 8 121

8.9 Reverberation

The intermixing of original sound with a reflecting sound is called reverberation. In other
words, the prolongation of the original sound is called reverberation. This phenomenon
occurs when the distance between the source of sound and the reflecting surface is less
than17.2 m. A number of echoes of original sound are heard during a reverberation.
Thunder that follows lightning flash is an example of reverberation. Reverberation is
heard in a newly built room or an empty room but not in a furnished or occupied room
because the materials kept in a furnished room absorb the sound.

Reasonable fact-3

Reverberation is heard in big rooms and halls.

In big rooms and halls, if the sound gets reflected less than 17m near from the source of
sound, the original sound and the reflected sound mix to cause the sound prolonged.
So, reverberation is heard in big rooms and halls.

Reasonable fact-4

Reverberation is not heard in occupied rooms.
Reverberation is not heard in occupied rooms because the materials in the room
absorb sound a lot.

Differences between Echo and Reverberation

S.N. Echo S.N. Reverberation

1 The repetition of the sound is called 1. The prolongation of sound is called

echo. reverberation.

2. The distance for an echo to take 2. The distance for reverberation to take

place is 17.2m or greater than 17 m. place is less than 17 m.

Reasonable fact-5

Sound absorbing materials are kept in the walls of cinema hall.
Sound absorbing materials are kept in the walls of cinema hall to minimize the
reverberation. These materials reflect sound less and absorb more. Therefore, the
quality of sound heard by the people becomes clear.

Reasonable fact-6

Recording studios are designed to produce reverberation.
Reverberation makes music melodious providing continuity. So, recording studios are
designed to produce reverberation.

122 Oasis School Science and Environment - 8 PHYSICS

Reasonable fact-7

Reverberation is heard in newly built or empty rooms.
Reverberation is heard in newly built or empty rooms because there are no sound
absorbing materials.

8.10 Echolocation

Echolocation is the process of locating an object with respect to the surroundings. Different
animals like whales, dolphins, bats and some birds use echo to locate their body, to find
their prey and to navigate without the use of eyes.

Fig. 8.4 (a) Measuring depth of sea by (b) Bats and dolphins make high pitched sounds while flying

using SONAR or moving which bounce off objects in the form of echoes

The depth of the ocean can also be calculated with the help of this method. The method

of finding the depth or position with respect to the surrounding objects is called Sound

Navigation and Ranging (SONAR). The device which is used to find the depth is called

Fathometer. The depth can be calculated by the following formula:

Depth (s) = Speed of sound in the given medium × Time
2



∴ s = v×t
2

Worked out Numerical 3
A boy shouts in front of a hill and he hears the echo after 0.2 seconds. Find the distance
between the boy and the hill. The speed of sound in air is 332 m/s.
Solution:

Speed (v) = 332 m/s
Time (t) = 0.2s

PHYSICS Oasis School Science and Environment - 8 123

Distance (s) = ?

We have,
s = v×2t = 332×20.2 = 33.2 m.
∴ The distance between the hill and the boy is 33.2 m.

Activity 2 ear
2
• Take two plastic tubes. Take a mirror and a card-
board and arrange them as shown in the figure. Fig. 8.5
Keep tube 1 constant with watch and move the
second tube 2 in different angles. Find the angle
between the cardboard and these tubes. Write 1
down the conclusion of this activity.

Activity 3

• Take a stop watch and shout the word 'Hello !" in front of a cliff or a wall nearby
your school. Record the time taken to hear the echo.

• Calculate the distance between you and the cliff/wall.

SuMMarY

• Sound is a form of energy which produces the sensation of hearing.

• Sound is produced due to vibration of a material medium.

• Sound can propagate through solids, liquids and gases. But it cannot propagate
through vacuum.

• The wave in which the particles of the medium vibrate to – and – fro (back and
forth) in the direction of the wave is called a longitudinal wave.

• A compression is that part of a longitudinal wave in which the particles of the
medium are closer to one another and a rarefaction is the part of the wave in
which the particles are farther apart.

• The distance between any two consecutive compressions or rarefactions is
called wavelength.

• The number of complete cycles made in one second is called frequency.

• The speed of sound is maximum in solids and minimum in gases.

• The bouncing back of sound when it strikes a hard surface is called reflection of
sound.

• The repetition of sound caused by the reflection of sound waves is called echo.

• The prolongation of the original sound is called reverberation.

• Echolocation is the process of locating an object with respect to the surroundings.

124 Oasis School Science and Environment - 8 PHYSICS

exercise

1. Choose the best answer from the given alternatives.

a. Sound cannot travel in .................................. .

i. air ii. water

iii. iron rod iv. vacuum

b. Sound is produced ................................ .

i. by shouting ii. by heating

iii. by the vibration of the body iv. by cooling

c. Reverberation occurs if the distance between the source of sound and the
reflecting surface is ................................ .

i. less than 17 m ii. more than 17 m

iii. equal to 17 m iv. all of above

d. Depth of the ocean can be calculated by the formula ................................ .

i. v3×t ii. v×t iii. v2 ×t iv. 2vt
e. The speed of sound is maximum in ................................ .

i. solid ii. liquid

iii. gas iv. vacuum

2. Tick (√) the correct statement and cross (×) the incorrect one.

a. Sound wave is a transverse wave.

b. The SI unit of wave length is metre (m).

c. The speed of sound is minimum in solid and maximum in gas.

d. The intermixing of an original sound with reflected sound

is called reverberation.

e. The minimum distance to hear an echo is less than 16.6 m.

3. Answer the following questions.

a. What is sound? How is it produced?
b. What is sound wave? Why is sound wave called a longitudinal wave?
c. Define frequency and wavelength.
d. Write down the relation between speed of sound, frequency and wavelength.
e. What is reflection of sound?

PHYSICS Oasis School Science and Environment - 8 125

f. What is echo? How is it produced?

g. What is meant by reverberation?

h. Write down the applications of echo.

i. Name any two animals that use echo.

4. Differentiate between:
a. Compression and Rarefaction

b. Frequency and Wavelength

c. Echo and Reverberation

5. The velocity of sound in three different media A, Medium Velocity of sound

B and C are given in the following table. Identify A 330 m/s
them under the category of solid, liquid and gas. 5,000 m/s
1500 m/s
B

6. Give reason. C

a. We cannot hear each other on the surface of

the moon.

b. Echo is not heard in a small room.

c. The ceiling and walls of cinema hall are covered with sound absorbing
material.

d. If we catch a ringing bell, the sound is stopped.

7. Numerical Problems

a. The speed of sound in a medium is 360 m/s and the frequency is 45 Hz.

Calculate the wavelength. [Ans: 8m]

b. The frequency and wavelength of a sound wave are 100 Hz and 3.3 m

respectively. Calculate the speed of the sound wave. [Ans: 330 m/s]

c. The speed and wavelength of a sound wave is 330 m/s and 8.25 m respectively.

Calculate the frequency of the sound wave. [Ans: 40 Hz]

d. A girl shouts in front of a cliff and she hears the echo after 0.4

second. Find the distance between the girl and the cliff. The speed of

sound in air is 330 m/s. [Ans: 66 m]

e. A man shouts in a well and echo is heard after 0.15 second. Calculate the

distance between the man and the surface of water. (Speed of sound

in air = 332 m/s) [Ans: 24.9 m]

f. Ram shouts in front of a cliff. The distance between the cliff and Ram is

150 m. If the speed of sound in air is 332 m/s, calculate the time to hear

the echo. [Ans: 0.9 s]

126 Oasis School Science and Environment - 8 PHYSICS

9uNIt Estimated teaching periods : Th Pr
2 0

Magnetic compass

MagNetISM

Objectives

After completing the study of this unit, students will be able to:
• explain molecular theory of magnetism.
• define magnetic induction and demonstrate it with explanation.
• state causes of demagnetization.
• state the methods of conserving magnetism.
Course of Study
• Introduction to magnetism
• Molecular theory of magnetism
• Magnetic induction
• Demagnetization
• Methods of conserving magnetism
Points to be Focused/Questions to be Discussed
• What is a magnet?
• What is meant by magnetism?
• State molecular theory of magnetism.
• What is magnetic induction?
• What is meant by demagnetization?
• How can we conserve magnetism?

PHYSICS Oasis School Science and Environment - 8 127

9.1 Introduction

A magnet is a substance which has the property of attracting iron, cobalt, etc. The
substances like iron, cobalt, nickel, etc. which are attracted towards the magnet are called
magnetic substances. The force that the magnet exerts on magnetic substances is called
the magnetic force. The discovery of a magnet is attributed to a shepherd boy, Magnes,
who while roaming on Mt. Ida, found that his iron-strapped sandals got stuck to the
mountain. He could not walk easily and tried to find the cause of the problem. A wise
man found that the mountain was made of black stone with iron in it which had a special
property to attract iron. That black stone was an ore of iron called magnetite. The stone
was called lodestone. It was used by navigators for finding their way in the sea. In our
daily life, magnets are widely used in electric appliances such as loud speakers, electric
motors, dynamos, toys, etc.

(a) Magnetite (b) Lodestone

Fig. 9.1

A magnet is a substance which attracts iron, cobalt, nickel, etc. and always rests in the
north-south direction when suspended freely. The Chinese knew that the naturally
occurring magnet called lodestone, when suspended freely, always points to north-south
direction. The Chinese sailors used lodestone as a magnetic compass for navigation.

The lodestones found in nature have weak force. They are mostly found with irregular
shapes and sizes. Therefore, artificial magnets are made from iron, steel, etc. These
magnets are widely used in various appliances. The magnets which are made by humans
in artificial methods are called artificial magnets.

Bar magent Horse-shoe shaped magnet Magnetic compass U-shaped magnet

Fig. 9.2

navigation /,nævɪˈɡeɪʃn/ - the skill or process of planning the route of a ship

128 Oasis School Science and Environment - 8 PHYSICS

Differences between natural magnet and artificial magnet

Natural magnet Artificial magnet

1. The magnet which is found in nature is 1. The magnet made by artificial method is

called natural magnet. called artificial magnet.

2. It is less powerful than artificial magnet. 2. It is more powerful than natural magnet.

9.2 Molecular Theory of Magnetism

If a magnet is broken into smaller pieces, these smaller pieces also contain the magnetic
properties. Depending on this fact, a theory was introduced by a British physicist Sir
Alfred Ewing. Molecular theory of magnetism states that, "Each molecule of a magnet or
a magnetic substance is an independent magnet which is called molecular magnet." In a
magnet, the molecular magnets are arranged in a parallel way of alignment in the same
direction. Hence, one end of the magnet has many free ends of north pole of molecular
magnets and another end has free south poles. Due to these free ends, a magnet can
attract iron, cobalt, etc.

In a magnetic substance, molecular magnets are arranged in the form of chains. In a
chain, north pole of molecular magnet is connected to the south pole of another molecular
magnet and so on. Thus, there is no any free end of the magnetic poles in a magnetic
substance.

Arrangement of molecular Arrangement of molecular
magnets in a magnet magnets in a magnetic substance

(a) (b)
Fig. 9.3 Showing molecular theory of magnetism

From the molecular theory of magnetism, we can conclude the following points:

1. A magnetic substance cannot attract another magnetic substance since the molecular
magnets are arranged in closed chains.

2. Poles of a magnet are strong since they have the free ends of the molecular magnets.
In the middle of the magnet, the molecular magnets are arranged one after another.
As a result, there is the cancellation of the magnetic property among the molecular
magnets and hence the attractive power is weak in the middle of the magnet. This is
only due to the effect of end points of the magnet.

3. When a long magnet is broken, the broken pieces also behave like a complete magnet.
As the magnet is broken, the broken part has the free ends of the molecular magnets

PHYSICS Oasis School Science and Environment - 8 129

and hence the new poles are developed. Due to the existence of the molecular
magnets, we cannot make a monopole or single pole of the magnet.

4. When a magnet is hammered, the magnetic properties are lost. In so doing the
molecular magnets are disturbed and they form a closed chain structure. As a result,
the magnet loses its property.

Reasonable fact-1

Magnetic substance does not show magnetic properties.
In a magnetic substance , the north pole of one molecular magnet is attached to the
south pole of another molecular magnet in the form of a closed chain. So, the magnetic
substance does not show magnetic properties.

Reasonable fact-2

North and South poles of the magnet has more energy.
In a magnet, molecular magnets are arranged in parallel in a certain direction. All the
north poles are arranged in one side and all the south poles are arranged in another
side which form north pole and south pole respectively. So, north and south poles of
the magnet has more energy.

Reasonable fact-3

The poles of a magnet cannot be separated.
Each magnetic molecule has south (S) and north (N) poles. A magnet consists of
numerous molecular magnets. So, when long magnet is broken, the poles of magnet
cannot be separated.

Activity 1

• Take a blade and break it into two parts as shown in Fig. 9.4
the figure.

• Now, take a bar magnet. Rub the magnet against the
piece of the blade so that it becomes a magnet.

• Break the piece of blade carefully into two parts and
check the magnetic property of these pieces.

• What can you conclude from this activity?

From this activity, it can be concluded that magnetic poles exist in pairs and they
cannot be separated. A magnet does not have only one pole.

9.3 Magnetic Induction

When an unmagnetized steel bar is held above the iron nails, it does not attract them
(fig. 9.5(a)). But when a bar magnet is kept near the unmagnetized steel bar, the bar gets

130 Oasis School Science and Environment - 8 PHYSICS

magnetized. As a result, the steel bar attracts iron nails (fig. 9.5(b)). It shows that the steel
bar develops magnetic properties when it is kept near a magnet. This process is called
magnetic induction. The process by which a magnetic substance develops magnetic
properties when it is placed near a magnet is called magnetic induction.

Bar magnet

Stand Stand Steel bar behaves
Unmagnetised as a magnet
steel bar Iron nails
Iron nails

(a) (b)

Fig. 9.5

When the magnet kept near the steel bar is taken away, the steel bar loses magnetic
properties. As a result, the iron nails sticking to the steel bar fall down. It shows that the
steel bar behaves as a magnet so long as it remains inside the magnetic field of a strong
bar magnet.

Activity 2 Fig. 9.6

• Bring a bar magnet and keep it on a table. Now,
attach an iron nail to one end of the magnet. Bring
another nail in contact with the previous nail. The
nail is attached to the previous nail. In this way, it
forms a chain of nails as shown in the fig. 9.6. What is
the reason behind it? Now, take the bar magnet away
from the nails. What do you observe? Why do nails
separate from each other while taking the magnet
away from the nails? What can you conclude from
this activity? This activity shows that the magnetism
acquired by induction is not permanent.

9.4 Demagnetization

When a magnet is heated, dropped or hammered, it loses magnetic property. The process
by which a magnet loses its properties is called demagnetization. The disturbance in the

PHYSICS Oasis School Science and Environment - 8 131

chain of molecular magnets in a magnet is the main cause of demagnetization. When the
molecular magnets in a magnet arrange into ring form, the magnet does not behave as a
magnet. A magnet gets demagnetized by either of the given activities:

i. By heating a magnet
ii. By hammering a magnet
iii. By dropping a magnet
iv. By rubbing the like poles of magnets
v. By storing magnets haphazardly without using keepers

Reasonable fact-4

Magnetic properties are lost when a magnet is heated.
When a magnet is heated, the array of order of molecular magnets will be disordered.
So, magnetic properties are lost when a magnet is heated.

Reasonable fact-5

When a magnet is hammered, it loses magnetic properties.
When a magnet is hammered the array of order of molecular magnets will be
disordered. So, magnetic properties are lost when a magnet is hammered.

9.5 Methods of Conserving Magnetism Bar magnet
Keeper
Magnets are highly useful. We should
conserve magnetism or magnetic energy.
Some methods for conserving magnetism
are given below:

i. We should not heat the magnet. Fig. 9.7

ii. We should not drop the magnet.

iii. We should not hammer the magnet.

iv. We should not rub the like poles of magnets.

v. We should not store magnets in moist places.

vi. We should store magnets by using keepers of iron.

vii. We should store magnets by joining their unlike poles.

132 Oasis School Science and Environment - 8 PHYSICS

Activity 3
• Bring a bar magnet, an iron nail and a magnetic compass.
• Place the iron nail on a table and keep the north pole of the magnet near the

nail. Now, place the magnetic compass on another side of the nail. Which pole
is formed on the other side of the nail?
• Now, remove the magnet from the nail. Bring the south pole of the magnet near
the nail. Which pole is formed on the other side of the nail?
• Write down the conclusion of this activity.

SuMMarY

• The force that a magnet exerts on magnetic substances is called the magnetic
force.

• A magnet is a substance which attracts iron, cobalt, nickel, etc. and always rests
in the north-south direction when suspended freely.

• The magnets which are made by humans by artificial methods are called
artificial magnets.

• Molecular theory of magnetism states that, "Each molecule of a magnet or
a magnetic substance is an independent magnet which is called molecular
magnet."

• In a magnetic substance, molecular magnets are arranged in the form of chains.

• Due to the existence of the molecular magnets, we cannot make a monopole or
single pole of the magnet.

• The process by which a magnet loses its properties is called demagnetization.
The disturbance in the chain of molecular magnets in a magnet is the main
cause of demagnetization.

• A magnet gets demagnetised by heating, dropping, hammering, rubbing the
like poles of magnets, etc.

• Magnets are highly useful. So we should conserve magnetism.

PHYSICS Oasis School Science and Environment - 8 133

exercise

1. Choose the best answer from the given alternatives.

a. A magnet attracts ………..........…. .

i. plastic ii. stone

iii. wood iv. iron

b. A freely suspended bar magnet always rests in ……….....…. direction.

i. east – west ii. north-south

iii. west – south iv. south – east

c. Which of the following is a natural magnet?

i. lodestone ii. bar magnet

iii. magnetic compass iv. U-shaped magnet

d. Which of the following is a temporary magnet?

i. magnetic needle ii. electromagnet

iii. bar magnet iv. compass

2. Tick (√) the correct statement and cross (×) the incorrect one.

a. Magnet is used by navigators to find their way in the sea.

b. Copper is a magnetic substance.

c. We can separate the poles of a magnet.

d. The pole of the magnet that points towards north is called

south pole.

e. We should not hammer the magnet.

3. Fill in the blanks using appropriate words.

a. A magnet is made of numerous molecular ........................ .
b. A magnet has ........................... poles.
c. The .......................... of a magnet cannot be separated.
d. The process of losing magnetic properties is called ........................ .
e. We should not rub ........................ of magnets.

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4. Answer the following questions.
a. What is a magnet?
b. State molecular theory of magnetism.
c. What is a magnetic substance? Draw a neat and labelled figure showing the
arrangement of molecular magnets in a magnetic substance.

d. What is a molecular magnet? Draw a neat figure showing the arrangement of
molecular magnets in a magnet.

e. What is meant by magnetic induction?
f. What is demagnetization? Name any three activities that cause

demagnetization.
g. Write any four methods to conserve magnetism.

5. Differentiate between:

a. Magnet and Magnetic substance
b. Magnetic induction and Demagnetization
6. Give reason:

a. A freely suspended bar magnet always shows north–south direction.
b. A magnet has stronger force of attraction at the poles than in the middle.
c. A magnet loses its properties if it is hammered.

7. Study the given diagrams and answer the following questions.

Fig. (a) Fig. (b)

i. Which one is a magnet? Why?
ii. What type of material is shown in fig. (a)?
iii. Which theory is explained by the above diagrams?

8. Describe an activity to prove that magnetic poles cannot be separated.
9. Describe an activity to demonstrate magnetic induction.

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10uNIt Estimated teaching periods : Th Pr
3 1

Electric heater

eleCtrICItY

Objectives

After completing the study of this unit, students will be able to:
• describe the structure and utilities of simple cell and dry cell.
• introduce household electrification (wiring system) and domestic

electric appliances.
• introduce fuse and MCB.

Course of Study
• Simple cell and dry cell
• Structure and utilities of simple cell and dry cell
• Household wiring system
• Some domestic electric appliances (electric lamp, heater, electric bell,

radio, television, telephone, mobile and computer)
• Fuse and MCB

Points to be Focused/Questions to be Discussed
• What is a simple cell? What are its defects?
• What is a dry cell ? Why is it widely used?
• What is meant by household wiring system?
• What are domestic electric appliances?
• What is a fuse? What is it made of?
• What is MCB? Why is it called the advanced form of fuse?

136 Oasis School Science and Environment - 8 PHYSICS

10.1 Introduction

Electricity is a form of energy which is produced due to the continuous flow of electrons
through a conductor. Every atom is made up of two types of charged particles. They
are protons (positively charged particles) and electrons (negatively charged particles).
Metallic conductors such as copper, aluminium, etc. contain a large number of free
electrons. When these electrons flow continuously in a certain direction, electric current
is produced. Electricity is one of the key sources of energy. It can easily be converted into
light energy, heat energy, sound energy, magnetic energy, and so on.

10.2 Sources of Electricity

In modern world, we use a large number of electronic appliances such as computer,
television, radio, camera, washing machine, refrigerator, calculator, watch, etc. Electrical
energy is required to operate these appliances. Similarly, electrical energy is essential to
run industries, to operate fans, to obtain light, heat, etc. The devices from which we obtain
electricity are called sources of electricity. The three major sources of electricity are as
follows:

1. Cell

2. Photo cell or Solar cell

3. Dynamo or Generator

Dry cell Photo cell or solar cell Generator

10.3 Cell Fig. 10.1

A cell is a device which converts chemical energy into electrical energy. There are mainly
two types of cells. They are:

a. Primary cell and

b. Secondary cell

a. Primary cell: The cell which converts chemical energy into electrical energy
and cannot be recharged is called primary cell. This cell cannot function if the
chemicals in it are exhausted once. Simple cell and dry cell are the examples of
primary cell.

exhaust / ɪ ɡ ˈ z ɔ ː s t / - completely used or finished

PHYSICS Oasis School Science and Environment - 8 137

b. Secondary cell: The cell which can be used again
and again after recharging is called secondary cell.
For example, lead acid cell, nickel-cadmium cell,
etc. In secondary cell, electrical energy is stored in
the form of chemical energy and when required, the
chemical energy is changed into electrical energy.

10.4 Simple Cell Fig. 10.2 Lead acid cell

A simple cell has two metallic plates dipped in the acid solution. One of the two plates is
copper and another is zinc. The acid solution is dilute sulphuric acid (H2SO4). The zinc
plate acts as a cathode and works as a negative terminal whereas the copper plate works
as a positive terminal in the form of anode. It produces a maximum potential difference
of 1 volt.

Copper wire

Bulb

Zinc plate Copper plate
Beaker
Dil.H2SO4

Fig. 10.3 Simple cell

The dilute sulphuric acid dissociates into hydrogen (H+) and sulphate (SO4– –) ions in acid
solution as follows.

H2SO4 2H+ + SO4– –

When zinc rod is dipped into the dilute sulphuric acid, zinc sulphate is produced along
with two electrons. In this way, zinc rod gets two electrons and becomes a negative
terminal, i.e.

Zn + SO4– – ZnSO4– – + 2e-

When copper rod is dipped into the dilute sulphuric acid, hydrogen ions (H+) are attracted
towards copper plate and produce hydrogen gas. The copper plate loses electrons and
becomes a positive terminal.

H+ + e- H

(From copper)

H + H H2

When a zinc plate and a copper plate are connected by a conductor, electrons flow
continuously. In this way, current is produced in the simple cell.

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Activity 1

• Take a clean beaker and keep 100 ml of water in it. Now, add 10 ml of dilute
sulphuric acid in it. Take a copper plate and a zinc plate and immerse both
plates in the acid solution.

• Connect these plates with a bulb by using conducting wires. Does the bulb glow?

Defects in a Simple Cell

There are two main defects in a simple cell. They are as follows:

a. Local action b. Polarization

a. Local Action

The zinc plate used in the simple cell is not pure. It contains impurities like carbon,
iron, copper, etc. These impurities act like carbon plate, iron plate, etc. So, the local
current starts to flow which resists the flow of current in the external circuit. This
defect is called local action. It decreases the life of a simple cell.

Remedy: Local action can be minimized by using pure zinc plate or by using the
mercury coated zinc plate.

b. Polarization

Polarization is another defect of the simple cell. It is the process in which hydrogen
gas is formed on the copper plate and remains in the form of a layer. This layer stops
the reaction between acid and the copper plate. As a result, current does not flow
through the circuit.

Remedy: Polarization is minimized by using depolarizer like potassium dichromate
(K2Cr2O7), manganese dioxide (MnO2) or copper sulphate (CuSO4).

Differences between Polarization and Local action

Polarization Local action

1. It is the defect of a simple cell in which 1. The impurities present in the zinc like

hydrogen gas formed during the carbon iron, copper, etc. cause to flow

chemical reaction gets deposited on local current which resists the flow

the surface of copper plate and stops of current in the external circuit and

the flow of electric current. reduces the life span of the cell. This

effect is called local action.

2. Polarization can be minimized by 2. Local action can be minimized by using
cleaning the copper plate regularly pure zinc or by using mercury coated
with a brush or adding potassium zinc plate.
dichromate (K2Cr2O7) or manganese
dioxide (MnO2) in the solution of
simple cell.

PHYSICS Oasis School Science and Environment - 8 139

10.5 Dry cell + Positive terminal
Air space
Simple cell has a liquid, i.e. sulphuric acid.
So, it is difficult to carry a simple cell from

one place to another. Dry cell has no solution Paste oafnMd nCO2, ZnCl2,
and hence is easy to carry and handle. There NH4Cl
is no risk of spilling acid from the dry cell.
Graphite rod (anode)

Dry cell consists of a zinc container and

a carbon rod with a brass cap. The zinc Zinc can (cathode)
container is filled with a moist paste of Metal jacket
ammonium chloride. The carbon rod is
placed at the centre of the zinc container. – Insulator
The carbon rod acts as the positive terminal
and zinc container acts as the negative Negative terminal
Fig. 10.4 Structure of a dry cell

terminal of the cell. It is surrounded by a

closely packed mixture of carbon powder and manganese dioxide (MnO2) in a muslin bag.
The top of the cell is sealed with wax to avoid evaporation of moisture of the ammonium

chloride. But there is a hole for the gas to escape formed in it.

Within the cell, chemical reaction occurs with zinc, ammonium chloride, manganese
dioxide and carbon. Here, manganese dioxide acts as a depolarizer. When two terminals
of the cell are connected by a conducting wire, electricity flows from one terminal to
another. The strength of a dry cell is 1.5V. When the reaction in the dry cell is complete,
the cell no longer supplies electricity. Thus, it stops working. If the cell is kept unused for
a long time, it stops working due to the local action.

Reasonable fact-1

Dry cell is widely used in the comparison of simple cell.
Dry cell has no solution and can be formed in any shape according to the requirement.
So, dry cell is widely used in comparison of simple cell.

Reasonable fact-2

Polarization does not occur in dry cell.
Manganese dioxide (MnO2) is used in dry cell. So, polarization does not occur in dry
cell.

Reasonable fact-3

The life span of a dry cell decreases gradually though it is not used.
Due to local action, the life span of dry cell decreases gradually though it is not used.

140 Oasis School Science and Environment - 8 PHYSICS

Activity 2

• Take a dry cell and cut it longitudinally. Observe its internal strucuture.
• Draw a neat and labelled figure of the dry cell and submit to your science

teacher.

Differences between Simple cell and Dry cell

Simple cell Dry cell

1. In a simple cell, the acid solution, i.e. dilute 1. In a dry cell, the acid solution, i.e. dilute

sulphuric acid (H2SO4) is used. sulphuric acid (H2SO4) is not used.

2. It is difficult to carry a simple cell from one 2. It is easy to carry a dry cell from one place

place to another. to another.

10.6 Household Wiring System

The AC circuit made in the industries, factories, Earthing wire
houses, etc. is called domestic electric circuit. The

electric power generated by the turbine is connected

by cables through high transmission line at very high Neutral wire Live wire
voltage. The voltage is decreased to 220 V by using
step down transformer and supplied to the household

purposes. To operate any electrical appliance, we

need two connecting wires. They are live wire (L),

and neutral wire (N). A third wire is also connected Fig. 10.5

in the power circuit which is called earthing wire (E).

Earthing wire saves us from electric shock and prevents the flow of excessive current, i.e.

overloading through the appliances.

The fuse which is connected before the kilowatt hour (KWh) Fig. 10.6
meter is known as corporation fuse. This fuse protects the meter
from getting damaged. Other fuses which are connected after
the kWh meter are called consumer's fuses. These fuses are
generally connected after main switch box. Main switch helps to
cut off the current throughout the household circuit. The main
switch box is made up of iron and a connection is made from the
iron box (main switch box) to the earth by a conductor which is
known as earthing. Finally, the wires which are emerging out
of the main fuse box are connected to the distribution board as
required.

PHYSICS Oasis School Science and Environment - 8 141

Corporation Consumers' fuse Socket
Fuse L

L N
E
N
Bulb

E

Meter Main Main Fan
box switch box fuse
(kWh meter)
Distribution board

Fig. 10.7 Domestic electric wiring

All the electric appliances are connected with separate circuit in parallel way. So, separate
switches are used to control the flow of current. All the current flows through the fuse
of the meter box. So, it should be of the value 15 A. It is very important to note here that
usually there are separate circuits in a house, the lighting circuit with a 6 A fuse and the
power circuit with a 16 A fuse. The lighting circuit is one which allows less amount of
current to flow but the power circuit draws more current through it. The lighting circuit
is used for running electric bulbs, tube lights, fans, radios, etc. On the other hand, power
circuit is used for running electric iron, room heater, refrigerator, etc. Each distributive
circuit is provided with a separate fuse so that if a fault like short circuiting occurs in one
circuit, its corresponding fuse blows off but the remaining circuits remain unaffected.

10.7 Some Domestic Electrical Devices

In the present day world, electricity is the most essential source of energy. Almost all
scientific equipment are operated by using electricity. The equipment which are operated
by electricity are called electrical equipment. These equipment convert electrical energy
into some other forms of energy during their operation. We use a variety of electrical
equipment at our homes. Some of the common household electrical appliances of our
daily use are as follows.

i. Electric lamp Electric bulb CFL

The electrical equipment which Fig. 10.8
converts electrical energy into light
and heat energy is called electric
lamp. There are two types of electric
lamps. They are filament lamp and
fluorescent lamp. The fluorescent
lamp is commonly known as tube
light. Nowadays, CFL (Compressed
Fluorescent Lamp) is used instead of
tube light.

142 Oasis School Science and Environment - 8 PHYSICS

ii. Electric heater

The electrical appliance that converts electrical energy into heat energy is called
electric heater. It is used for cooking food, making the room warm, heating water,
pressing (ironing) clothes, etc.

Electric heater Immersion rod Electric iron Electric kettle

iii. Electric bell Fig. 10.9

The electrical appliance that converts electrical energy into sound energy is called
electric bell. An electromagnet is used in the electric bell. In an electric bell, a
temporary magnet is produced by using electricity and the magnet attracts the iron
armature and hits the gong repeatedly. As a result, sound is produced. Nowadays,
electric bell is widely used in houses, offices, schools, etc.

Hammer

Gong

Electromagnet

Fig 10.10 Electric bell

iv. Radio and television

Radio is an important means of one-way communication. It can be operated by
using battery or electricity. Similarly, television is an important means of one-way
communication. It is an audio-visual electrical equipment which is operated by electricity.

Radio Television
Fig 10.11

hazard / ˈ h æ z ə d / - a thing that can be dangerous or cause damage

PHYSICS Oasis School Science and Environment - 8 143

v. Telephone and mobile

Telephone and mobile are the most important electrical equipment of two-way
communication. They are operated by electricity.

Telephone Mobile

Fig 10.12

vi. Computer

Computer is the most important electronic equipment. Modern life is almost
impossible in the absence of computer. It is used to operate e-mail, internet, etc. It
is also used to write books, store data, play music, store and play videos and many
more. Computer is also operated by electrical energy.

Computer Laptop

Fig 10.13

10.8 Fuse and MCB

A fuse is a thin wire made of tin (63%) and lead (37%) having low melting point and high
resistance. The capacity of a fuse is measured in ampere (A). A fuse breaks the circuit
by melting itself when the current exceeds the safe value. Hence, it prevents the electric
appliances in the circuit from getting damaged and other electric hazards.

Fuse wire

Porcelain fuse Cartridge fuse Symbol of fuse

Fig. 10.14

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