Blooming Science-9-2077 final final for press

a) Mechanical advantage (MA) = ?

b) Velocity ratio (VR) =?

c) Efficiency (η) =?

According to the formula,

a) MA = L = 1200N = 4
E 300N

b) VR = R = 24cm = 4.8
r 5cm

c) = MA × 100%
VR

= 4 × 100%
4.8

= 40 × 100% = 5 × 100% = 83.33%
48 6

Hence, in that wheel and axle MA is 4, VR is 4.8 and η is 83.33%.

Moment of a Force

When a force is acted on a body, it results either displacement Turning effect Turning effect

of a body or its rotation about a point or axis. This rotation or

turning effect of force may be clockwise or anticlockwise.

The figure given below shows the turning effect of a force

applied on the body. In figure (a) the rotation is anticlockwise Force Force (b)
while in (b) it is clockwise. (a)

The turning effect of the force acting on a body about an axis is called moment of the force.

Apply a force in the handle of a door to open it. Now close the door, then apply a force near the
hinge to open it. Do you find any difference in the force applied while opening the door? We find
that less force is needed to open the door when it is applied at its handle.

For this activity it is clear that moment of a body also depends on the perpendicular distance of
the line of action of the applied force from the axis. Thus, the moment of force is equal to the
product of the magnitude of the force and the perpendicular distance of the line of action of the
force from the axis of rotation or fulcrum. Mathematically,

Moment = F × s

Here, F = force applied

s = perpendicular distance of the line of action of the force from the fulcrum

The SI unit of moment is N-m but N-cm is also in use. M F
s
If the force is not applied perpendicular on the lever, the value
of moment decreases.

Blooming Science Book 9 51

Law of Moment

Suspend a metre scale horizontally from its s E = Effort
midpoint as fulcrum. Now suspend some weights on
both sides of the midpoint and adjust their distance
such that the scale again becomes horizontal when
the product of effort and effort distance, and that of
load and load distance are calculated, it is found that both are equal.

Mathematically,

Effort x Effort distance = Load × Load distance

(Anticlockwise moment) (Clockwise moment)

It concludes that:

In the equilibrium condition of a lever, the sum of the anticlockwise moment is equal to the sum
of the clockwise moment. This is the law of moment.



Solved problem

In the given figure a lever is shown. Three weights 20N, 40N and 10N are suspended in it.

Now calculate:

a) Clockwise moment

b) Anti clockwise moment Scale

c) Can the lever be in the condition of balance in this
situation?

d) What will be the location of 10N weight by keeping Effort Load
(Anti-clockwise) (Clockwise)
other loads unchanged to balance the lever?

Stand

Here,

First weight (L1) = 20N

Second weight (E1) = 40N

Third weight (E2) = 10N

Distance between L1 and fulcrum (Ld1) = 30cm 20cm
5cm
Distance between E1 and fulcrum (Ed1) = 5cm 30cm

Distance between E2 and fulcrum (Ed2) = 20cm

Now,

a) Anti-clockwise moment = L1 × Ld1 20N 40N 10N

= 20 × 30 = 600N cm

b) Clock-wise moment = E1 × Ed1 + E2 × Ed2
= 40 × 5 + 10 × 20

= 200 + 200 = 400N cm

52 Blooming Science Book 9

c) The lever will not be balanced, as the clockwise moment and anti-clockwise
moment are not equal

d) To balance the lever by changing the location of 10N weight

Clockwise moment = Anti-clockwise moment

Or, E1 × Ed1 + E2 × Ed2 = L1 × Ld1

or, 40 × 5 + 10 × Ed2 = 20 × 30

or, 300 + 10 Ed2 = 600

∴ Ed2 = 600 - 200 = 40cm
10

Thus, the lever can be balanced by keeping the 10N weight at a distance of 40cm

from the lever.

Let’s Learn

1. Value of VR in single movable pulley is two while in a single fixed pulley it is one. This
is because in single movable pulley, load is supported with two segments of string but in
single fixed pulley the load is supported with a single segment only. We know that VR =
No. of segments of string supporting the load.

2. A machine is never 100% efficient, because due to friction some of the input energy wastes in the
form of heat energy and some of input energy is used in lifting the weight of machine parts.

3. Usually tall trees break in storms. This is because perpendicular distance of the line of
action of the force from the fulcrum is more in tall trees. It causes more turning effect of
the force applied by wind on tall trees.

4. Winding roads are made on hills to make the surface slanted. It increases the effort
distance by remaining the load distance constant. In this condition less effort is required
to roll vehicles on the hill.

5. Probability of breaking branch increases when person moves towards the end of the branch
of tree. This is because, the distance of the line of action of the force from the fulcrum
increases in it. In this condition, the turning effect of the force applied by the person on
the branch increases.

6. Magnitude of input work is greater than output work because some of the input work
wastes due to the friction.

7. Magnitude of velocity ratio is greater than that of mechanical advantage of a simple
machine. This is because, mechanical advantage is affected by friction but velocity ratio
is not affected.

8. Long spanners are used to open a rusted knot or big knot. This is because the knot can be
opened only when more turning effect of the force is produced on it. It is possible by long
spanners only due to the more distance between fulcrum and the line of action of the force.

9. Lubricants (oil and grease) are used in machines to reduce friction. This helps to increase
mechanical advantage and efficiency of the machines.

Blooming Science Book 9 53

Main Points to Remember

1. The simple instruments, which make our work easier, faster and convenient to do, are
called simple machines.

2. Simple machines help us by magnifying force, accelerating work and by changing the
direction of force.

3. Principle of machine states that in an ideal machine, effort x effort distance = load x load
distance.

4. Mechanical advantage of a machine is a ratio of load overcome to effort applied in that
machine.

5. Velocity ratio is defined as the ratio of distance covered by the effort to the distance of a
machine covered by load in that machine.

6. MA is affected by friction but VR is not affected by friction hence value of VR is always
greater than M.A.

7. Efficiency of a machine is the ratio of output work to input work in that machine.

8. Friction is reduced in machines by lubricating, making the sliding surfaces smooth and by
using ball bearings.

9. In pulleys, the velocity ratio is calculated by number of pulleys used or by the number of
rope segments that support the load used.

10. In an inclined plane, length of slope (l) is effort distance and height of slope (h) is load
distance, hence velocity ratio is calculated by the ratio of l to h.

11. In a wheel and axle, velocity ratio is calculated by the ratio of circumference of big
cylinder to the circumference of small cylinder.

12. Moment of a force is the turning effect produced by that force.

13. Moment is affected by two factors, they are:

a) The amount of force applied.

b) The perpendicular distance between fulcrum and the line of action of force applied.

14. Law of moment states that “In the condition of equilibrium of a lever the clockwise
moment acting on it is equal to the anticlockwise moment acting on it.”

PRO J ECTWORK

Visit a workshop or furniture industry near by you. Observe different tools used and make a
table to mention their types & uses.

54 Blooming Science Book 9

Exercise

1. Choose the correct answer from the given alternatives.

a. Formula to calculate mechanical advantages.

a. E b. L×Ld = E×Ed c. ED d. none of above
L LD

b. Out put work is equal to ..................................

a. E×Ed b. MA c. L×Ld d. All of above

c. Efficiency is affected by..................................

a. load b. effort c. gravity d. friction

d. ................................. is turning effect of force.

a. moment of force b. force c. momentum d. speed

e. Which machine is not affected by friction?

a. compound machine b. perfect machine c. simple machine d. hydraulic machine

2. Answer the following questions:

a) Define simple machine. Classify it.

b) Write the advantages of simple machine.

c) Define mechanical advantage and efficiency of a machine. What factor affects on
the mechanical advantage of a machine?

d) What is meant by an ideal machine? What are output work and input work?

e) What is meant by velocity ratio of a lever is 3 and efficiency of pulley is 60%?

f) What should be done to make a simple machine more efficient?

g) What is moment? What are the two factors which affect moment?

h) State law of moment of the force.

3. Give reasons:
a) Value of velocity ratio is greater than value of mechanical advantage.
b) Value of efficiency is never 100% or more in practice.
c) Input work is always greater than output work.
d) It is easier to climb up a slanted slope than a vertical slope.
e) Usually tall trees break and fall down in storms.
f) Usually long spanner are preferred more than short spanner to unscrew a very tight
and rusted knot.
g) Possibility of breaking branch increases when a person goes to the tip of the branch.
h) Metal cutters have long handles.
i) It is easier to lift the same load by using three pulley system than two pulley system.

j) Wheel and axle is called a continuous lever.

Blooming Science Book 9 55

4. There is no gain in mechanical advantage in the case of a single fixed pulley. Explain, why
the pulley is then used?

5. In the case of block and tackle arrangement, mechanical advantage increases with the
number of pulleys. Why?
L

6. The diagram shows a lever of uniform mass supported at Effort

the middle point. Six coins of equal mass are placed at
mark A on the left hand side.
AB

(a) Can you balance the lever with 12 more coins of
equal mass? If yes, where would you place them? If
not why?

(b) With six coins still on the left, three coins are put on the right at mark B. Where on
the right should you pile an addition of six coins to get a balance?

7. Write two uses of each inclined plane, pulley and wheel and axle.

8. What should be done to lift the same load by applying less effort on an inclined plane?

9. State the principle of a lever.

10. In which inclined plane, AB or BC is it easier to do work? Explain with reason.

B

1000N 1000N
6m 2m 12m

AC

11. a) Two person A and B are carrying a ladder as in the figure. Who will feel heavier
while carrying the ladder?

b) The mechanical advantage is 2, what does it mean?

AB

Ladder

L

Numerical problems

1. A machine of V. R.2 is used to raise a load of 200 N. If the effort required is 150 N.

a) What is the M.A. of the machine ?

b) What is the efficiency of the machine ? (Ans: 1.3, 65%)

56 Blooming Science Book 9

2. An effort of 250N raises a load of 1000N through 5m in a pulley system. If the effort

moves 30m, what is the mechanical advantage, velocity ratio and the efficiency of the

pulley system ? (Ans : M.A. = 4, V.R. = 5 η = 80%)

3. A load of 1000N is lifted by using 250N effort in a combined double pulley block system.
Calculate the mechanical advantage, velocity ratio and efficiency of the machine.
[ Ans: M.A. = 4, V. R. = 4, η. = 100%]

4. A force of 250N is applied on a body. If the distance between the point of application of

force and the fulcrum (axis of rotation) is 50cm, What is the moment of force about the

fulcrum ? [Ans:125Ncm]

5. In given figure, what is the load acting in an anticlockwise direction round the fulcrum so

that the lever is in equilibrium ? [525N]

L=? 2cm F 10cm E=105N
E

6. If the efficiency of a 3 wheel pulley is 75%, how much effort is needed to lift a load of

900N with the help of this pulley system? Calculate. [400 N]

7. What effort is needed to balance a load of 1000N in a wheel and axle with efficiency 80%,

if the radius of the wheel is 1m and that of the axle is 20cm [Ans. 250N]

8. If 800 N load is lifted vertically calculate MA and E. The efficiency of a Effort
pulley system is 80% and VR is 4. [Ans:3.2, 205 ]

9. If the pulley system given in figure has efficiency 75%. What is the value of
effort when it is in equilibrium? [Ans: 2N)

10. Calculate the effort required to balance the wheel-barrow 3N
given in the figure. [ Ans: 500N]
E

L= 1000 N
F

30 cm 60 cm N

11. From the given fig. of an inclined plane calculate. E = 100

a) M.A, V.R. & Efficiency [Ans: 2, 2, 100%] 4m 2m
b) Input & Output work [Ans:400 J]

c) What should be the length of the inclined plane if the

same load has to be pulled with 50N effort for the same

efficiency as above? [Ans: 8m] 200 N

Blooming Science Book 9 57

Chapter WORK, ENERGY AND POWER

4

Learning Outcomes Estimated Periods: 8+2

On the completion of this unit, the students will be able to:

• define work and its types.
• identify the different forms of energy.

• describe the interrelationship among work, energy, power and their
units.

• solve simple numerical problems related to work, energy and power.

Work

In common poeple’s concept the word work refers to almost

any kind of physical or mental activities such as to carry a box

across the room or to do homework. But in science, work is

done when a force is applied to a body and the body moves by a

certain distance in the direction of the force. For example, work

is done when a bullock pulls a cart on a road or an engine pulls

a train on a railway line or a student lifts a bag full of books. a pe(rIsmoangpeu)s4h.i1ng a wall does no
However a body holding a load in hand does no work. Similarly

work, if the wall does not move. If a body moves some distance by applying an external force,

then it is called work done by a body in the direction of force.

If something is lifted from the ground, then work is done. More force is needed to move a body
of large mass through a distance. More work is done when more force is applied. Similarly when
the body acted upon by force displaces more distance, more work is done. Thus, work done on a
body depends on two factors such as:

a. The magnitude of the applied force, and

b. The distance through which a body moves.

Work done by a force on a body is equal to the product of the magnitude of the force and the
distance moved by the body in the direction of the force.

Suppose a force F is acting on a body at A in the line AC moves the body from A to B. If the
displacement of the body be d (i.e. AB = d) along the direction of the force, then the work done
W is given by, W = F × d

F

A d BC

From the above definition, it is clear that whatever great force may be, no work will be done
unless a force acts through a distance.

58 Blooming Science Book 9

So, the work is said to be done when a force acting on a body produces some displacement in the
direction of the force.

If the body is displaced in the direction other than the direction of the applied force, then work
done is given by the product of the component of the force in the direction of the displacement
and the displacement itself.

FF

qq
FCosq

d

For the object as shown in fig, the component of force in the direction of the displacement ‘d’ is

FCosq. Box
Force
Therefore, work done (W) = Fcosq × d
or W = FdCosq

If the force acts perpendicular to the direction of displacement then q =\
90o, so no work is done as W = F × s × Cos 90o = 0 ( Cos90o = 0)

For example, if a porter carries a box on his head and moves on the displacement(d)
horizontal ground then the work done is zero because he has to apply the
force in the direction opposite to gravity but displacement is horizontal.
so, q = 90o and therefore, work done = 0

Unit of Work

The work is a scalar quantity. It has only magnitude but no direction.

The unit of work is a product of the unit of force and the unit of distance. In SI system, the unit
of force is Newton (N) and that of distance is metre (m). Hence, the unit of work is newton metre
(Nm) which is called Joule (J).

SI unit of work is Joule.

One Joule is the amount of work done when a force of one Newton acting a body moves it,
through a distance of one metre in the direction of force.

\ 1 Joule (J) = 1 Newton (N) × 1 metre (m)

One Joule is equal to 1 kilogram metre2/second2

1J = N × m [1N = 1kg m/s2]

1J = kg m/s2 x m

1J = kg m2/s2

The CGS unit of work is erg.

1 joule = 107 erg
1 erg = 10-7 Joules

Blooming Science Book 9 59

Types of Work A healthy adult man requires 2500- ?Do
There are two types of work done. They are 3000 calories of food everyday whereas
a. Work done against gravity woman requires around 2000 calories You
b. Work done against friction per day. Only 260-300 calories are brunt Know
by our body during playing of football
Work Done Against Gravity for 1/2 an hour.

If a body moves in the direction of force applied, work is done by the force. For example, if a
body be allowed to fall from a certain height, the body falls downward due to the gravity of the
earth. Here the body moves in the direction of the gravity.

Activity
To calculate the work done against gravity

1. Take a box of mass 10 kg.
2. Climb up a staircase.
3. Count the number of steps and measure the height of each step.
4. Multiply the number of steps by the height of each step to get the total vertical

distance walked.
5. Calculate the work done.

In above activity, work is done against the force of gravity.

To calculate the work done in moving up the staircase, only the vertical distance is
measured. Because the force is used against the force of gravity to lift the box vertically
upwards. Thus, the force is used vertically upward, but no work is done against the force
of gravity while walking the horizontal surface of the steps.

When a body under the action of force moves in a direction perpendicular to the direction
of the force, no work is done. In climbing the staircase, while walking the horizontal
surface of the step, the direction of the displacement and the force of gravity are mutually
perpendicular.

Solved Numerical Problem

A girl of mass 60 kg climbs up a staircase of 2.5m high. Calculate the work done by her?

Solution:

Mass (m) = 60kg

Height (h = d) = 2.5m

Work done (W) = ?

According to the formula,

W = F × h

= m × g × h [∴ F = mg]

= 60 × 9.8 × 2.5 [∴ g = 9.8 m/s2]

= 1470 J

60 Blooming Science Book 9

Work Done Against Friction

When a body is pushed or pulled along a rough surface and it moves, work is done against the
force of friction.

When a block kept on the floor is pushed with a small force, it does not move. If we go on
increasing the magnitude of the force, it then starts to move. Similarly, a moving object when left
freely, it comes to rest after moving some distance. From the above discussion we can think that,
there must be certain force acting between two surfaces in contact opposing the motion. This
opposing force is called the force of friction.

F F

Force of friction Force of friction

Fig: Sliding object Fig: Rolling object

So, when we have to slide or roll the object on any surface we have to do extra work overcome
the friction. This work is called the work done against friction.

Activity

To demonstrate the work done against friction

Force

0 N
4 O
8
12

1. Put a wooden block on the table.
2. Attach a spring balance to it.
3. Pull a spring balance a little, but do not move the block.
4. Now slowly increase the pull till it begins to move and pull it to some distance.
5. Measure the distance covered by the block.
6. Record the force required to pull the block in the spring balance.
7. Use formula W = F x d to calculate the work done on pulling the block on the table.
8. Pull the same block on the very rough surface. Find the force required covering the

same distance and calculate the work done on pulling the block on a rough surface.

Solved Numerical Problem
A man applies a force of 200 N against the friction to push the wheel cart through a distance
of 5m. Calculate the work done by a man.

Solution:

Force (F) of friction = 200N

Distance moved (d) = 5m

Work done (W) = ?

Blooming Science Book 9 61

According to the formula,
W = F × d
= 200 N × 5m
= 1000 Nm
= 1000J
∴ Work done by the man is 1000 J.

Energy

All living beings including human beings need food. Food provides energy to perform various
activities. Thus the energy from the food enables us not only to breathe and think but also to do
jobs like running, jumping, lifting or playing. Can we live without food for a long time?

A body having energy can do work. Thus, energy is the ability of a body to do work and is equal
to the work done on the body. So it is also measured in Joules.

Types of Energy

There are many different forms of energy such as mechanical, heat, light, magnetic, electrical,
sound, chemical, atomic etc. These forms of energy are explained here in brief.

Mechanical Energy

The energy possessed by a body by virtue of its motion or position is called mechanical energy.
It is of two types;

i. Potential energy ii. Kinetic energy

Potential Energy (Energy due to position)

The energy possessed by a body by virtue of its position or state (configuration) is called potential

energy. For example water stored in a dam, stretch catapult, a leg raised to

hit a football etc. It is calculated as P.E =mgh mgh

Measurement of Gravitational Potential Energy.

When a body is lifted vertically through a height, work is done against the h
weight and this work is stored up in the body as the potential energy. Then
the gravitational potential energy of a body of mass ‘m’ situated at a height
h is equal to the amount of work done in lifting it from the ground to that
height against the force of gravity.

Now, force applied to lift the body, F=mxg

Work done in lifting the body through a vertical height = F × h

Then, work done = F × h

= mg × h

So, the potential energy of the body at a height h from the ground is mgh.

∴ P.E = mg h

62 Blooming Science Book 9

Solved Numerical Problem

A metal ball of mass 5 kg is allowed to drop freely from a height of 15 m above the ground.
Calculate the potential energy possessed by the ball when initially at rest, (g = 9.8 m/s2).

Solution:

Mass (m) = 5 kg

Height (h) = 15m

Acceleration due to gravity (g) = 9.8 m/s2

Potential energy (P.E.) =?

According to the formula,
P.E = mgh
= 5 × 9.8 × 15
= 735 J
∴ The potential energy possessed by a metal ball at rest in 735 J.

Kinetic Energy (Energy due to motion)

The running motor is used to run a turbine for generating electricity. Kinetic energy of a body
depends on its mass and velocity. A stream of slow flowing water possesses less energy. A stream
of fast flowing water possesses high energy.

It is a common experience, that the fast moving body can do work when it strikes another body. A
fast moving stone can work on a window pane and break it. Similarly when a hammer strikes a nail,
it exerts a force on the nail and moves it through a certain distance ( a hammer striking a nail thrusts
it into wood). Thus an object in motion has ability to do work and therefore it has energy.

The energy possessed by a body by virtue of its motion is called kinetic energy.

Examples of kinetic energy are moving bullets, rotating wheels of a moving car, a moving fan,
running water, wind, a moving hammer and fast moving electrons in a television tube. Kinetic
energy is very helpful to us. The force of wind (air in motion) can be used to generate electricity
Wind turns the blades of a wind generator. The rotating blades are connected to a machine that
produce electricity. This machine changes the KE of the wind into electrical energy.

A massive, fast moving body has more kinetic energy than the light, slow moving body.

Kinetic Energy Expression Proof: (Relation between work and energy)

Consider a body of mass ‘m’ moving with initial velocity ‘u’. Let external force ‘F’ be applied on
it producing an acceleration of ‘a’ thereby changing its velocity to ‘v’ after moving the distance
‘s’. Then, we know from equation of motion relating v, u, a and s:

v2 = u2 + 2as

or v2-u2 = 2as F mu mv

or as = 1 (v2-u2)............................(i) s
2

Then, work done on the body is W = F × s

Blooming Science Book 9 63

or W = ma × s

or W = mas ...........................(ii)

Using equation (i) in (ii) we get,

W= m 12(v2-u2)

or W = 1 m(v2-u2) .....................(iii)
2

If the body was initially at rest, then u = 0

Therefore W = 1 m(v2-02)
2

or W = 1 mv2
2

This work done causes the motion of the body. So, K.E = work done

i.e. K.E. = 21mv2 proved.

Differences between Potential Energy and Kinetic Energy

Potential Energy Kinetic Energy

1. It is the energy posessed by a body 1. It is the energy a body has because of its
because of its position or configuration motion.
(change of shape).

2. Its value can be calculated by mgh i.e. PE. 2. Its value can be calculated by 1/2 mv2 i.e.

= mgh. K.E. = 1/2 mv2

3. It depends on the acceleration due to 3 It depends on the velocity of the body.
gravity at a place and also on the height
of the body with respect to the reference
level.

4. e.g. Water stored in a dam possesses 4. e.g. Water falling from a dam possesses
potential energy. kinetic energy.

Solved Numerical Problems

1. The K.E. of a motorbike of mass 350kg increases its velocity from 72km/hr to 90km/
hr doing work on it. What work was done?

Here, mass of bike (m) = 350kg.

initial velocity (u) = 72km/hr

= 72 × 1000
= 60 × 60
720
36

= 20 m/s.

64 Blooming Science Book 9

Final velocity (v) = 90 km/hr.

= 90 × 1000
60 × 60
900
= 36

= 25m/s.

Now,

Work done on the motorbike (W) = change in K.E

= 1 m(v2-u2)
2
= 12× 350{(25)2-(20)2}

= 175{625-400}

= 175 × 225

= 39375J

2. A bullet of mass 500 g is fired with a uniform velocity of 200 km/h. Calaculate its
kinetic energy.

Solution: Mass (m) = 500g.

= 500 kg = 0.5kg
1000

Velocity (v) = 200 km/h

= 200 × 1000 m/s = 500 m/s
60 × 60 9

Kinetic energy (KE) = ?

According to the formula

KE = 1 mv2
2
500 500 2
= 1 × 1000 × 9 = 771.7 J
2

Chemical Energy

Foods contain stored energy. Green plants prepare food by photosynthesis, In photosynthesis
solar energy is converted into chemical energy and is stored in the food. The chemical energy
contained in the food that we eat provides the energy required to do work in our daily life. That’s
why all living beings need food to live.

Similarly, fossil fuels such as coal, petrol, diesel, kerosene, wood etc. contain stored chemical
energy. A car engine uses fuel i.e. chemical energy. The chemical energy contained in fuel
produces heat when burnt.

When a matchstick is striked against the rough surface on the matchbox, chemical energy present
in the chemical of the match head is used and it is converted into sound and light energy.

Blooming Science Book 9 65

Electrical Energy

Electrical motors, fans, washing machine, air conditioner, TV, calculator, fax, computer, radio
etc. use electrical energy to operate. Electrical energy is produced by the flow of electrons.
Electrical energy can be converted into other forms of energy such as heat, light etc. easily, safely
and cheaply. So electrical energy has wide applications in the modern life. It is also used to run
lifts and machines in factories.

Light Energy

Light energy is the form of energy which produces sensation in our eyes to produce the image
of an object. Light energy travels in the form of electromagnetic waves having both electric and
magnetic characteristics. Light energy does not require any medium for its propagation. So, it
can travel in vacuum as well.

The major source of light energy is the sun. However, in our daily life, there are a large number of
artificial sources of light. The oil lamp, the candle, the electric bulb, mercury tube etc. are some
examples of artificial sources of light which we use.

Heat Energy

This is energy which makes the men and the animals feel about hotness and coldness. It gives the
sensation of hotness and coldness. Our body itself is a source of heat. Heat is continuously generated
in our body. Some of the chemical energy that is obtained from the food we eat is converted into
heat required for our body. Due to this heat, the temperature of our body remains at 37oC.

In addition to internally generated heat in our body, we need heat energy for various purposes.
For example, for cooking food; for keeping rooms, houses, and environment warm; for running
heat engines in vehicles, etc. Various sources of heat energy are the sun, wood burning, bio gases,
petroleum products, electricity, etc. However, the most abundantly used heat source is the sun.
The sun emits a huge amount of energy in the form of heat which keeps our environment, homes,
houses and buildings warm for us.

Sound Energy

Sound is the form of energy which produces sensation in our ears. In fact, the sound energy is
the mechanical energy. It requires a mechanical medium for its propagation. It travels in the form
of a wave (to be discussed later). When the sound wave travels through a medium, it sets the
molecules of the medium into vibration (to and fro motion about a fixed point). During vibration,
energy is transmitted from one particle to the another and hence from one place to another.

Magnetic Energy

The magnetic energy is the energy associated with a magnetic field. The magnetic field is the space
around a magnet where another magnet experiences a force. A great advantage of the magnetic energy
is that it can be stored for long time without losing the information it carries. Now a days, the magnetic
energy is used widely in audio and video recording. In audio and video recording, the audio and video
information in the form of electrical energy is converted into magnetic energy. The magnetic energy is
stored in a magnetic material locally called a tape. During playing, the magnetic energy stored in the
tape is converted into electrical energy without loss of the magnetic energy previously stored.

66 Blooming Science Book 9

Nuclear Energy

A huge amount of energy is produced when fission or fusion of the atomic nuclei takes place. It
is known as atomic energy. The atomic energy is used for producing electrical energy. The heat
and light energy of the sun is produced by the nuclear fusion.

When a heavier nucleus splits into lighter nuclei (fission) or light nuclei fuse together (fusion)
a large amount of energy is released in the form of heat, light etc. Such energy having nuclear
origin is called nuclear energy. That is, the energy possessed from a nuclear fission or fusion is
called nuclear energy.

Energy Transformation

One form of energy may be converted into another. This change of energy from one form to
another is known as transformation of energy. For example, water in the reservoir of a dam has
potential energy. When water falls on the turbine in the power house, the potential energy of the
stored water is converted into kinetic energy. The kinetic energy of moving water rotates the blades
of the turbine. The kinetic energy of the turbine is used to run the generator. The generator converts
the kinetic energy into electrical energy. The electricity generated at the power house is transmitted
at very high voltages to the consumer site where it is again converted into electricity at an ordinary
voltage. The electricity is used in different electrical appliances such as iron, heater, bulb, fan, radio
etc. In a fan, electric energy is converted into mechanical energy. In an electrical heater and iron,
electrical energy is converted into heat. In an electric bulb, electrical energy is converted into light.
In telephone, electrical energy is converted into sound.

Potential energy Kinetic energy Electrical
Rotation of turbine energy
Water in dam

There are many energy transformations of great importance taking place around us. A few examples
of energy transformations and the devices which bring about these transformations are given in
table.

Energy Transformation Devices
1. Heat energy into mechanical energy 1. Steam engine
2. Sound energy into electrical energy 2. Microphone
3. Solar energy into electrical energy 3. Photoelectric cells
4. Electrical energy into mechanical energy 4. Motor
5. Chemical energy into electrical energy and then light 5. Electric torch
6. Mechanical energy into electrical energy 6. Bicycle dynamo
7. Electrical energy into sound energy 7. Loud speaker
8. Electrical energy into mechanical energy 8. Electric fan
9. Electrical energy into kinetic energy. 9. Washing machine

Blooming Science Book 9 67

Conservation of Energy

Whenever energy is transformed from one form to the other or transferred from one body to

another, it is found that there is no loss of energy in the process. The total amount of energy

remains constant. A K.E.=0 but P.E. = mgh
ball
both K.E. And P.E. but
Energy can neither be created nor destroyed, it can simply h B total energy = mgh

be transformed into energies of different forms, the total
energy before transformation is equal to the sum of the

different energies obtained after transformation. This is P.E. = 0, K.E.≠ 0 but total
known as the principle of conservation of energy. C energy does not alter

In the diagram, a ball is at height ‘h’. In conditional ground

‘A’ its total energy is equal to PE due to that height. When it is released it gains velocity but

loses the height. In condition ‘B’, its total energy is KE+PE but energy is not increased but

remains constantsas in ‘A’. When is touches the ground at ‘C’, it has no energy but the energy

is transformed into sound, heat, etc and is absorbed by surrounding. In this way energy remains

constant and can’t be created nor can be destroyed.

Activity

To demonstrate the conservation of energy
Hammer a piece of iron for a few times
Touch the iron.

The iron becomes warm. The energy changes during the hammering a piece of iron are as
follows,

Chemical energy → Potential energy → Kinetic energy → Heat
energy

(Food in our body) (In hammer when lifted) (in hammer during falling) (In iron)

Power

Suppose two girls, one strong and other weak, draw water from a well 5 metre deep. If both have
drawn one bucket of water, obviously both of them have done equal amount of work. However,
the strong girl has done her work in a short interval of time than the weak girl. So power is not
the amount of work that is capable of doing, but it is the speed with which the agent is able to do
the work. Therefore, power is the rate at which it performs work.

Power is the rate of doing work. It is also defined as the rate of transformation of energy.

Power is measured by the amount of work done in one second. If W is the total work done in time
t, then the power P is given by,

Power (P) = Work (W)
Time (t)

Power is a scalar quantity. The SI unit of power is Watt (W). One Watt is the power which
performs 1 Joule of work in one second, i.e.,

1 Watt (W) = 1 Joule
1 Second

68 Blooming Science Book 9

An electric heater has power of 1500w. It means it can convert 1500 J of electrical energy into
heat and light energy in 1s.
A power of one watt is a small unit. Larger power are measured in kilowatt and megawatt.
1KW = 1000 W = 103W
1MW = 1000 KW = 100,00,00 W = 106W

Horse Power

Power can also be measured in horse power. This is a traditional system to measure power.
Mathematically, [1h.p. = 746 watts.]

Differences between Work and Power

Work Power

1. It is the product of force applied and the 1. It is the rate of doing work.
distance covered in the direction of force.

2. It is not related with time. 2. It is related with time.

3. It is measured in joule. 3. It is measured in watt.

Differences between Energy and Power

Energy Power

1. Energy is the capacity of a body to perform 1. Power is the rate of doing work.
work.

2. Energy of the body is measured by the 2. Power is measured by the work done in

total work done by the body. Time is not unit time. Total work done divided by

considered. time taken gives the power.

3. In SI unit, the unit of energy is Joule. 3. In SI unit, the unit of power is watt.

4. Energy can be transferred from one from to 4. Power cannot be transferred.
another.

5. Energy exists in different forms. 5. Power does not exist in different forms.

Solved Numerical Problems

1. A pump set is used to lift water to a reservoir of 4000 litres capacity placed 10m high.
If it takes 1 hour to fill the reservoir, calculate the power of the pumpset. (Assume
g = 10m/s2)

Solution:

Mass of water (m) = 4000 kg (∴ 1 litre of water has a mass of 1kg.)

Weight of 4000 kg of water (F)= 40,000 N (4000 kg × 10m/s2 = 40,000 N)

Height (h) = 10 m

Time (t) = 1 hour = 3600 sec

Work done (w) = ?

Blooming Science Book 9 69

Power (P) = ?

(i) To find work done

Work done (w) = Force (F) × Height (h)

= 40,000 N × 10 m

= 400,000 Nm

= 400,000 J

(ii) Power developed = Work done
Time taken

= 400,000
3600

= 111.11 W

∴ The power of the pumpset is 111.11 W.

2. Sajana having 50 kg mass runs upstairs and reaches 3 metre high from the first floor
in 8 seconds. Calculate the work done against the gravity and the power developed
by her. (Assume g = 10m/s2)

Solution:

Mass (m) = 50 kg

Height (h) = 3m

Time (t) = 8s

Acceleration due to gravity (g) = -10m/s2

(i) Work done (W) = ?

(ii) Power (P) = ? (i)

To find work done,

Work done (W) by Sajana = F × h

= mg × h (∴ F = mg)

= 50 × (-10) × 3 = 15,00 J

(ii) To Find Power,

Power (P) developed by Sajana = W
t

= 1500
10

= 150 W

∴ Work done against the gravity is 1500 J and power developed by Sajana is 150 W.

70 Blooming Science Book 9

Let’s Learn

1. A man standing with 100kg load does not perform work. It is because the product of
froce and distance covered is work. A man standing with heavy load has d = 0 so he does
no work.

2. Kinetic energy increases if mass increases because KE is directly proportional to mass of
a body as KE = 21mv2.

3. A body with higher speed possess more KE, it is because KE is also directly proportional
to the speed of a moving body. Higher the speed, move will be KE or vice versa.

4. If 60w is written in an electric bulb it consumes or converts 60 joules of electrical energy
into heat and light energy in one second.

Main Points to Remember

1. If the body does not cover any distance, no work is said to be done.
2. Work is said to be done by a force when the force applied on a body moves it in the

direction of force.
3. Work done by force is the product of the magnitude of the force applied and the distance

moved in the direction of force.
Work done (W) = Force (F) x Displacement (d)
4. Work is done against gravity when a body is lifted up.
5. Work is done against friction when a body is pushed or pulled on the surface.
6. The amount of work done is the amount of energy transferred.
7. 1 Joule is the work done when a force of 1 Newton moves the body by 1 metre.
8. Potential energy and kinetic energy are two forms of mechanical energy.
9. Different forms of energy are chemical, mechanical, electrical, heat, sound, magnetic,

nuclear etc.
10. Energy is the capacity to do work. It is measured in joule.
11. Energy possessed by a body by virtue of its motion is called kinetic energy.
12. Energy possessed by a body is virtue of its position or change in its form is called potential

energy.
13. Gravitational potential energy of a body of mass m placed at a height h from the surface

is mgh.
14. The conversion of energy from one form to another is called energy transformation.
15. Energy can neither be created nor can be destroyed but it can be transformed from one

form to another and the total energy remains conserved. It is called conservation of energy.

PRO J ECTWORK

Find out some devices used at your home which transform one form of energy into another.
Make a table with their names and energy transformation.

Blooming Science Book 9 71

Exercise

A. Choose the correct answer from the given alternatives.

1. F.d gives the magnitude of

a. energy b. work c. power d. none of above

2. Joule is SI unit of

a. power b. energy c. work d. both b and c

3. Kinetic energy is calculated by

a. 1 ma2 b. mgh c. 1 mv2 d. mdg
22

4. If mass of a body remains constant but it’s velocity is increased by 3 times by how many
times it’s kinetic energy increases?

a. 3 times b. 9 times c. 6 times d. 12times

5. Lifting the load is example of:

a. work against gravity b. work against friction c. both a and b d. none of above

B. Answer the following questions.

1. Define work. In which unit is it expressed?

2. Explain types of work done with examples.?

3. What do you mean by one Joule of work? Write the formula relating the work, force and
displacement?

4. Define energy. Write two examples in which chemical energy is converted into heat
energy.

5. Define potential energy. What kind of energy does a flying kite possess?

6. State the law of conservation of energy and explain it with an example.

7. What is power? Define horse power.

8. In an electricity bulb 100 w is written. What does it means?

9. What is transformation of energy? Explain with two examples.

10. Distinguish between:

a. Work and power b. Energy and power

c. Work and energy d. Potential energy and kinetic energy

11. Describe the energy change involved in the following examples;

a. Lighting a torch light

b. Lighting match stick

c. Producing nuclear energy

d. A waterfall is used to turn a paddle wheel. The paddle wheel works a dynamo
which in turn lights a bulb.

72 Blooming Science Book 9

12. Name the device in each case for the following energy transformation.

a. Chemical into light and heat b. Mechanical into electrical

c. Chemical into electrical d. Kinetic into electrical

e. Electrical into light f. Light into electrical

g. Kinetic into potential h. Electrical into mechanical

13. State which possesses potential energy;

a. A stone placed at the top of a slide b. Stretched rubber

c. Red hot iron d. Stretched bow

e. A toy car is wound up f. Stone lying on the road

14. Name the energy possessed by the following;

a. Sun light b. Bullet released from the gun

c. Dry cell d. Water stored in dam

e. Explosive f. Stretched spring

Numerical Problems

1. A bullet of mass 50 g is moving at a speed of 200 km/hr. What is its kinetic energy? (If a

bullet of mass 50 grams is flying at the velocity on 200km per hour, how much energy is

stored ?) [Ans : 77.16J]

2. Calculate the work done by a man in carrying a mass of 16 kg over his head when he
covers a distance of 25 m in the (i) horizontal direction (ii) vertical direction.

(Take g = 10 m/s2 ) [Ans : (i) 0, (ii) 4000 J]

3. How much energy is needed to lift a body of mass 20 kg to a height of 10m?
[Ans : 2000J]

4. What will be the power of a girl who can perform 1000 J of work in 5 seconds ? [Ans 200 W ]

5. A crane can lift up a load of 600 N from the ground 5 m above in 10 seconds. Calculate its
working capacity in (a ) Watt (b) Kilowatt and (c) Horse power.

[Ans: (a) 300 W (b) 0.3 KW (c) 0.40 HP]

6. A machine was used to lift a heavy rock of weight 720 N while constructing a road in a
hill. If the rock was lifted to a height of 20 m in 24 seconds, (i) how much work was done
by the machine while lifting the rock? (ii) What is the power of the machine ?
[Ans : (i) W = 14400J, (ii) P = 600 W]

7. A football of mass 500gm falls from a height 5m and rebounds from the floor to a height

of 3m find (a) The K.E. at the moment of striking the ground (b) The K. E. at the moment
{Hint,21 mv2 = 21m (2gh)} [25J, 15J]
of leaving the ground.

Blooming Science Book 9 73

8. The weight of a body is 10 kg. Calculate the amount of work done by the body in climbing

8.5 meters height of a building (g = 10 ms-2). [850J]

9. Water is falling from a falls of height 20 m onto the blades of a turbine at the rate of 500

kg per minute. Calculate the power given to the turbine. [1666.60 W]

10. A stone of mass 3 kg is thrown with a velocity of 50 ms-1 which stops at a distance of 20

m. Calculate the work done by it when it suddenly stops. [3750J]

11. A man weighing 100 kg has power 500 W. Calculate the time taken to climb up the stairs

of 10 m. [19.6s]

12. An electric bulb consumes 3600 Joules of energy in two minutes. Calculate its power.
[30W]

13. How many times kinetic energy of a body changes when its velocity increased 2 times

without change in its mass? [KE increases 4 times]

14. How many times kinetic energy of a body changes when its mass is increased by 3 times

without changing its velocity? [3 times]

74 Blooming Science Book 9

Chapter LIGHT

5

Learning Outcomes Estimated Periods: 3+1

On the completion of this unit, the students will be able to:

• define refraction of light and demonstrate the refraction of light.

• define electromagnetic spectrum and describe the experiment to show light waves
having different frequency.

The branch of physics that deals with the study of light and its phenomenon is called optics. Light is a
form of energy which gives sensation of vision. All the luminous bodies like the sun, burning candle,
electric bulbs, etc. are the sources of light. The sun is the main source of light for us. A ray is a small
path along which light travels. It is denoted by a sign, →. A group of rays is called a beam.

Ray Parallel beam Divergent beam Convergent beam

Refraction of Light

Light travels in a straight line in a medium. When a beam of light passes from one medium to
other medium, it bends. The bending of light, when it passes from one medium to other medium
is called refraction of light.

In the figure, a ray of light AO in air is incident on the Air A Normal
surface XY of a glass slab. It makes an angle AOM with M

the normal MN drawn at the point O. The ray bends Incident ray i
along OB in the glass slab making an angle NOB with X
O Y
r Denser Medium
the normal MN. Here AO is incident ray and OB is

refracted ray. ∠AOM = i and ∠NOB = r are called angle N C
of incidence and angle of refraction respectively. Angle Glass slab B Refracted ray
of incidence (i) is the angle between incident ray (AO)
and normal (MN) and angle of refraction (r) is the angle

between retracted ray (OB) and normal (MN). The point O is called point of incidence.

Rarer and Denser Media

The medium through which the light passes is called an optical medium. The optical medium
which is less dense is called rarer medium and the optical medium which is more dense is called
a denser medium. For example: in air and water, the air is a rarer medium and water is a denser
medium. Thus, the rarer and denser mediums are relative terms.

Blooming Science Book 9 75

Laws of Refraction of Light

1. The incident ray, the normal and the refracted ray at the point of incidence all lie in the
same plane.

2. When a ray of light passes through a rarer medium to a denser medium, the refracted ray
bends towards the normal and when the ray of light passes through a denser medium to a
rarer medium, the refracted ray bends far away from the normal.

3. Snell’s law:

It states that the ratio of sine of angle of incidence to the sine of angle of refraction of two
mediums is always constant. This constant is known as the refracted index (µ).

i.e. sin i = µ
sin r

4. An incident ray passing through the normal always goes straight.

Cause of Refraction of Light

When light passes from one medium to another, it refracts. It is because of variation in speed of
light in different mediums. The speed of light is maximum in vacuum (or air), when light passes
from air to a denser medium, its speed decreases and vice-versa. Due to this change in the speed
of light in different mediums, refraction takes place.

Activity

To study the effect of refraction of light

Take a beaker and place a coin at its bottom. Move back from the vessel till the coin just
disappears from sight.

What should be done to see the coin from the same position without moving your head or the
coin or the beaker?

Ask your friend to pour water into the vessel without disturbing the position of the coin. The
coin becomes visible as the water level rises. In the above activity, the walls of the vessel
block the light from the coin when there is no water. On pouring water, the rays of light
coming from the coin are refracted away from the normal as they pass from water to air and
reached the eyes. Hence, the coin appears to be slightly raised from its real position.

Beaker Beaker
Water
Coin
Coin

When light rays strike the boundary of separation of two media, then there is sudden change
the physical properties of the media.

76 Blooming Science Book 9

incident ray reflected ray

refracted ray

The angle through which the path of a light ray deviates from the original direction and the

percentage of light that is absorbed, reflected or refracted depend both on the nature of the two

media involved and the direction of the incident ray. Scan for practical experiment

Refractive Index

(i) According to snell’s law,

µ (refracted index) = sin i
sin r
visit: csp.codes/c09e07

(ii) The refractive index of a medium is defined as the ratio of speed of light in air (or vacuum)
to the speed of light in material medium.

RI (µ) = speed of light in air (c)
speed of light in medium (v)

Refractive indices of different substances with respect to air (i.e. when light is passing from air
to the medium) are given in the following table.

Medium (substance) Refractive Index
Ice 1.31
Water 1.33
Glass 1.5
Alcohol 1.30
Diamond 2.42
Paraffin 1.44
Glycerine 1.47
Turpentine 1.47

Blooming Science Book 9 77

Solved Numerical Problem
1. From the given figure, find out the refractive index of the glass.

Here, angle of incidence (i) = 80o 80o Air
O

angle of refraction (r) = 41o

refractive index (µ) =? 41o

We know, m = sin i = sin 80o = 0.984 = 1.50 Glass
sin r sing 41o 0.656

∴ Refractive index = 1.50.

Consequences of refraction

Stick appears bent when partially dipped in water?

Water acts as a denser medium while the air acts as rarer Stick Air (r) Eye
medium. When a straight stick is partially dipped in water, rays

from straight stick under the water travel in the straight line in

water and they get refracted while coming out of the water in the A1
Water (d)
air. The refracted rays enter the observer’s eyes and it looks bent.
A

Real and Apparent Depth R Air Eye
O r B
Any object in a denser medium when viewed
from a rarer medium appears to be in lesser r A
depth than its real depth. Such lesser depth Apparent depthI
is known as apparent depth. This is due to Real depthWater
refraction.
i
Let M be a point object lying at the bottom of M
a pond. A ray of light MO incident along the
normal to the surface of liquid. OM = Real
depth.

Another ray MA bends away from the normal and goes along AB. The two refracted rays AB and OR
appears to come from a point I, which is called virtual image of M.

So, OI is the apparent depth which is shorter than the real depth. Mathematically, it can be proved that

Refractive index (µ) = real depth
apparent depth

78 Blooming Science Book 9

Twinkling of Stars: star (apparent Star (Real position)

The earth’s atmosphere consists of a number position)

of layers of the air of varying densities. The

upper layer is rarer and the lower layer is atmosphere Layers of the air

denser. Therefore, the rays coming from the

stars bend towards the normal and hence the Observer
stars appear slightly raised in position in the

sky. Earth
The layer of the air in atmosphere do not

remain stationary. They constantly intermingle with each other causing the fluctuation in density

of different layers of the atmosphere. This causes the shifting of apparent position of star at

random. The light coming from the stars therefore presents quivering appearance giving the

impression as if the stars are twinkling.

Apparent Position of the Sun:

B

Apparent position

A Earth
Sun

The sun even when below the horizon can sometimes be seen. The upper layers of atmosphere
are rarer and lower layers are denser.

So when a ray of light from the sun A enters into the atmosphere, it gets refracted at each layer. The
ray bends towards the normal and the sun appears slightly higher above the horizon than it really is.

Critical Angle and Total Internal Reflection

Consider a ray of light passing from a denser medium to a rarer medium, i.e. glass to air.

In the figure (i) a ray is refracted away from the normal and r > i. If the angle of incidence is
increased, the angle of refraction also increases as in the figure (ii), if it is continued such that
the angle of refraction is 90o, now the situation is critical for refracted ray because for a small
increase in angle of incidence, the refracted ray will bend towards the denser medium (glass) and
if angle of incidence is slightly decreased it will be refracted to the air medium.

Thus, the angle of incidence in a denser medium is called a critical angle when the angle of
refraction at a rarer medium is 90o. Fig (iii) ∠AOP = critical angle (C)

N B N NN
r Air
Or B r=90o O Air
O i Air O Air B B
A
i P i=c A i>c
AP A

PP

Glass slab Glass slab Glass slab Glass slab

(i) (ii) (iii) (iv)

Blooming Science Book 9 79

If the angle of incidence is greater than the critical angle, all the rays of light reflected back to the
same medium. This reflection is called total internal reflection. Light pipe and binocular work
on the basis of total internal reflection. Sparkling of diamond is also because of total internal
reflection. The figure (iv) shows the total internal reflection.

Conditions for occurring total internal reflection

(i) Light should pass through a denser medium to a rarer medium.

(ii) The angle of incidence should be greater than critical angle.

The reciprocal of sine of critical angle in the medium angle also gives the refractive index.

i.e. µ = 1 c where c is critical angle.
sin

Practical uses of total internal reflection
1. Optical fibres work on the basis of TIR and are used in telecommunication.
2. Light pipes are used to view the internal organs of the human body, without surgery.
3. Used to carry computer, telephone and television signals in the form of pulses.

The critical angle of some substances are given below.

Substances Critical angle (c)
Ice 50o
Water 49o
Alcohol 48o
Paraffin 44o
Turpentine 43o
Glycerine 43o
Glass 42o
Diamond 24o

Dispersion of Light

You have seen that, during raining season, just after rain, if there is sunshine then we can see
rainbow. Do you know why?

The white light of sun is actually Some time we see rainbow in the sky just ?Do
a combination of seven colors
of light. When this white light You
incident in drops of rain water, the after the rain. It is due to dispersion of Know
light is dispered into seven colours
and rainbow is formed. This light in the drops of water suspended in the
phenomenon of splitting of white
light into its seven constituent atmosphere. The rainbow is formed always

in opposite direction of sun, ie, if sun is at

east, rainbow is seen at west.

colours is called as dispersion of light. The seven colours are in the pattern of VIBGYOR ie

violet, Indigo, blue, green, yellow, orange and red.

80 Blooming Science Book 9

a) Dispersion of light in prism

A prism is a transparent refracting medium C D
bounded by two plane surfaces inclined at Angle of
prism
some angle.

In the figure, A prism with traingular base E
Refracting face
is shown. The faces BCDF and ACED A

are refracting faces where the ray of light Base B F
incidents. The angle between refacting faces

is called angle of prism. The <ACB is the

angle of prism. Usually, a prism is represented by a triangle base.

From the figure alongside, it is seen that when white ray of light ‘AO’ incidents the refacting
face, it splits into seven different colours. The seven colours are seen in screen as a spectrum of
white light. The spectrum is the band of seven colours produced by dispersion of white light.

Cause of dispersion of light:

We know that the speed of white light in air(or vacuum) is C

3x108m/s. It is the maximum speed of light. The velocity of

light decreases when it travels in other medium. A white ray of

light when refracts inside the prism, its velocity decreases. The

seven different colours of light have seven different velocities O R
inside the prism, this causes the colour of light to split inside A O
Y
the prism. We have seen that ‘Red’ colours is seen at top of G
B
spectrum, it is because red colour has highest velocity (longest
I
wavelength 7x10-7m). The violet colours is seen at bottom of V

spectrum since violet has least velocity (shortest wavelength B D

4x10-7m) and deviates more. The speed of othes colours are in

intermediate between ‘Red and Violet.’

Activity

Take a prism and focus a white light in it. Thermometer
We will get spectrum of white light. Now put
the thermometer just above the red colour R
for sometime. What change will you seen in O
reading of thermometer? Y
G
You will see temperature reading in themometer B
rises. Do you know why? I
V
Just above the red light, there is infrared wave
which is not visible but gives heat or warmth
to us. We can feel the warmth of infrared but
it can’t be seen. Due to presence of infrared
above red light, the reading in the thermometer
rises.

Similarly, below the violet ray there is ultravoilet wave which is also invisible to us.

Blooming Science Book 9 81

b) Recombination of spectrum of colours: When the spectrum produced by a prism is made
to pass through another inverted prism, all the colours mix with each other to produce a white
light. This is called as recombination of spectrum of colours.

white
light

White
light

up right Inverted
prism prism

Electromagnetic Spectrum

Electromagnetic waves are those waves which do not need a material medium for their propagation
and can travel even though vacuum. Light waves and radio waves are electromagnetic waves
because they don’t require material medium for their propagation.

The gamma rays, X-rays, ultraviolet rays, infrared radiations, microwaves and radiowaves are
all electromagnetic waves.

The electromagnetic waves are transverse wave in nature. We have known that wavelength or
frequency is associated into every kind of waves.

The classification of electromagnetic waves according to frequency or wavelength is known as
the “electromagnetic spectrum”.

The electromagnetic wave of the shortest wavelength is g-ray and the longest wavelength is radio
waves.

Our eyes are sensitive to electromagnetic spectrum only for the wave of wavelength between 4
× 10-7 meters and 7.5 × 10-7 m. The electromagnetic waves in this region are called visible light.
So light is an electromagnetic wave whose wavelengths lies on visible region.

Electromagnetic spectrum

γ-ray X-ray Ultra-violet Visible rays Infrared Micro-wave Radio-wave
10-5
λ=10-13m λ=10-11 λ=10-9 λ=10-7 10-3 10-1 103m

82 Blooming Science Book 9

Nature of Electromagnetic Wave

The part of the electromagnetic spectrum which is visible to human eyes, is called visible spectrum.
We have seen that the visible light is only a small part of the whole electromagnetic spectrum.
The waves of longer wave length than the red colour of the visible spectrums are infrared rays,
microwaves and radio waves. The waves of wavelength shorter than the violet colour of the
visible spectrum are ultraviolet rays, X-rays and g-rays.

If electromagnetic radiation is not transmitted through a medium, it may be reflected or absorbed.
Again, different types of radiations are reflected or absorbed by different materials; metals reflect
most radiations but not X-rays and gamma rays. These rays can pass through thin metal sheets
but are absorbed by thick ones.

The various types of radiant energy have different wave-lengths and frequencies. These two
characteristic multiplied are equal to the velocity. As the velocity is constant for any particular
medium, it follows that the higher the frequency of a wave, the shorter the wave-length, and vice-
versa. High frequency waves, have high energy. The velocity (v), frequency (f) and wave-length
(λ) of electromagnetic waves are formulated by the formula. v = f × λ

This is called wave equation, where v is the velocity (in m/s), µ is the frequency (in Hz) and λ,
the wave-length (in m). The electromagnetic frequencies extend from 103 Hz for radio waves
to above 1019 Hz for gamma rays. Wave lengths range from more than 10-3 m for radio waves
to less than 10-13m for the gamma rays. Table shows the frequencies, and wave-lengths of the
electromagnetic spectrum.

Electromagnetic Spectrum

Electromagnetic Spectrum Wave length (A) in meters Frequency (µ) in hertz
Gamma rays 10-13 - 10-11 (short wave-length) 1019 and above
X-rays 1017 - 1019
Ultraviolet radiation 10-11 - 10-9 1015 - 1017
Visible light 5 × 10-9 - 5 × 10-7 1014 - 1015
Infrared radiation 4 × 10-7 - 8 × 10-7 1011 - 1014
Microwaves 8 × 10-7 - 8 × 10-5 109 - 1011
Radio-wave 103 - 109
10-5 10-3
10-3 - 103

The properties of Electromagnetic Waves are as given below:
1. They are transverse in nature.
2. They can travel through a vacuum and in a medium.
3. The speed of electromagnetic waves is 3 × 108m/s in vacuum.
4. They show the property of reflection, refraction, interference, diffraction and polarization.
5. They can cast a shadow if there is any obstacle.
6. All the electromagnetic waves follow v = f × λ

Electromagnetic waves whose wave length are just shorter or just longer than those of visible
light are called ultraviolet light (UV) and infrared light (IR), respectively.

Those waves with much longer wave length that are used for radio and Television transmission
are known as radio waves.

Blooming Science Book 9 83

X-rays are another form of electromagnetic wave mostly used in medical purpose.

Whatever may be the wave length or frequency of electromagnetic wave, they travel through
space at the same speed and have the same essential nature.

Uses of X-rays Uses of UV-rays

To detect the broken bones in our body. For test of purity of ghee, egge, ornaments,
etc.

It is used for radiotherapy For sterilizing medical instruments.

It is used in airports, custom office and in It is absorb by body to produce vitamin D.
security system.

Let’s Learn

1. Light wave is called electromagnetic wave because it can travel even in vacuum. The
wave which does not need material medium to travel is called an electromagnetic wave.

2. They ray of light bends when it passes from one medium to the other. It is because the
velocity of light is different in different media. Light rays bend due to change in velocity
when travel from one medium to another.

3. A rectangular glass slab does not disperse white light. Hence, all colours of white light
emerge in the same direction i.e. parallel to the incident ray consequently, there is no dispersion.

4. An air bubble in a jar of water shines brightly. This is because light entering from water to
air bubble undergoes total internal reflection. For the observer, this light appears to come
from the bubble. So, it shines.

5. In a metallic ball painted black with lampblack shines in water, air molecules are enclosed
in the space of lamp black, when the ball is immersed in water, light undergoes total
internal reflection at the film of air. So, the metallic ball painted black with lamp black
appears shining when immersed in water.

6. The radio waves are not harmful because they have very low frequency and have less
penetrating power.

Main Points to Remember

1. The phenomenon of bending of light which occurs when it passes from one transparent
medium to another is called refraction of light.

2. The laws of refraction of light are:

(i) The incident ray, normal and the refracted ray lie on the same plane.

(ii) The refracted ray bends away from the normal when the ray passes from denser
medium to rarer medium. Similarly, the ray bends towards the normal when it
passes from rarer medium to denser medium.

(iii) For a given pair of media, the ratio of the sine of the angle of incidence (sin i) to the
sine of the angle of refraction (sin r) is constant. This is also called Snell’s law. The
constant is called refractive index (µ).

84 Blooming Science Book 9

3. Refractive Index (µ) = Sin i = Speed of light in vacuum
Sin r Speed of light in medium

= Real depth of liquid
Apparent depth of liquid

4. The critical angle (C) for a pair of media is the angle of incidence in the denser medium
for which the angle of refraction in the rarer medium is 90o.

5. The light reflects totally into denser medium when a light travelling from a denser
medium to rarer medium makes an angle of incidence greater than its critical angle. This
phenomenon is called total internal reflection of light.

6. Red light has maximum velocity and violet light has minimum velocity.

7. Light travels in the form of transverse wave.

8. The band of all electromagnetic waves according to increasing wave lengths are called
electromagnetic spectrum.

9. UV, X-ray and gamma rays have shorter wave length and high frequency than that of
visible light.

10. Infrared, radio waves and micro waves have longer wavelength and lower frequency than
that of visible light.

11. The electromagnetic waves of shorter wave lengths are dangerous.

12. An object becomes hot when absorbs infra red ray.

PRO J ECTWORK

Take a circular cardboard and paint it into seven different colours. If this
cardboard is spin, the seven colours will mix together to form a white
light, such a disk was made by Isaac Newton to prove white light is made
up of seven different colours. So, it is called as Newton’s Disk. (Seven
different colours are violet, indigo, blue, green, yellow, orange and red-
VIBGYOR)

Newton's Disk

Exercise

A. Choose the correct answer from the given alternatives.

1. The speed of light in a vacuum is

a. 3x108 kms b. 2.2x108ms c. 3x108 ms d. 3x108cms

2. Which of the following has maximum refractive index?

a. glycerine b. diamond c. paraffin d. water

Blooming Science Book 9 85

3. Which of the following is critical angle of paraffin ? Glass block
a. 50° b. 44° c. 46° d.47°
4. Which of the following is refractive index of glass.
a. 1.36 b. 1.47 c. 1.76 d. 1.5
5. Ray of which colours is found at top of a rainbow?
a. violet b. blue c. red d. yellow

B. Answer the following questions:
1. What is optics? Define ray and beam of light.
2. What is refraction of light? What are the laws of refraction?
3. Figure shows a ray of light entering a rectangular block of glass.

(ii) Copy the diagram and draw the normal at the point
of incidence.

(iii) Sketch the approximate path of the ray through the
block and out on the other side.

4. Complete the following diagrams;
Air

Water Water 42o
(i) (ii)
Glass

(iii)

5. In the figure given, coin in the bowl cannot be seen. How can you
see the same coin in the same place of bowl from the same place
of eye? Justify your answer with diagram.

6. Explain with reason;

(i) Why does the bottom of a pond look less deep than its real Coin
depth?

(ii) An inclined glass rod placed in water appears bent, why?

iii) A ray of light bends as it passes from one medium to another, why?

(iv) A straight stick appears bent when partially immersed with water, why?

(v) Stars appear twinkling, why?

8. Study the diagram and answer the questions given below: N
(i) What term is used to denote the ∠POM?

(ii) What effect is caused to the ray OQ when ∠POM is A O Q Air
increased? B
Water
(iii) Write the name of the phenomenon that takes place when
∠POM is increased? 49o
P

M

86 Blooming Science Book 9

(iv) What two conditions are necessary for the above phenomenon to occur?

(v) Write one use of this phenomenon.

9. What do you understand by the total internal reflection? State the conditions required for

it to occur?

10. Draw a diagram to show that the apparent depth of water in a tank is less than its real depth.

11. What is a critical angle? Draw a diagram to show what happens to a ray of light travelling

in water when it strikes the surface of the water at the critical angle.

12. Define the refractive index of a medium. What do you mean by the statement that the

critical angle of glass is 42o?

13. Write two common properties of the electromagnetic spectrum.

14. Why does violet light appear at the bottom of the spectrum?

15. What is UV? What are the advantages and disadvantages of UV light?

16. What are X-rays? Write two uses of it.

17. How will you show the presence of infrared rays in the electromagnetic spectrum?

18. Distinguish between mechanical wave and electromagnetic waves.

19. Show the differences between ultraviolet rays and infra-red rays.

20. Study the given figure and answer the questions.

a) Name the phenomena shown.

b) Why is red colour at top and violet at bottom?

c) Does this phenomena occur in glass slab? If not, why?

d) What happens if another prism(inverted) is placed in R
the path of spectrum? O
Y
e) Write the colours of pattern VIBGYOR. G
B
I

V

Numerical problems

20. If the refractive index of diamond is 2.42. Find its critical angle.[Ans: 24o]
21. The criticle angle of glass is 42o. Find the speed of light in this medium. [Ans: 2 ×108m/s]
22. Calculate the frequency of x-ray whose speed in air is 3 × 108m/s and its wave length is

10-8 m.
[Ans: 3 × 1016Hz]
23. Refractive index of water is 4/3. Calculate the velocity of light in water. [Ans: 2.2 ×108m/s]
24. The apparent depth of water in a pond is 2m. The refractive index of water is 4/3. Calculate

the real depth of water in the pond. [Ans: 2.6m]
25. A ray of light is incident at an angle of 45o. If refractive index of medium is 1.5.

Calculate angles of refraction. [Ans: Sin-1(0.47).

Blooming Science Book 9 87

Chapter SOUND

6

Learning Outcomes Estimated Periods: 8+2

On the completion of this unit, the students will be able to:

• explain the nature of sound wave.

• impart knowledge on infrasonic, audible and ultrasound waves and their sources

• explain intensity and pitch of sound.

• illustrate the refraction of sound and find speed of sound.

• describe reflection and refraction of sound with examples and mention their effects in
daily life.

Introduction

Sound is a form of energy which is produced by the
disturbances in any media. Sound transmits in the form of
wave. Sound waves carry energy called sound energy. A
source is necessary for the production of sound and medium
is essential to propagate it. The flute, the violin, the madal,
the guitar, the drum, the bell, the vocal cord of animal,
the radio, the television etc are some examples of sources
of sound. Waves are produced when material substances
vibrates. Sound also exhibits the properties of reflection and refraction like light. The bodies
which vibrate and produce sensation of hearing to ears are the sources of sound.

Nature of Sound Waves

Sound waves propagate through solids, liquids and gases. The propagation of sound waves in air
medium is described as follow:

When loudspeaker produces sound, its cone vibrates. The
vibration of its cone disturbs the air molecules close to it. This
in turn disturbs the neighbouring molecules. In this way the
vibrating cone of the loudspeaker alternately compresses air
molecules surrounding it. The backward and forward movement
of the molecules in air medium causes series of compressions
and rarefaction, which constitute the sound waves. The sound
wave is propagated in the form of kinetic energy in the air
medium. Thus sound waves are also called mechanical waves.
Sound waves cannot propagate through vacuum. But light waves being an electromagnetic wave
can travel in vacuum.

88 Blooming Science Book 9

On the basis of medium required, waves are mechanical and electromagnetic. The following are
the difference between them:

Mechanical waves
1. It requires material medium to travel
2. It’s speed is low.

3. It is transverse and longitudinal

Electromagnetic waves
1. It can travel in vacuum.
2. It has high speed.

3. It is transverse only

The wave, are of two types on the basis of nature of vibration of particles. They are transverse
wave and longitudinal wave.

Transverse Wave

If the direction of the vibration of molecules and the direction of propagation of waves are at

right angle then it is called transverse wave. direction of direction of
particle vibration propagation
For example waves in string, waves in surface of water,
light waves etc

Longitudinal Wave

If the direction of the propagation of wave and the Fig: showing vibration of particles
vibration of the particles of medium is same, then the
wave is called longitudinal wave. For example sound
wave in air.

Activity

To demonstrate a longitudinal wave
1. Take a slinky spring. It should be a long spiral spring.
2. Hold it horizontally as in figure and gently stretch it.
3. Jerk the slinky spring and generate the pulse or hump in it.
4. Observe the moving hump.

CRC R C

5. The humps travel along the spring and also return after reflection at the other end. It
looks like a wave. The slinky spring itself does not move. The hump carries energy in
the spring, disturbing different neighbouring parts of the slinky spring. In the process
transfer of energy takes place.

Blooming Science Book 9 89

Comparison of Transverse and Longitudinal Waves

Transverse Waves Longitudinal Waves

1. Vibrations of particles of a medium 1. Vibrations of particles of a medium are
are perpendicular to the direction of along the direction of propagation of
propagation of wave. wave.

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

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

4. Speed of this wave is more. 4. Speed of this wave is less.

5. It can travel in vacuum. 5. It cannot travel in vacuum.

Some terms related to waves

Crest and Trough Crest
Both transverse and longitudinal waves are represented
graphically by a ‘sine curve’. Transverse waves travel Mean position
in the form of trough and crest. When the displacement Trough
of a particle is maximum above from its mean position,
the particle is said to be at the crest of the wave. When
the displacement of a particle is maximum below, the
particle is said to be at trough.

Compression and Rarefaction

During the propagation of longitudinal wave, the particles of the R CRC R
medium come closer at a certain region called compression, whereas
at a certain region, the particles of the medium go farther away. This Fig: Longitudinal waves
is called rarefaction.

Wave Length

The distance between one compression to another compression of wave propagation or the

distance between two consecutive crest or trough is called wave length. It is denoted by lambda

(λ) and its unit is metre. Crest Crest
l

For Transverse Wave

The distance between the two consecutive crest or two l
consecutive through is called the wavelength.
Trough Trough

For Longitudinal Wave

The distance between the centre of two consecutive rarefaction or two consecutive compressions
is called the wavelength of the longitudinal wave.

l

RC RC
l

90 Blooming Science Book 9

Amplitude (a)

In transverse wave, the vibration of molecules is perpendicular to the wave propagation. The
height of the crest or the depth of the trough is amplitude. More energy is needed to propagate
large amplitude waves. So, small stone produces small and large stone produces large amplitude.

Crest Crest

a Displacement a a B
AO AO
a Trough B a

Trough

Frequency(F): The numbers of complete vibrations made in one second or number of complete
waves produced in one second is called as frequency. It s SI unit is hertz (Hz). It is denoted by ‘f ‘

Time period(T): The total time taken to from a complete wave is called time period. It’s SI unit
is second. It is caculated by T= 1/f.

Speed

The distance covered by a wave per unit time is called wave speed.

Velocity = Frequency × Wave length

v=f×λ

Proof of v = f × l [Wave Equation]

From the definition of time period, we have the distance travelled in time period
T = wavelength

From the definition of time period, we have the distance travelled in 1 second = l
T

But the distance travelled in one second is called the speed (v) so

v= l
T
1
or, v = T × l

or, v=f×l ; because, f = 1
T

This gives the relation of speed, frequency and wavelength of a wave. This is also called wave

equation.

Speed of sound in different media (at 0 oC)

S.N. Medium Velocity (m/s) S.N. Medium Velocity (m/s)
In solids In liquids
6000 1400
1 Granite 5000-7000 7 Water 1210
2 Steel 8 Alcohol 6000
3 Aluminium 5100 9 Granite 1260
4 Glass 5000 10 Hydrogen 330
5 Wood 4000-5000 11 Air 258
6 Brick 3600 12 Carbon dioxide

Blooming Science Book 9 91

Factors affecting velocity of sound in air

The following factors affect the velocity of sound in air.

Pressure: At constant temperature the change in pressure has no effect on the velocity of sound
in the air.

Temperature: The velocity of sound varies directly as the square root of the absolute temperature
of the air.

∴ Velocity of sound ∝ temperature

Thus, higher the temperature, higher the velocity of sound. That’s why, velocity of sound is more
in hot air than in cold air.

Density: The velocity of sound in air is inversely proportional to the square root of the density
of the gas.

∴ Velocity of sound ∝ 1
density

It means, sound travels faster in air of lower density.

Humidity: The terms humidity refers to the water vapour (moisture) content of air in the
atmosphere. As the density of water vapour is less than that of dry air, density of moist air is
lesser than that of dry air. We know velocity of sound in gas varies inversely as the square root of
the density of the gas. Therefore, sound travels faster in moist air than in dry air.

Direction of wind: The velocity of sound is higher in the direction of wind and less in opposite
direction.

Spectrum of Sound Waves

The frequency of sound differs from its source. Frequency is inversely proportional to the

wave length f ∝ 1 . The wave of sound that a human ear can hear is called audible sound. It

λ

ranges between 20Hz to 20 KHz. The waves with frequencies below 20Hz are called infrasonic

or subsonic wave (waves resulted from earthquake and volcano) and more than 20,000 Hz

(20 KHz) are called ultrasonic waves. Bats, dolphin can produce and hear ultrasound. Dog and

cat cannot produce ultrasound but can hear it. Whale, rhino and elephant can hear and produce

infrasound. Adult usually above 25year can’t hear ?Do

Practical uses of Ultrasound You
1) To detect cracks in metal casting. sound of frequency 17,000Hz and Know

2) For cleaning the parts of machine. above, which is heard by teenagers.

3) For sterilization. Such a sound is used in many public

4) For emulsion. places like malls, departments stores

5) Ultrasound is useful for relieving. to drive the teenagers away.

neuralgic, rheumatic pains, pains due to arthritis, etc.

6) For the extraction of broken teeth.

7) For bloodless surgery.

8) The most revolutionary use of ultrasound is in the treatment of mental patients.

92 Blooming Science Book 9

To determine the Depth of Sea

Fathometer generates ultrasonic sound waves of frequency 50 KHz. Ultrasonic waves are sent
towards sea bed. When these waves reach the sea bed, they are reflected back to fathometer. Let
v is the velocity of sound in water and t is the total time taken by the ultrasonic wave to move
from fathometer to the sea bed and then from sea bed to fathometer.

So that ultrasound travels the depth of sea twice. Transmeter
Therefore, distance moved by the ultrasound = 2 d
Time taken = t Reciever

Now, from the definition of speed = distance moved t
time taken
2d d incident reflected
or, v= t sound sound wave

or, vt = 2d Sea bed

or, d= vt
2

The speed of sound (v) for a medium is known, time taken (t) can be measured directly. So depth
of sea can be found using above formula.

Solved Numerical Problems
1. If the frequency of the sound is 100Hz and its wavelength 3.3 m. Calculate the velocity

of the sound.

Solution:
Frequency (n) = 100Hz
Wave length (λ) = 3.3m
Velocity = ?
According to the wave euqaiton,
v = n × λ
= 100 × 3.3
= 330 m/s
Therefore, the velocity of sound is 330 m/s.

2. An ultrasonic wave is sent from a ship towards the bottom of the sea. If the time
interval between the sending and receiving of the wave is 6 seconds and the velocity
of sound in sea water is 1500 m/s. Calculate the depth of the sea.

Solution:

Time (t) = 6s

Velocity (v) = 1500 m/s

Distance (d) = ?

According to the formula,

Blooming Science Book 9 93

d = vt
2

= 1500 × 6
2

= 4500 m

∴ The sea is 4500 metres deep.

Characteristics of Sound

The sound can be characterized from each other in the following aspects (a) Pitch (b) Intensity
(c) Timbre (d) Tone and Note

Pitch

The shrillness or flatness of the sound is called the pitch. Shrill sound has high pitch and flat
sound has low pitch. It depends upon frequency. The voice of ladies and babies is usually of high
pitch than men. The sound of female has frequency 8.5 KHz and that of male is 6.5 KHz.

Pitch is that characteristic of sound by which we can distinguish sharp sound from flat sound.
Frequency and pitch are not same. In fact pitch is sensory (auditory) perception. Pitch is not a
physical quantity because it can’t be measured and expressed in unit. It helps us to differentiate a
male voice from a female voice, the buzzing of mosquito from roaring of lion etc.

Pitch is inversely proportional to thickness of the string of guitar

Stretched wire is said to be in tension. The tension on the string also affects the pitch of sound.
The string of guitar produces sharp sound when it is well stretched or is in high tension and
produces flat sound when it is under low tension.

Hence, pitch is directly proportional to tension on the string

Relation of Pitch with Wavelength

We know that pitch of sound is directly proportional to frequency. But wavelength is inversely
proportional to frequency. Hence, more is the wavelength less is frequency and hence low pitch
and less is the wavelength high is the frequency and hence high pitch. i.e. Pitch is inversely
proportional to wavelength.

Intensity of Sound

The rate at which sound waves carry energy from source is intensity of sound. The amplitude
of wave increases if it has higher energy. Increase in intensity of sound increases the loudness of
sound and decrease in intensity of sound decreases the loudness of sound. It is measured in (dB)
decibel unit. For every increase in 10 decibel, intensity of sound increases by ten times.

The intensity of sound is defined as the energy carried by sound waves, per unit area per unit time.

Let, E be the energy carried by the sound from area ‘A’ of the source in time ‘t’ Then.

Intensity (I) =AE× t

or, I = E × 1

t A

94 Blooming Science Book 9

or, I = P × 1 ; because, P = E = power

A t

or, I = P ........................(i)

A

From this expression the intensity of sound can be defined as the power carried by sound per unit
area. The SI unit of intensity is Joule per square meter per second or watt per square meter (J/m2s)
or (watt/m2). Here, we consider the area through which sound waves are propagated normally.

The sound propagates in all directions from the source of sound that forms a sphere at a point
away from the source.

Therefore to find intensity at a distance ‘r’ from the source of sound, we consider a sr

sphere of radius ‘r’ then surface area of the sphere is 4pr2. i.e. A = 4pr2.
P
We have I = 4pr2

\ I ∝ 1 ................................(ii)

r2
But the energy transmitted by the source is directly proportional to the square of amplitude ‘a’

of sound wave. Therefore, the intensity of the sound is directly proportional to the square of the

amplitude of the sound

i.e. I ∝ a2...................................(iii)

Intensity of some sound

Sound Loudness (dB)

Threshold of hearing 0
Moving leaves 10
Whisper 20
Conversation (2m) 50
Normal Conversation(1m) 60-65
Telephone bell 80
Motor cycle 90
TElhercetsrhicolddriollfsp(a12inm) 90
Jet aircraft taking off (25m) 130
140

Factors on which intensity or loudness depends

1. Amplitude of vibration: Intensity and hence loudness is directly proportional to the square
of the amplitude. A louder sound has a greater amplitude and a higher intensity than a
fainter sound. \ I ∝ a2

2. Distance from the source: Intensity at a point varies, inversely as the square if its distance
from the source.

3. Size of sounding body: Larger the size of a sounding body, greater is the intensity and
louder the sound produced. the temple produces louder sound than the home bell due to
same reason.

Blooming Science Book 9 95

4. Density of the medium: Intensity is directly proportional to the density of the medium.
That’s why when air in a bell jar is gradually exhausted, the sound produced by a ringing
bell placed in the bell jar becomes more and more faint.

5. Presence of other bodies: Reflector of sound amplifies the loudness, while absorbing
material reduces the loudness.

6. Wind: Loudness is greater in the direction of wind than in the opposite direction.

Differences between Intensity and Pitch

Intensity Pitch
1. It is related to the loudness of sound. 1. It is related to the sharpness of sound.
2. It depends on energy or amplitude of sound. 2. It depends on frequency.
3. Its unit is decibel. 3. It has no unit.
4. It is measurable quantity. 4. It is non-measurable quantity.

Quality or Timbre

This characteristic distinguishes notes of same pitch and loudness produced by different musical
instruments. As a matter of fact, a note is not single “sine curve” but it is combination of different sine
curves. This distinguishes two notes of the same pitch and loudness produced by two different sources.
Hence, a person can be recognized by the quality of voice. If a harmonium and a violin are played to the
same pitch and loudness, even an untrained ear can distinguish the two musical notes by their quality.

The quality or timbre of a sound is that characteristics by virtue of which we can distinguish two
sounds of same pitch and same loudness produced by two different musical instruments or by
two different person’s voice.

For example, two same musical sound notes produced on two different instruments, like piano
and harmonium can be easily distinguished, by their quality or timbre. By the quality of sound
we can recognize voice of a person even without seeing him.

Tone and Note

The musical sound produced by an instrument in general is highly complex. It differs in frequency,
amplitude, timbre, tone and note.

Tone: Sound of single frequency is known as tone.

Note: Musical sound made up of several frequencies is known as a note.

Persistence of Hearing

The sound persists in human ear for a time period of one tenth of a second i.e. for 0.1 second even
after sound stops. This ability of our ear is known as persistence of hearing.

Reflection of Sound

The returning back of sound wave in same medium when a sound
wave is incident in the reflecting surface is called reflection
of sound. It flowers the laws of reflection of light. Echo and
reverberation are the products of reflection of sound.

96 Blooming Science Book 9

Echo

Sound also follows the laws of reflection just like light. When sound is created and its reflected
wave returns after striking a wall or any object to the listener, we say an echo is heard. To hear
an echo, the distance between a person and the reflecting surface should be at least 17m. This
condition is easily met in large halls and open fields. Hence, when sounds are created or speech
is uttered, the formation of echoes created confusion. To avoid this, walls of the auditorium are
covered with sound absorbing materials so that echoes can be minimized. Repetition of same
sound once or more number of times due to reflection and reaches the listener after a time period
of one tenth second i.e. 0.1 sec is called an echo. If same or different sound notes are received
by human ear in less than 0.1 second then they are not audible as original sound. Instead they
are audible as noise or different sound. Therefore, the echo is heard only when the same sound
reaches the ear after 0.1 second.

Conditions required for Echo

i. The distance between the source of sound and reflecting body should be more then 17m.

ii. The loudness of sound should be more enough.

iii. The area of reflecting body should be large.

Reverberation

If the distance between the source of sound and reflecting body is less than 17m, original sound
and reflected sound mix and the sound is prolonged. This phenomenon is called reverberation. It
is heard in an empty room or newly built room. In a furnished room, furniture and other materials
absorb sound a lot, hence reverberation not heard. In musical concept, it is more beneficial as
it makes music melodious providing continuity. To reduce reverberation, the windows of a hall
should be opened. This is because the sound passes out through the window instead of being
reflected. Good sound absorbents also reduce the reverberation.

If there are numbers of reflecting surfaces close to one another, a series of reflection occurs.
Because of this, original sound and reflected sound mix to cause the sound prolonged. This is
called reverberation.

Differences between Echo and Reverberation

Echo Reverberation

1. An echo is the repetition of original sound heard 1. A reverberation is a long drawn impression

after the sound is reflected from a far surface. of sound which dies away gradually.

2. For echo to occur, the reflecting surface 2. For reverberation two near surfaces must be

must be at least 17m away from the source there (sources of sound and the reflecting

of sound. surface is too short i.e. less than 17m).

3. Sound is reflected only once. It occurs in 3. The sound is reflected several times by two

the hills, big halls and conference rooms. or more near surfaces, because of which we

hear the first sound lengthened. It occurs in

empty ordinary rooms.

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Refraction of Sound

Temperature is the main factor which makes air medium non-uniform, forming different layers.
The phenomenon of bending of sound waves in layers of air at different temperatures is called
refraction. Two causes of refraction of sound are formation of different layers of air medium due
to change in temperature and variation of speed of sound with temperature.

Why sounds are more audible at night than in a day?

At night, ground cools more rapidly than the air above. Surface layer becomes cooler than that at a
higher level, speed of sound is greater in warm air than in cold so sound travels faster at upper level
than lower level, so refraction occurs downwards to earth to this, sound is refracted towards earth
and in day it is refracted away from earth. Thus sound is more audible at night than in a day.

high At Night
At Day

Density Cold air Density warm air Sound
increases Sound increases wave
wave
less
warm air
cold air

Noise and Music

The sound received impression are divided into two categories. They are noise and musical
sound. A sound which produces an unpleasant and jarring effect on the ear is called noise and a
sound which produces a pleasing sensation on the ear is called musical sound. However there
is no sharp line of demarcation between the two categories because this distinction is quite
subjective. What may be ‘music’ for one may be ‘noise’ for another. Noise if reflected at regular
intervals may produce a pleasant sensation, for example pattering of rain drops on a tin shed, the
hum of a distant market place, etc.

Music (a) Noise (b)

Theoretically, musical sound is characterized by a regular, continuous vibration with no sudden
discontinuity in it. Further, it must be periodic. All musical sounds have regularity and rhythm
in them, while a noise does not have any of these characteristics. A noise is an abrupt, harsh,
discontinuous, not periodic, irregular sound. The difference may be apparent from fig (a) and (b).

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Differences between Musical Sound and Noise Sound

Musical sound Noise sound
1. It produces a pleasant effect on the ears. 1. It produces an unpleasant effect on the ears.
2. It has regularity and rhythm. 2. It has abrupt, harsh and discontinuous sound.
3. Changes in amplitude are not sudden. 3. Changes in amplitude may be sudden.

Noise Pollution

Pollution is presence of matter or energy in an unusual or unintended place. It adversely affects
the environment. Among different types of pollution, noise pollution is one of the major. Noise
can damage the ears and there will be temporary or permanent hearing loss depending upon the
intensity and duration of sound. A noise level of more than 80 dB for more than a few minutes can
cause reduction in the auditory sensitivity. In some factories and other work situations, workers
are constantly subjected to high noise levels. The workers there suffer chronic hearing loss. Some
major effects of noise pollution are given below:

1. It increases blood pressure and sugar level in blood.

2. It makes people tired, nausea etc.

3. People find it difficult to concentrate on work.

4. It may make people deaf and can cause ear problems.

5. People find it difficult to sleep soundly in noisy places.

6. Communication based on sound is disturbed in noisy places.

Let’s Learn

1. When a person speaks in a room, the energy produced is used to produce vibrations in air
particles in the room. As volume of air in the room is less than of air in open, the amplitude
of vibrations of air particles in the room is more than the amplitude of vibrations of air
particles in open. Hence, it is possible to hear more easily and clearly a person speaking
in room than in the open air.

2. Sound produced in a big hall repeats itself for sometimes. This is because of multiple
reflection of sound wave from walls of the hall.

3. For the propagation of sound, material medium is needed. Since there is no air on the
moon, two astronauts cannot talk on the surface of the moon as they do on the earth.

4. Velocity of sound in air is about 332 m/s but velocity of light is 3 x 108 m/s. Both the sound
and light waves travel same distance while reaching to a place on the ground, hence light
reaches the ground at first and then sound. It is why, the thunder of lightening is heard
some moments after the flash of light is seen.

5. Bats emit ultrasonic waves. These waves are reflected by obstacles in their path. The
reflected waves are received back by the bat. Such waves carry information about distance,
size, direction and nature of obstacles. Hence, bats can ascertain distance, size, direction
and nature of obstacles without seeing.

6. The intensity of sound is inversely proportional to the square of the distance from the source.
Therefore the intensity of sound decreases as the listener goes further from the source. So,
sound is feeble and feeble with increase in distance between the source and listener.

Blooming Science Book 9 99

7. Hard hitting of a bell makes the amplitude of the vibration larger. Consequently the
intensity of sounds increases because intensity of sound is directly proportional to the
square of the amplitude. Therefore, hard hitting of a bell produces a louder sound.

8. Density of moist air is less than that of dry air. Since, velocity of sound in a gas is inversely
proportional to the square root of the density of the gas, velocity of sound is more in moist
air than that in dry air. Therefore, sounds are heard louder on a wet day than on a dry day.

9. The repetition of sound caused by reflection from a rigid obstacles like cliff, hill or walls
of a building is called echo. For the echo to be heard clearly, the distance between the
source of the sound and the reflecting body should be at least 17m. In small room, this
distance is less than 17m. Hence, we cannot hear echo in a small room.

10. The reverberation time of a hall filled with people will be reduced much more because
clothes of audience absorb sound a lot. In an empty hall, the sound is reflected from walls
almost without any absorption. Hence, sound in a hall filled with people sounds more dull
than in the same empty hall.

11. The speed of sound wave is minimum in gas medium and maximum in solid medium. In
solids, molecules are very close to each other due to which energy of sound wave quickly
passes from one molecule to another molecule. Hence, speed of sound wave becomes more.
In gases, molecules are not close together and hence energy of sound wave takes more time
to pass from one molecule to another molecule. Hence, speed of sound is less in them.

Main Points to Remember

1. The path of energy is called wave.

2. The propagation of disturbance (wave), without involving transfer of any material within
it is called wave motion.

3. In transverse wave, the vibration of particles of a medium is perpendicular to the direction
of propagation of wave.

4. In longitudinal wave, the vibration of particles of a medium is along the direction of
propagation of waves.

5. Material medium is required for mechanical waves and material medium is not required
for electro-magnetic waves.

6. One compression and one rarefaction in longitudinal wave and one crest and one trough
in a transverse wave is called a complete wave.

7. The maximum displacement of a particle from its mean position in a wave is called its amplitude.

8. The number of complete waves, set up in a medium in one second is called its frequency.

9. The distance between two consecutive crest and trough or that in between two consecutive
compression and rarefaction in a wave is its wave length.

10. The velocity with which, wave propagates in a medium is called wave velocity.

11. Speed of sound in solids is the maximum and that in gases is the minimum.

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