Oasis Science and Technology 9

Meaning of 60% efficiency of a machine

A machine has 60% efficiency means that 60% of the input work is converted to useful output
work and the remaining 40% of the input work is wasted to overcome the friction and to move
the parts of the machine.

Differences between Practical machine and Ideal machine

S.N. Practical machine S.N. Ideal machine

1. The efficiency of a practical machine 1. An ideal machine has 100% efficiency.
is less than 100%.

2. In a practical machine, output work 2. In an ideal machine, output work is

is less than input work. equal to input work.

3. In a practical machine, MA is less 3. In an ideal machine, MA is equal to VR.
than VR.

Worked out Numerical 1

A load of 800 N is lifted by a machine applying 400 N effort. If the effort distance and load
distance are 90 cm and 30 cm respectively, calculate the mechanical advantage, velocity ratio
and efficiency of the machine.

Solution:

Given, Load (L) = 800 N

Effort (E) = 400 N

∴ Mechanical advantage (MA) = L = 800 N =2
E 400 N

Effort distance (E.d.) = 90 cm

Load distance (L.d.) = 30 cm

∴ Velocity ratio (VR) = E.d. = 90 cm =3
L.d. 30 cm

Now, Efficiency (ŋ) = MA × 100%
VR

= 2 × 100%
= 3

66.66%

∴ The MA, VR and ŋ of the machine are 2, 3 and 66.66% respectively.

3.8 Types of Simple Machines

1. Lever 2. Pulley 3. Wheel and axle
4. Inclined plane 5. Screw 6. Wedge

PHYSICS Oasis School Science - 9 43

1. Lever

A lever is a rigid bar which is free to rotate about a fixed point called fulcrum. The weight
to be lifted is called load and the force applied on the lever is called effort. The distance
between the fulcrum and the load is called load arm and that between the fulcrum
and the effort is called effort arm. Scissors, wheel-barrow, forceps, crowbar, etc. are some
examples of lever.

Effort

Load

Effort arm

Load arm Fulcrum
Fig. 3.2 Typical lever

Depending on the position of load, effort and fulcrum, levers are classified into three
types. They are:
a. First class lever (Fulcrum is situated in between the load and the effort)
b. Second class lever (Load is situated in between the fulcrum and the effort)
c. Third class lever (Effort is situated in between the load and the fulcrum)

Principle of Lever
According to this principle, when a lever is in the state of equilibrium, the product of the

load and load arm is equal to the product of the effort and effort arm.

∴ Load × Load arm = Effort × Effort arm


Worked out Numerical 2

Study the following diagram and calculate MA, VR and ŋ of the lever.

600 N 200 N
60 cm
20 cm
Fig. 3.3

44 Oasis School Science - 9 PHYSICS

Solution:

Effort = 200 N

Load = 600 N
= 20 cm + 60 cm = 80 cm = 0.8 m
Effort arm = 20 cm = 0.2 m
= ?
Load arm = ?

MA

VR

ŋ = ?
We have, MA =
Load = 600 =3
Effort 200

VR = Effort distance = 0.8 =4
Load distance 0.2

Now, ŋ = MA × 100%
VR

= 3 × 100%
4

= 75%

∴ The MA, VR and ŋ of the lever are 3, 4 and 75% respectively.

2. Pulley

A pulley is another simple machine which is commonly used to lift heavy loads. A pulley
is a circular disc having a groove on its circumference through which a rope can pass. It
is installed in cranes to lift construction materials, vehicles, etc. In rural areas, pulley is
used to lift water from wells. A pulley makes our work easier by changing the direction
of force and by multiplying the force. In a pulley, load is attached at one end of the rope
and the effort is applied at another end. There are three types of pulleys. They are:

a. Single fixed pulley
b. Single movable pulley

c. Block and tackle system

Effort

Force

Force Fig. 3.4 (b) Single movable pulley Fig. 3.4 (c) Block and tackle system
Fig. 3.4 (a) Single fixed pulley

PHYSICS Oasis School Science - 9 45

a. Single fixed pulley: If a pulley does not move up and down with the load, it is
called a single fixed pulley. In a single fixed pulley, the distance covered by an
effort is always equal to the distance covered by the load. So, VR of a single fixed
pulley is always one, i.e. number of pulley in the system. A single fixed pulley can
be considered as an equal arm lever in which the effort arm and load arm are equal
to the radius of the wheel. So, a single fixed pulley is an example of first class lever.

Although there is no gain in MA and VR, yet single fixed pulley is widely used
because:

(i) it makes our work easier by changing the direction of the force applied, and

(ii) the person can use his/her own body weight, while applying the effort
downwards.

b. Single movable pulley: If a pulley moves up and down along with the load, it is
called a single movable pulley. In a single movable pulley, one end of the rope is
attached to a fixed hook and effort is applied to another end. In this pulley, effort
distance is two times more than the load distance. So, VR of a movable pulley is 2,
i.e. number of rope segments used to support the load.

In a single movable pulley, load is equal to twice the effort. So, a movable pulley
doubles the effort we exert. Therefore, the single movable pulley acts as a force
multiplier.

A single movable pulley is usually used in combination with a single fixed pulley. It
is because a fixed pulley allows the effort to be applied in the downward direction
whereas the movable pulley doubles the effort applied.

Differences between a Single fixed pulley and a Single movable pulley

S.N. Single fixed pulley S.N. Single movable pulley
1.
It does not move up and down 1. It moves up and down along
along with load. with load.

2. It is used to change the direc- 2. It is used to multiply the force as

tion of the effort to a more con- it doubles the effort we exert.

venient direction.

3. The MA and VR of a single 3. The MA and VR of a single mov-

fixed pulley is 1. able pulley is 2.

4. It is used to raise a small load. 4. It is used to lift a heavy load.

c. Block and tackle system: If two or more pulleys are used in a combined form,
it is called a block and tackle system. Such type of pulley system makes our work
easier by multiplying the force and changing the direction of force. The VR of a
block and tackle system is equal to the number of pulleys in the system.

The block and tackle system of pulley consists of two sets of pulleys, each set
containing two or more pulleys. These pulleys are mounted on a common axle and

46 Oasis School Science - 9 PHYSICS

are of the same radius. The set of pulleys fixed to the rigid support is called block
whereas the set of pulleys which is movable and attached to the load is called tackle.

fixed pulley block

fixed end of rope T spring balance
T T reading the effort force

T E

moving pulley block

slotted masses load
A block and tackle with VR=4

Fig. 3.5 Block and tackle system

Activity 1

• Observe different types of pulleys in your science laboratory. Draw their diagrams
and classify them in terms of fixed, movable or block and tackle system.

MA, VR and ŋ of a pulley

MA = Load
Effort

VR = No. of pulleys (except single movable pulley)

= No. of rope segments that supports the load

ŋ = MA × 100%
VR

Worked out Numerical 3

A load of 500 N is lifted by using a single movable pulley. If the effort applied is 300N,
calculate the efficiency of the pulley.

Solution: Load = 500 N


Effort = 300 N

VR = 2 (Since the pulley is single movable pulley)

MA = ?

ŋ = ?

PHYSICS Oasis School Science - 9 47

We have, MA = Load = 500 = 1.66
Effort 300

ŋ = MA × 100%
VR

= 1.66 × 100 % [∴ VR = 2]
2

= 83.3%

∴ The efficiency (ŋ) of the pulley is 83.3%.

3. Wheel and axle Wheel

Wheel and axle is a system of two co-axial cylinders Axle
having two different radii in which the bigger one
is called a wheel and smaller one an axle. It works Effort Load
like a lever. Spanner, paddle of bicycle, steering
of vehicles, screw driver, door knob, etc. are some Fig. 3.6 Wheel and axle
examples of wheel and axle. In a wheel and axle,
load is attached to the axle and effort is applied to
the rope of the wheel. When the wheel completes
one rotation, the axle also completes the same.

In a wheel and axle, the common axis of wheel and
axle acts as a fulcrum which lies in between load and
effort. So, wheel and axle is considered as the first
class lever.

MA, VR and ŋ of a wheel and axle

MA = Load
Effort

VR = Effort distance
Load distance

Circumference of wheel (2πR)
= Circumference of axle (2πr)

∴ VR = Radius of wheel (R)
Radius of axle (r)

MA
ŋ = VR × 100%

Worked out Numerical 4

In a wheel and axle, the radius of the wheel is 20 cm and that of the axle is 4 cm. If a load of
1000 N is lifted by using an effort of 300 N on it, calculate MA, VR and ŋ.

steering /ˈstɪərɪŋ/ - the machinery in a vehicle that is used to control the direction it goes

48 Oasis School Science - 9 PHYSICS

Solution:

Radius of the wheel (R) = 20 cm

Radius of the axle (r) = 4 cm

Load = 1000 N

Effort = 300 N

MA = ?

VR = ?

ŋ = ?

We have,

MA = Load = 1000 = 3.3
Effort 300

VR = R = 20 =5
r 4

ŋ = MVRA × 100% = 3.3 × 100%
5

= 66.0%

∴ The MA, VR and ŋ of the wheel and axle are 3.3, 5 and 66% respectively.

Wheel and axle: A Continuous Lever C

Wheel and axle is a developed form of a lever. In the given Y
diagram, the effort is acting at point X on the wheel. The load X
is at Y and the centre of the wheel C acts as a fixed point called
fulcrum. The figure shows that fulcrum lies between load and Fig. 3.7 Wheel and axle as
effort. So it can be considered as the first class lever. In a lever, the a continuous lever
effort can be applied to a certain angle but less than 90°. In this
case, the effort can be applied in a continuous manner until the
load reaches the required height. Owing to these reasons, wheel
and axle is called a continuous lever.

Activity 2

• Take a wheel and axle system and calculate its MA, VR and ŋ.

4. Inclined plane h l

An inclined plane is a simple machine which Fig. 3.8 Inclined plane
is a sloping surface. It is used for lifting the Oasis School Science - 9 49
heavy loads applying less effort. The road
which is made on the hilly region is an example
of inclined plane. Similarly, an inclined plank
used for loading a truck and a staircase also
act as inclined plane. The length of an inclined
plane acts as the effort distance and the height
as the load distance.

PHYSICS

In an inclined plane, the length of the inclined plane is always more than its height. So,
VR of an inclined plane is always more than one. Please note that the smaller the angle
of inclined plane, the smaller is the effort needed to move up a load. In other words, the
longer the inclined plane, the smaller is the effort needed to move up a load on it.

MA, VR and ŋ of an inclined plane

MA = Load
VR = Effort
ŋ =
Length of inclinded plane (l)
Height of inclinded plane (h)

MA
VR × 100%

Worked out Numerical 5

Calculate MA, VR and ŋ of the given inclined plane. The length and height of the inclined
plane is 15 m and 8 m respectively.

Solution: = 600 N 400 N
Load = 400 N
Effort = 15 m 600 N
Length of inclined plane (l)

H eight of the inclined plane (h) = 8m

MA = ?

VR = ? Fig. 3.9

ŋ = ?

We have,

MA = Load = 600 = 1.5
Effort 400

VR = l = 15 = 1.8
h 8

ŋ = MA × 100% = 1.5 × 100% = 83.3%
VR 1.8

∴ The MA, VR and ŋ of the given inclined plane are 1.5, 1.8 and 83.3% respectively.

5. Screw
A screw is a modified inclined plane with a raised spiral line along its surface. The raised

spiral line is called a thread. The distance between any two successive threads is called
pitch. Screw is turned and pressed into wood, metal, etc. to fasten two things together.
Jack screw is used to lift heavy vehicles like truck, bus, etc. Screw nail, jack screw, driller,
etc. are some examples of screw.

50 Oasis School Science - 9 PHYSICS

Screw nail Jack screw Driller
Fig. 3.10

6. Wedge

A wedge is a piece of metal, wood, etc. with one thick end and another sharp pointed end.
It is used for cutting and splitting, drilling holes and pushing or lifting heavy objects. In
a wedge, effort is applied from the thick or blunt end. All tools and weapons with sharp
edges like axe, knife, blade, chisel, nail, etc. are the examples of wedge.

Axe Knife Wedge Chisel

Fig. 3.11

3.9 Moment Door Hinge
Handle
In our day to day activities, we open a door, turn on tap or Door frame
tighten up a nut with a spanner. In all these cases, we exert a
force which is called turning force. The force which rotates a
body about a fixed point is called turning force.

The turning effect produced by a force is called moment or Hinge

torque. It is the product of force and perpendicular distance More force
Less force
between the line of action of force and the axis of rotation Fig. 3.12 Door with two handles

of the body. The SI unit of moment is newton metre (Nm).

Moment of a force can be calculated by the given formula.

Moment = Force × Moment arm
F×r
=

fasten /ˈfɑːsn/ - to close or join together the two parts of sth Oasis School Science - 9 51
PHYSICS

When the force is applied in inclined position,

Moment = F × r a
θ
= F × a sinθ.


r = a sinθ

3.10 Law of Moment Fig. 3.13

In the equilibrium condition, the sum of clockwise moment is equal to the sum of anticlockwise

moment, i.e.

Sum of clockwise moment = Sum of anticlockwise moment

Fig. 3.14

Clockwise moment is the moment produced by a force in the clockwise rotation. It can be
taken as a negative moment. Similarly, the moment produced by a force in the anticlockwise
rotation is called anticlockwise moment. It can be taken as a positive moment. Some examples
of turning effect in our daily life are as follows:

Clockwise (negative) moment Anticlockwise (positive) moment

Fig. 3. 15

i) Long spanner is used to open a rusted nut so that the moment increases. It helps
to open the nut easily by applying less effort.

Nut Spanner Effort Reasonable fact-1

Fig. 3.16 Spanner As we move towards a tip of the tree branch, it is
more likely that the branch will break, why?
As we move towards the tip of the tree branch, the
weight of the man produces more moment due to
increase in perpendicular distance. This gives more
moment on the base of the branch. As a result, the
branch will break.

moment /ˈməʊmənt/ - the turning effect produced by a force
equilibrium /ˌiːkwɪˈlɪbrɪəm/ - a state of balance, especially between opposite forces

52 Oasis School Science - 9 PHYSICS

ii) The probability of breaking of a tall tree is more than a short one during storm as the
tall tree has longer moment arm than that of the short tree. So, the tall tree experiences
greater turning effect or moment and breaks down easily during the storm.

iii) In houses, the doors are installed with the hinge at one side and the handle is
fixed at a larger distance from the hinge. So, less effort is required to produce large
turning effect. But if the handle is fixed near the hinge, more force is required to
produce the same turning effect.

iv) The handle of a screw driver is made wider to produce larger moment, so the
screw moves easily.

v) The steering of trucks and buses is made larger than that of cars to produce a large
turning effect by increasing the moment arm.

vi) In a screw jack, which is used to lift heavy vehicles, a long handle is used to
produce a large turning effect by increasing the moment arm.

Reasonable fact-2 Reasonable fact-3

Generally spanner is made short to unscrew a There is no gain in mechanical advantage of
small nut and made long for a big nut, Give reason. a single fixed pulley. However it is used in
Small nut can be unscrewed with small turning practice. Give reason.
effort (moment), so the moment produced by Single fixed pulley changes the direction of force
short nut is sufficient to unscrew the small nut, in a convenient manner. In our daily life, there
whereas to unscrew big nut, larger moment are a lot of examples where changing direction of
is necessary which can be obtained by a long force is more important than gain in mechanical
spanner. So, long spanner is used to unscrew the advantage. In such cases, a single fixed pulley is
big nut. used.

SUMMARY

• The devices which are used to make our work easier, faster and to change the direc-
tion of force are called simple machines, e.g. beam balance, scissors, pulley, etc.

• Lever, pulley, wheel and axle, inclined plane, screw and wedge are the simple ma-
chines that we use in our daily life.

• The ratio of load to the effort is called Mechanical Advantage (MA).
• Velocity Ratio (VR) is the ratio of the distance travelled by effort to the distance trav-

elled by load.
• The percentage ratio of output work to input work is called efficiency (ƞ) of a

machine.
• A lever is a rigid bar which is free to rotate about a fixed point called fulcrum.
• A pulley is a circular disc having a groove on its circumference through which a rope

can pass.
• Wheel and axle is a system of two co-axial cylinders having two different radii in

which the bigger one is called a wheel and smaller one an axle.
• A wedge is a piece of metal, wood, etc. with one thick end and another sharp pointed

end.
• The machine without any friction that has 100% efficiency is called an ideal machine.
• The turning effect produced by a force is called moment or torque. The SI unit of

moment is newton - meter (Nm).
• According to the principle of moment, the sum of clockwise moment is equal to the

sum of anticlockwise moment in equilibrium condition.

PHYSICS Oasis School Science - 9 53

Exercise

Group-A
1. Define simple machine.
2. Why are simple machines used?
3. What is mechanical advantage? Write down its formula.
4. What is velocity ratio? Write down its formula.
5. Define input work and output work.
6. What is efficiency of a machine?
7. What are the factors that affect the efficiency of a simple machine?
8. How can the efficiency of a simple machine be increased? Write.
9. What is a perfect machine?
10. What is the relationship among the efficiency, output work and input work of a machine?
11. What is a pulley?
12. Define fixed pulley and movable pulley.
13. What is the velocity ratio of a single fixed pulley?
14. What is the velocity ratio of a single movable pulley and the pulley having more than

two wheels?
15. What is an inclined plane?
16. What is a wheel and axle?
17. Give any two examples of wheel and axle.
18. What is moment? Write down its formula.
19. What are two factors that affect the turning effect of force (moment)?

Group-B
1. Write any two differences between mechanical advantage (MA) and velocity ratio (VR).
2. Derive the relation among MA, VR and efficiency.
3. What is meant by the statement that the MA of a simple machine is 3?
4. What is meant by the saying that the VR of a machine is 5?
5. What is meant by the statement that efficiency of a simple machine is 60%?
6. What is its efficiency? Is it possible to make a perfect machine in our daily life?
7. What is meant by a practical machine? What is the relation between MA and VR in a

practical machine?
8. Write any two ways to reduce the friction in a machine.

54 Oasis School Science - 9 PHYSICS

9. Write any two differences between input work and output work.
10. Write any two differences between practical machine and ideal machine.
11. Write any two differences between fixed pulley and movable pulley.
12. The MA and VR of pulley increases on increasing the number of pulleys in the system.
Why?
13. The efficiency of a machine can be increased by applying oil or grease. Give reason.
14. Give reason:

a) Scissors and pulleys are called simple machines.
b) Mechanical advantage has no unit.
15. Mechanical advantage is always less than velocity ratio in a practical machine, why?
16. The VR of a single movable pulley is 2. Give reason.
17. The efficiency of a machine cannot be 100%. Justify this statement.
18. Wheel and axle is also called a continuous lever, why?
19. The probability of breaking the branch of a tree is more as we move to the top. Give
reason.
20. A long spanner is used to open the rusted nut, why?
21. The door handles are kept near the edges of the door planks, why?
22. The steering wheel of a truck is larger than that of a car. Give reason.

Group-C
1. Write any three applications of simple machines in our daily life.
2. Describe the structure of a pulley with a neat and labelled figure.
3. How is MA, VR and efficiency of a wheel and axle calculated? Explain.

4. Calculate the efficiency of a lever if its MA and VR are 3 and 4 respectively.

[Ans: 75%]

5. Calculate the efficiency of a four wheeled pulley system if it is used to lift a load of 500 N

by using an effort of 300 N. [Ans: 41.66%]

6. Calculate the effort applied to the given diagram for its equilibrium state. Also calculate

MA, VR and η of the lever. [Ans: 200 N, MA = 3, VR = 4, η = 75%]

600 N Effort=?
60 cm
20 cm

PHYSICS Oasis School Science - 9 55

7. A 20 cm long spanner is used to open a rusted nut of a motor cycle by applying an effort
of 60N. Calculate the moment produced. [Ans: 12 Nm]

8. A 25 cm long spanner is used to open a rusted nut. If the moment produced is 8Nm,

calculate the effort applied. [Ans: 32 N]

Group-D

1. In a wheel and axle, the radius of the wheel is 50 cm and that of the axle is 20 cm. If a load
of 2000 N is lifted by applying an effort of 1000 N, calculate MA, VR and ŋ.

[Ans: VR = 2.5, MA = 2, ŋ = 80%]

2. In an inclined plane, an effort is applied to lift a load of 1000 N. The MA and VR of the

inclined plane are 2 and 3 respectively. Calculate the efficiency and effort applied.

[Ans: 66.66%, 500N]

3. Study the given figure and calculate MA, VR and η.

300 N

450 N
[Ans: MA = 1.5, VR = 2, η = 75%]

4. Study the given figure and calculate MA, VR and η.

400 N

32 m 8 m
1200 N

[Ans: MA = 3, VR = 4, η = 75%]

5. What is block and tackle system? Draw neat and labelled figure showing block and
tackle system having four pulleys.

6. Although there is no gain in MA and VR, yet single fixed pulley is widely used. Give any
two reasons to justify this statement. Draw a neat and labelled figure showing a wheel
and axle.

56 Oasis School Science - 9 PHYSICS

4UNIT Estimated teaching periods

Theory 8

Practical 2

Work, Energy

and Power Albert Einstein

Objectives

After completing the study of this unit, students will be able to:

• differentiate and relate between work, energy and power with their units.
• describe the types of energy.
• solve the problems related to work, energy and power.

4.1 Introduction

We use the terms 'work' and 'energy' in everyday life. When we work for a long time, we get
tired and cannot work anymore. We say we have no energy to do any more work. The capacity
of doing work is called energy and work is said to be done when a body moves in the direction
of the force applied. Both energy and work are measured in joule (J). Similarly, the rate of
doing work is called power. In SI system, it is measured in watt (W). Energy, work and power
are closely related to each other.

4.2 Work

In general, the term work is used to describe any activity in which muscular or mental effort
is exerted. But in physics, work is defined very precisely. In physics, work is said to be done
only when a body moves in the direction of the force applied. Work done on a moving body is
equal to the product of force exerted on the body and the distance covered by the body in the
direction of force or displacement, i.e.

Work done (W) = Force (F) × Displacement (s) Final position
of the body

∴ W = F × s

Initial position
of a body

Force (F)

Displacement (s)

Fig. 4.1

When we sit on a chair and study for 2-3 hours, we are not doing work from a physicist's point
of view. Similarly, no work is done by a person standing on a place for a long time. Work is said
to be done when the force acting on a body produces motion in it in the direction of the force.

PHYSICS Oasis School Science - 9 57

If a horse pulls a cart and covers some distance, it does some work. Thus, the work done by a
body depends on following two factors:

i) the magnitude of the force applied (F)
ii) the displacement produced by the force (s)

(a) A man pushing a wall (b) A man pushing a car

Fig. 4.2

If the force applied and displacement make an angle θ, following is the required expres-
sion for the work done.

Work done (W) = F Cos θ × s F

θ

s
When θ = 900, i.e. force is perpendicular to the displacement, the work done is zero.

[W = F Cos θ × s = F Cos 900 × s = F × 0 × s = 0 ]

For example, the work done on a body moving in a circular path is zero because when the
body moves in a circular path, the centripetal force acts along the radius of the circle and
it is in right angle to the motion of the body.

The SI unit of force is newton (N) and that of displacement is metre (m). So the SI unit of work
done is newton metre (Nm). It is also called joule (J). In CGS system, work is measured in erg.
Work is a scalar quantity.

One joule work

The work done is said to be 1 joule if 1 N force can displace a body through 1 m distance in the
direction of the force applied.

Relation between joule and erg

We have,

Since, W = F × s

1 J = 1 N × 1m

58 Oasis School Science - 9 PHYSICS

W = F × s Initial position Final position
1 J = 1 N × 1 m of a body of the body

= 105 dyne × 100 cm [∵ 1 N = 105 dyne]

= 107 dyne. cm 1N
= 107 erg. Force (F)

∴ 1 J = 107 erg 1m
Fig. 4.3 One joule work done

4.3 Nature of Work Done

The value of work done may be positive, negative or zero which is described as follows:

a) Positive work [When θ is acute (<900)]

W = F × s = F × s Cos θ

∴ When θ is actue (<900), Cos θ is positive. Hence, the work done is positive. For example,

i) When a body falls freely under the effect of gravity, θ = 00, Cosθ = Cos00 = +1.
Therefore, the work done by gravity on a body is positive.

ii) When a cart is pulled by applying a force at an acute angle, the work done is
positive.

iii) When a spring is stretched, the work done is also positive.

b) Negative work [When θ is obtuse (>900)]

W = F×s = F×s Cos θ

∴ When θ is obtuse (>900), Cos θ is negative. Hence, the work done is negative. For
example,

i) When a ball is thrown vertically upward, θ = 1800, Cos1800 = –1. Therefore, the
work done by gravity on a body thrown upwards is negative.

ii) When a body is moved over a rough horizontal surface, the work done by the
frictional force is negative.

iii) When brakes are applied on a moving vehicle, the work done by braking force is
negative.

c) Zero work [When θ is a right angle (=900)] s

W = F × s = F × s Cos θ O θ = 900
F
∴ When θ is 900, Cosθ = Cos 900 = 0. Hence, the work done is zero.
For example, Fig. 4.4

i) When a person carrying some load on
his head moves on a horizontal platform,
θ = 900. So the work done by him is zero.

ii) When a person carrying a briefcase moves horizontally,
work done by the person is zero.

PHYSICS Oasis School Science - 9 59

iii) When a body moves in a circular motion, the work done by the centripetal force
is zero. This is because θ = 900 and W = F × s Cos θ = F × s Cos 900 = 0. Similarly, no
work is done when a satellite revolves around the earth.

No work is done when a body moves in a circular path.

4.4 Types of Work

1. Work done against gravity

2. Work done against friction

1. Work done against gravity mg
h
Gravity is the force by which a body is attracted towards the
centre of the earth. The work done against this force is called Fig. 4.5
work done against gravity. For example, the work done by lifting [ Force = weight]
an object. Let us consider a body of mass 'm' is lifted to a height [ Force (F) = mg]
'h'. Then,
F
Work done against gravity (W) = Force × displacement
s
= Weight × height Fig. 4.6∴
∴

= m.g × h

∴ W = mgh

2. Work done against friction

The force which opposes the motion of a body
moving on the other body when they are in contact
is called friction. The work done against this force
is called work done against friction. For example,
a body dragged over a surface of another body.

Work done (W) = F × s

Worked out Numerical 1

Calculate the work done if a force of 10 N is applied to move a body through 10 m.

Solution:

Force (F) = 10 N
= 10 m
Displacement (s) = ?

Work (W) = F×s
= 10 × 10
We have, W = 100 J






∴ The work done (W) = 100 J.

60 Oasis School Science - 9 PHYSICS

Worked out Numerical 2

Calculate the work done when a person of 50 kg climbs a 20 m tall tree.

Solution:

Mass of the person (m) = 50 kg

Displacement (s) = 20 m

Work (W) = ?

We have,

W = F×s
mg × h
= 50 × 9.8 × 20 [∵ F = mg, s = h]
9800J [∵ 1 kg = 9.8 N]
=

=

∴ The work done (W) = 9800 J.

Worked out Numerical 3

A car of mass 1000 kg is travelling with acceleration of 9 m/s2. Calculate the work done
when it travels 50 m distance.

Solution: Reasonable fact-1
Mass of the car (m) = 1000 kg

Acceleration (a) = 9 m/s2 Priya is standing on the road for 3
= 50 m hours by carrying a load of 100N.
Displacement (s) = ? What is the work done by her?
Why?
Work done (W) = F × s The work done by Priya is zero in the
= m × a × s above condition, because the distance
We have, = 1000 × 9 × 50 covered by Priya is zero. Please note
= 450000 J that, according to the definition of
W work, distance should be covered in
the direction of force.






∴ The work done (W) = 4.5 × 105 J

4.5 Energy

When a body is capable of doing a work, it is said to possess energy. The reverse is also true.
Thus, energy is defined as the capacity of doing work. Energy is a scalar quantity. The SI unit
of energy is joule (J) and its CGS unit is erg. The amount of energy possessed by a body is
equal to the amount of work it can do when the energy is released. Whenever work is done,
energy is consumed.

reverse /rɪˈv‰ːs/ - the opposite of what has just been mentioned Oasis School Science - 9 61
PHYSICS

Activity 1

• We use many sources of energy like firewood, biogas, petrol, diesel, etc. List
the sources which you use at your home. Discuss in small groups how certain
sources of energy are dependent on the sun. Prepare a short report on it.

Some more practical units of energy are given below:

Unit Symbol Equivalence in joule
Erg erg 10-7 J
Calorie Cal. 4.2 J
Kilowatt hour kWh 3.6 × 106 J
Electron volt eV 1.6 × 10-19 J

4.6 Different Forms of Energy

There are several forms of energy. Some of them are explained below:

1. Mechanical Energy

The energy possessed by a body by virtue of its motion or position or configuration is
called mechanical energy. Mechanical energy is of two types:

i) Kinetic energy ii) Potential energy

i) Kinetic Energy (KE)
The energy possessed by a body due to its motion is called kinetic energy. The

bullet fired from a gun, a moving ball, a flying bird, a rolling stone, etc. are some
examples of the objects possessing kinetic energy.

Activity 2

• Take a heavy ball. Drop it on a thick bed of sand, a wet bed of sand
would be better. Drop the ball in the bed from the height of about 25
cm. The ball creates a depression.

• Repeat this activity from the heights of 50 cm, 1 m and 1.5 m. Ensure
that all the depressions are distinctly visible. Compare the depth in
each case. Which one of them is the deepest and which one is the
shallowest? Why? What can you conclude from this activity?



Derivation of the formula of KE

Let us consider a body of mass 'm' initially u=0

at rest (u = 0) is moving with an acceleration F a
'a'. Let 'v' be the final velocity acquired by
s
the body in moving a distance 's'. Fig. 4.7

According to the equation of motion,
We have,

62 Oasis School Science - 9 PHYSICS

v2 = u2 + 2as

Or, v2 = 02 + 2 as [since, u = 0]

Or, s = v2 …………….. (i)
2a

Again,

KE = Work done

= F× s

= m.a × v2 [∵ F = ma, s = v2 ]
2a 2a

= 1 mv2
2

∴ KE = 1 mv2
2

From the above formula, it becomes clear that:

i) the kinetic energy (KE) of a body is directly proportional to the mass (m) of the
body, i.e. KE ∝ m.

ii) the kinetic energy (KE) of a body is directly propertional to the square of the
velocity (v), i.e. KE ∝ v2.

Worked out Numerical 4

The KE of a moving body becomes four times the original if its velocity is doubled. Verify
this statement numerically.

Solution:

Let 'm' and 'v' be the mass and velocity respectively of a moving body. Then, kinetic
energy (KE) of the body is given by

KE = 1 mv2
2

If the velocity is doubled,

(KE)new = 21 m × (2v)2 = 1 m × 4 × v2
2

= 4 ( 1 mv2) = 4 (KE) [∵ KE = 1 mv2 ]
2 2

Thus, the KE of a moving body becomes four times the original one if its velocity is doubled.

Worked out Numerical 5

Calculate the kinetic energy of a body of mass 200 g moving with the velocity of 10 m/s.

Solution:

Mass of the body (m) = 200 g = 200 kg = 0.2 kg
1000

Velocity (v) = 10 m/s

PHYSICS Oasis School Science - 9 63

Kinetic energy (KE) = ?
We have,

KE = 1 mv2 = 1 (0.2) × (10)2 = 10 J
2 2

∴ The kinetic energy (KE) = 10 J.

ii) Potential Energy (PE)

The energy possessed by a body resting at a certain height due to its position or
configuration is called potential energy. For example, a stretched rubber, compressed
spring, leg lifted to kick, water stored in a dam possess the potential energy.

Activity 3

• Take a slinky. Ask your friend to hold one of its ends. You hold the other end and
move away from your friend. Now, you release the slinky. What happens? How
does the slinky acquire energy when it is stretched?

Derivation of the formula of PE
Let us consider a body of mass 'm' is raised to a height of 'h'. Then,

PE = Work done (W)

= F× s [∵ W = F×s] h
Fig. 4.8
= m.g × h [∵ F = mg and s = h]

∴ PE = mgh

This energy is also called gravitational potential energy.

Worked out Numerical 6

Calculate the potential energy of a body of mass 50 kg if it is raised to a height of 8 m.

Solution:

Mass of the body (m) = 50 kg
= 8m
Height (h) = ?

Potential energy (PE) = mgh

We have, PE


= 50 × 9.8 × 8 [∵ g = 9.8 m/s2]

= 3920 J

∴ The potential energy of the body (PE) = 3920 J.

64 Oasis School Science - 9 PHYSICS

2. Heat Energy

The energy produced due to the vibration of molecules of a body is called heat energy.
For example, burning firewood, an electrical heater, burning coal, etc. possess heat energy.
The heat produced by burning petrol in a motorcycle engine provides the energy needed
to run the motorcycle. When water is heated, it changes into steam. The steam possesses
heat energy which is used to rotate turbines and railway engines.

3. Light Energy

When a body is extremely heated, it emits light. Light is a form of energy which produces
the sensation of vision. For example, energy possessed by a glowing bulb, a burning
candle, etc. The sun is the natural source of light energy. The solar energy is used by
green plants for photosynthesis.

4. Sound Energy

Sound is a form of energy which is produced due to the vibration of a body. It travels
only through the material medium. An electric bell, a temple bell, an earphone, etc.
produce sound. Hence, these things are called the sources of sound energy.

5. Electrical Energy

The energy which is generated due to the flow of electrons through a conductor is called
electrical energy. When electrons flow through a conductor, current is produced which
is called electrical energy. There are different sources of electrical energy such as hydro-
electricity, cell, battery, dynamo, generator, etc.

6. Chemical Energy

Different substances such as food, coal, petrol, firewood, diesel, kerosene, etc. store
chemical energy. The energy stored in these substances is released when a chemical
change takes place. This type of energy is called chemical energy.

7. Magnetic Energy

The energy possessed by a magnet is called magnetic energy. It can be used to operate
electric bell, to produce hydroelectricity, etc.

8. Nuclear Energy

The energy produced from the nucleus of an atom is called nuclear energy. It can be
produced when two or more light nuclei combine to form a single heavy nucleus (nuclear
fusion) or when a heavy nucleus splits into two or more light nuclei (nuclear fission).
This energy is also called atomic energy.

4.7 Transformation of Energy

Transformation of energy is the process in which one form of energy is converted into another
form. For example, the conversion of electrical energy into light energy and heat energy by
using an electric bulb.

dynamo /ˈdaɪnəməʊ/ - a device for turning mechanical energy into electricity Oasis School Science - 9 65
PHYSICS

Transformation of energy in hydroelectric project

Water stored in a dam Falling water Rotation of turbine Electricity in coil

Potential energy Kinetic energy Kinetic energy Electrical energy

Transformation of energy while using an electric fan

Current in wire Formation of electromagnet in the coil Movement of fan blades
Electrical energy Magnetic energy Kinetic energy

Fig. 4.9 Hydroelectric project

Let us learn about some devices and objects with their energy conversion.

a. Solar cell : Light energy to electrical energy

b. Microphone : Sound energy to electrical energy

c. Battery or Cell : Chemical energy to electrical energy

d. Heater : Electrical energy to heat and light energy

e. Turbine : Mechanical energy to electrical energy

f. Dynamo/Generator : Mechanical energy to electrical energy

g. Electrical bulb : Electrical energy to light energy and heat energy

h. Electric motor : Electrical energy into mechanical energy

i. Loudspeaker : Electrical energy into sound energy

j. Electromagnet : Electrical energy into magnetic energy

k. Television : Electrical energy into light and sound energy

l. Burning of fuels : Chemical energy into heat and light energy

m. Explosion of crackers/bombs : Chemical energy into heat, light and sound energy

explosion /ˈɪkspləʊƷn/ - the sudden violent bursting and loud noise of sth

66 Oasis School Science - 9 PHYSICS

4.8 Principle of Conservation of Energy

According to this principle, "Energy can neither be created nor be destroyed but it can be
changed from one form to another."

For example, if 100 J of electrical energy is supplied to an electric bulb, it converts 10 J of
electrical energy into light energy and remaining 90 J of electrical energy into heat energy, i.e.

Electrical energy = Heat energy + Light energy

100 J = 90 J + 10 J

4.9 Power

Suppose an old man takes 20 seconds to do a particular work whereas a young man takes
only 8 seconds to do the same work. It is clear that the rate of doing work of the young man is
more than that of the old man. So, the rate of doing work is called power. It is a scalar quantity.
Power is measured in watt (W) or J/s or horsepower (h.p.). Power is calculated by the given
formula.

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

∴P = W
t


From the above formula, it becomes clear that power depends on two factors, viz. (i) the
amount of work done, and (ii) the time taken to do the work.

1. When a body does larger amount of work in a particular time, its power is said to be
greater and vice-versa, i.e. P ∝ W.

2. When a body takes lesser time to do a particular amount of work, the power is said to be

greater and vice-versa, i.e. P ∝ 1 .
t

One watt power

The power is said to be 1 watt if 1 joule work is done in 1 second.

Since, P = W
t
1 W =
1J

1s

The SI unit of power, i.e. watt (W) is an important unit of power as it is widely used in

electrical works. A bulb of 20W consumes electrical energy at a rate of 20 joules per sec-

ond. Another unit of power is horse power (h.p.) which is equal to 746 W. The relation

among different units of power is given below:

PHYSICS Oasis School Science - 9 67

1000 W = 1 kilowatt (kW)

1000 kW = 1 mega watt (MW)

746 W = 1 horse power (h.p.)

Meaning of 100 W written on an electrical bulb

Since, W Fact File-1
P = t
In an electric bulb 60W is written.
∴ 100 W = 100 J It means that the bulb consumes
1s 60J of electrical energy in 1
second and converts it into heat
Hence, 100W written on an electric bulb means and light energy.

that the bulb converts 100 J of electrical energy

into heat energy and light energy in 1 second.

Differences between Work and Power

S.N. Work S.N. Power
1. Power is the rate of doing work.
Work is the product of force and 1.
2. displacement in the direction of Its SI unit is watt (W).
3. force. It is affected by the time taken.

Its SI unit is joule (J). 2.

It is not affected by the time taken. 3.

Worked out Numerical 7

Calculate the power of a man of mass 50 kg if he can climb a staircase of height 3m in 10
seconds.

Solution:

Given,

Mass of the man (m) = 50 kg
Height of the staircase (h) = 3 m

Time (t) = 10 s

Power (P) = ?
We have, P
= W
t

= F×s
t
[∵F= mg and s = h]
= m.g.×h
t

= 50×9.8×3 [∵ g = 9.8 m/s2]
10

= 147 W

68 Oasis School Science - 9 PHYSICS

SUMMARY

• Work is said to be done when a body moves in the direction of the force
applied.

• Work done is said to be 1 joule if 1 N force can displace a body through 1 m
distance in the direction of the force applied.

• The capacity of doing work is called energy. Its SI unit is joule (J).

• The energy possessed by a body due to its motion or position or
configuration is called mechanical energy.

• The energy possessed by a body due to its motion is called kinetic energy.

• The energy possessed by a body resting at a certain height due to its
position or configuration is called potential energy.

• According to the principle of conservation of energy, "Energy can neither
be created nor destroyed but it can be changed from one form to another."

• The rate of doing work is called power. Its SI unit is watt (W).

• The energy produced due to the vibration of molecules of a body is called
heat energy. For example, burning firewood, an electrical heater, burning
coal, etc.

• Light is a form of energy which produces the sensation of vision.

• Sound is a form of energy which is produced due to the vibration of a body.

• The energy which is generated due to the flow of electrons through a
conductor is called electrical energy.

• The energy produced from the nucleus of an atom is called nuclear energy.

• Transformation of energy is the process in which one form of energy is
converted into another form.

• One watt power is the rate of doing one joule work in one second.

• The power of a bulb is 100 W means that the bulb converts 100 J of electrical
energy into heat energy and light energy in one second.

PHYSICS Oasis School Science - 9 69

Exercise

Group-A
1. What is work done? Write down its SI unit.
2. What are the conditions required for work to be done?
3. Define 1 joule work done.
4. What is meant by work done against gravity?
5. Define work done against friction.
6. Write down the formula to calculate the work done against gravity.
7. What is energy? In which unit is it measured?
8. Name any two forms of energy.
9. What is mechanical energy?
10. Define potential energy.
11. What is kinetic energy?
12. Name any two sources of electrical energy.
13. Name the form of energy contained by each of the given matter/actions:
i) bread ii) battery iii) ringing bell iv) flowing water v) petrol
vi) coal vii) glowing bulb viii) stretched rubber ix) water stored in a dam
x) bullet fired from a gun xi) sun xii) magnet xiii) a watch with wounded up spring
xiv) rolling ball xv) a stone released from the catapult
14. Name any two equipment that are operated by magnetic energy.
15. What is transformation of energy?
16. State "law of conservation of energy"?
17. Define power and write its SI unit.
18. What is 'one watt power'?
19. What is relation between work done and power? Write.
20. Which form of energy is present in a hot iron rod?
21. Which factors does the power of a body depend on?

Group-B
1. Write any two differences between force and work done.
2. Write any two differences between potential energy and kinetic energy.
3. Differentiate work done against friction.
4. Work cannot be done for a long time without having food. Give reason.

70 Oasis School Science - 9 PHYSICS

5. No work is done when a person is standing by carrying a load of 50 kg for one hour,
why?

6. Electrical energy is widely used among the various types of energy. Justify this statement.
7. Write any two differences between energy and power.
8. What happens to the kinetic energy when the velocity of a moving body is doubled?
9. The cricket player who catches the ball wears gloves. Give suitable reason.

Group-C

1. How is atomic energy produced? Which factors does the kinetic energy of a body de-
pend on?

2. What is transformation of energy? Write with examples. What is the relation between
work done and power?

3. Write down the transformation of energy while lighting a bulb in a torchlight by using

cells. Write any two applications of heat energy. Stone

4. Write down the transformation of energy while using a microphone and

a loudspeaker in a programme. Write any two applications of electrical

energy.

5. What happens to the kinetic energy when the velocity of a moving body Land
is doubled? Show with calculation.

Group-D

1. How does transformation of energy take place in each of the given actions?

i) Producing electricity in a hydropower station

ii) Producing electricity in a nuclear furnace

2. Prove that: i) PE = mgh ii) KE = 1 mv2
2

3. The velocity of a body of mass 60 kg reaches 15m/s from 0m/s in 12 seconds. Calculate

the kinetic energy and power of the body. [Ans: 6750 J, 562.5W]

4. The potential energy of a stone kept on the earth's surface is zero, give 200 kg
reason. Study the given diagram and calculate the potential energy

stored in the stone. What is the potential energy when the stone reaches 50 m

the ground surface?

[Ans: 98000 J, OJ] Land surface

5. A man of mass 70 kg climbs the Nautale Durbar of Basantapur. He climbs
15 steps in 1 minute. If the height of a 1 step is 15cm, calculate his power. (g = 9.8 m/s2).

[Ans: 25.22 W]

PHYSICS Oasis School Science - 9 71

5UNIT Estimated teaching periods

Theory 2

Practical 1

Light

Objectives Willebrord Snell

After completing the study of this unit, students will be able to:

• define refraction of light and explain the laws of refraction.

• describe dispersion of light and demonstrate the spectrum of light.

• explain the electromagnetic wave with its nature.

• describe the frequency and wavelength of electromagnetic waves (Gamma rays,
X – rays, UV – rays, visible rays, infra-red and Radar).

5.1 Introduction N

We see things in our surroundings due to the presence of I
light. Light is a form of energy which produces the sensation
of vision. It always travels in a straight path as long as the Air i
light rays travel in a medium with the same density. Light
travels from one place to another with a very high speed. It O
was found that the speed of light in a vacuum is about 3×108
m/s. This is the maximum speed of light. The speed of light Glass r Refracted ray
in air medium is slightly less than that in the vacuum. For
the sake of simplicity, the speed of light in air is taken to be R
equal to that of the speed in the vacuum. The speed of light Fig 5.1 Refraction of light
in glass medium is 2 × 108 m/s whereas the speed of light in
water medium is 2.25 × 108 m/s.

When light rays are made to go obliquely from one transparent medium to another, they
change their direction at the boundary separating the two media. For example, when light
rays travelling in air enter into water medium, they bend or change their direction on entering
the water medium. This process is called refraction of light.

5.2 Refraction of Light

When light rays pass obliquely from one transparent medium to another transparent medium,
they change their direction at the boundary separating the two media. The bending of light
when it passes obliquely from one transparent medium to another is called refraction of light.

Refraction of light is responsible for a number of optical illusions. For example, the stick or
pencil immersed partially in water appears to be bent. The depth of a pond appears less than

72 Oasis School Science - 9 PHYSICS

the real depth, mirage can be seen on coal-tarred roads or deserts in hot sunny days and so
on. Similarly, various optical instruments like camera, microscope, telescope, etc. work on the
basis of refraction of light through glass lenses.

5.3 Optically Denser and Rarer Medium

The speed of light in different media is different. A medium is a transparent substance like

air, glass, water, etc. in which light travels. The medium in which light travels faster is called
optically rarer medium. The medium in which light travels comparatively slowly is called
optically denser medium. For example, air is an optically rarer medium as compared to water
and glass. Similarly, glass is an optically denser medium as compared to water and air. The
density of the denser medium is more than that of the rarer medium. So the speed of light in
denser medium is less than that in the rarer medium. For example, the speed of light in glass
(denser) medium is 2×108 m/s and in air (rarer) medium is 3×108 m/s.

5.4 Laws of Refraction of Light

When light travels from one medium to another, it follows certain rules. These rules are
popular as laws of refraction of light. The laws of refraction of light are as follows:

Normal

A

Incident ray Air medium (rarer)
Angle of incidence(i)
i
900

O Glass medium (denser)

Angle of

refraction (r) r

Refracted

B ray

Fig. 5.2 Refraction of light

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

2. The ratio of the sine of angle of incidence to the sine of angle of refraction for a given pair
of media is constant, i.e.

sin i = µ (constant)
sin r

The constant is called refractive index and is denoted by µ where µ is a Greek letter and
pronounced as 'mew'.

The second law of refraction of light gives the relationship between the angle of incidence

and the angle of refraction. This relation was discovered by Willebrord Snell in 1621 AD.

Therefore, this law is also called Snell's law of refraction. This law explains the refraction

in the following conditions.

transparent /trænspærənt/ - allowing you to see through it
illusion /ɪˈluːʒn/ - something that seems to exist but in fact does not

PHYSICS Oasis School Science - 9 73

a. When a ray of light travels from an optically denser medium to a rarer medium, it
bends away from the normal and when it passes from a rarer medium to a denser
medium, it bends towards the normal.

Normal

Normal Denser Rarer (air) i Rarer air
i (glass) r Denser (glass)

Rarer Denser (glass)
(air) r

Away from normal Towards normal Through the normal
(a) (b) (c)

Fig. 5.3

b. A ray of light passing through normal does not deviate. In this case, the angle of
incidence is zero.

Activity 1

• Place a coin at the bottom of an empty bowl. Move your eye away from the
bowl till you can no longer see the coin. Now, ask your friend to pour some
water into the bowl. Observe what will happen. The coin becomes visible
to us. Why?

5.5 Refractive Index

The ratio of the sine of angle of incidence to the sine of angle of refraction for a given pair of
media is constant which is called refractive index. It is denoted by µ. Since the refractive index
is a ratio of two similar quantities, the sines of angles, it has no unit. It is a pure number.

sin i
sin r
Refractive index (µ) =

If we consider a ray of light travelling from air to glass medium, then the ratio
sin i/sin r is called the refractive index of glass with respect to air, i.e.

sin i = µair glass
sin r

Worked out Numerical 1

Calculate the refractive index of a glass medium if the angle of incidence and angle of
refraction are 45° and 28° respectively.

Solution:

Angle of incidence (i) = 450
Angle of refraction (r) = 280

74 Oasis School Science - 9 PHYSICS

Refractive index (µ) = ?

We have,

µ = sin i = Sin 450 = 1.5
sin r Sin 280

∴ The refractive index of glass with respect to air is 1.5.

Refractive index of a medium can also be defined as the ratio of speed of light in vacuum to
the speed of light in the medium, i.e.

Refractive index (µ) = Speed of light in vacuum (c)
Speed of light in medium (v)

Worked out Numerical 2

Calculate the refractive index of water if the speed of light in vacuum and water are
3 × 108 m/s and 2.2 × 108 m/s respectively.

Solution:

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

Speed of light in water (v) = 2.2 × 108 m/s

Refractive index (µ) = ?

We have, µ = vc = 3 ×108 = 1.36
2.2×108

∴ The refractive index of water is 1.36.

5.6 Cause of Refraction of Light

We know that the speed of light is different in different media; it is less in water and more
in air. When light travels from one medium to another, the speed of light changes and this
change is the cause of refraction of light. Hence, refraction of light takes place while going
from one medium to another because the speed of light in different media is different.

Refraction of light through a glass slab

Consider a rectangular glass slab PQRS having parallel faces PQ and RS as shown in the figure
on the next page. A ray of light AB in air medium is incident on the glass surface PQ at the
point B. Since the ray AB enters from air (rarer medium) to glass (denser medium), the ray
bends towards the normal and follows the path BC. At point C, refraction takes place again.
Since the ray BC enters from glass (denser medium) to air (rarer medium), the ray bends away
from the normal, emerging out of the glass and following the path CD. The ray AB is called
the incident ray, BC is called refracted ray and CD is called the emergent ray.

PHYSICS Oasis School Science - 9 75

Incident ray, AB A N Q
Refracted ray BC P i
Emergent ray, CD
Lateral shift, XY B Glass slab
r

Normal NN' and MM' N' M
Angle of incidence, ∠ABN
longer path

Anlge of refraction, ∠N'BC R C S
Emergent angle, ∠M'CD shorter path e Y
lateral shift

M' X D

Fig. 5.4 Refraction of light through a glass slab

In the above figure, it is seen that the emergent ray CD is parallel to the incident ray AB, but
the emergent ray is laterally displaced from the original path of the incident ray by the
perpendicular distance XY. The perpendicular distance between the original path of the
incident ray and the emergent ray is called lateral displacement or lateral shift. Lateral
displacement is directly proportional to the (i) thickness of the slab, ii) incident angle and iii)
refractive index of the glass.

5.7 Real Depth and Apparent Depth N

When the ray of light travelling in a denser medium A r
enters the rarer medium, it bends away from the normal. A

Due to this phenomenon, the bottom of a pond appears i Water
to be raised and the depth seems to be less than that of Apparent depth

the actual one. The actual depth of the pond is called real I N' Real depth
depth. But the virtual depth at which a body appears due Coin

to refraction of light is called apparent depth. Fig. 5.5 O

Therefore, a coin lying at the bottom of a cup full of water

appears to be raised. Similarly, the bottom of a swimming pool filled with water appears to be

raised. In the given diagram, when we observe from point S, point O seems to be coming from

I. So, AI is called apparent depth and AO is called real depth.

The refractive index in terms of real depth and apparent depth is given by

Refractive index (µ) = Real depth
Apparent depth

where µ is the refractive index of the denser medium with respect to the rarer medium. The
4
refractive index of water with respect to air is = 1.33. Therefore, the depth of a pond appears
3 3

4 th of its real depth while viewing it from vertically above.

apparent /əˈpaerənt/ - that seems to be real but may not be

76 Oasis School Science - 9 PHYSICS

Worked out Numerical 3

Calculate the refractive index of water if the real depth and apparent depth of the bottom of
a pond is 3 m and 2.25 m respectively.

Solution:

Real depth = 3 m

Apparent depth = 2.2 m

Refractive index (µ) = ?
Real detph
We have, µ = Apparent depth = 3 = 1.33
2.2

∴ The refractive index of water is 1.33.

5.8 Effects of Refraction of Light

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

a. Bending of stick in water Apparent bend

A stick immersed in water appears to be bent and short Apparent position
at the surface of water when it is viewed obliquely from of stick
above. The rays of light coming from the lower end of
Surface

the stick bend away from the normal as light passes

from water (denser medium) to air (rarer medium).

As a result, a virtual image of the part of the stick Water

below water is formed as shown in the figure. Thus

the immersed part of the stick appears to be raised and Fig. 5.6 Bending of stick in water

bent on the surface of water.

b. A coin placed at the bottom of a vessel full of water appears to be raised

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

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

i) A water pond or a pool appears less deep than its actual Coin (real position)
depth.
Fig. 5.7

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

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

virtual /ˈvƷːtʃʊəl/ - almost or very nearly the thing described Oasis School Science - 9 77
PHYSICS

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

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

Reasonable fact-1

If a spear is shot exactly at the place where the fish is seen in water, the
fish does not get killed, why?
The fish inside water appears slightly higher than its actual position due to
refraction of light. As a result, the fish does not get killed if a spear is shot exactly
at the place where the fish is seen in water.

2. When light travels from a rarer medium to a denser medium, it bends towards the normal.
A number of phenomena can be observed in our daily life based on this principle. Some
of them are as follows:

a. The sun is visible two minutes before the actual sunrise and two minutes after the
actual sunset

The actual sunrise takes place when the sun is just above the horizon. When the sun is
just below the horizon, the light rays coming from it suffer atmospheric refraction when
they enter a denser medium (lower atmospheric layers) from a rarer medium (upper
atmospheric layers). So they bend towards the normal at each refraction. Due to the
continuous refraction of light rays coming from the sun at each layer of atmosphere, they
follows a curved path as shown in the figure and reach the observer's eye. As a result, we
can see the sun two minutes earlier than it rises above the horizon in the morning due to
atmospheric refraction of light.

Apparent sun

Horizon

Observer

Earth

Rising sun

Fig. 5.8

It is also due to atmospheric refraction of light that we can still see the sun for two minutes
even after the sun has set below the horizon. Thus a day would have been shorter by four
minutes if the earth was without an atmosphere.

horizon /həˈraɪzn/ - the furthest that you can see, where the sky seems to meet the land or the sea

78 Oasis School Science - 9 PHYSICS

Fig. 5.9

b. The stars seem higher than they actually are
The stars seem to be higher in the sky than they actually are due to the atmospheric

refraction of light. The light rays coming from the stars suffer atmospheric refraction on
entering earth's atmosphere as light rays come from a rarer medium to a denser medium.
So they bend towards normal at each refraction. Due to the continuous refraction of light
rays at each layer of the atmosphere, they follow a curved path as shown in the figure.
As a result, the stars seem higher than they actually are.

Apparent position

Real position

Refractive index of
air is increasing

Fig. 5.10

c. Stars twinkle but planets do not twinkle

The twinkling of stars is due to atmospheric refraction of light rays coming from the
star. The rays of light coming from the star suffer atmospheric refraction due to varying
optical densities of air at various altitudes. The continuously changing atmosphere
refracts the light from stars by varying amounts and in different directions from one

twinkle /ˈtwɪŋkəl/ - to shine with a light that keeps changing from bright to faint to bright again

PHYSICS Oasis School Science - 9 79

moment to the next. When the atmosphere refracts more starlight towards us, the star
appears to be bright and when the atmosphere refracts less starlight, the star appears to
be dim. In this way, the amount of starlight reaching our eyes increases and decreases
continuously. As a result, stars twinkle at night.

On the other hand, planets do not twinkle at night. The planets are very close to the earth
as compared to the stars. So, the intensity of light that we receive from planets is very
large. So small variations in their position and brightness are not noticeable. Thus, the
brightness of a planet always remains the same. The continuously changing atmosphere
is unable to cause variations in the light rays coming from big planets. Therefore, planets
do not twinkle at night.

5.9 Critical Angle and Total Internal Reflection

A ray of light bends away from the normal when it passes from a denser medium to a rarer
medium. Let us observe the given diagram.

ab

90° c
ir d

a bc d

Fig. 5.11 Refraction of light and total internal reflection

For ray b, the angle of incidence is less than the angle of refraction. When the angle of
incidence in the denser medium is increased gradually, the corresponding angle of refraction
also increases. For a certain angle of incidence in the denser medium, the corresponding angle
of refraction becomes 900. In the diagram, the ray c gives the angle of refraction 900. This
particular angle in the denser medium is called critical angle.

Rarer r Rarer r = 900 Rarer ir
Denser i Denser i c

Denser

Fig. 5.12 Critical angle and total internal reflection

The angle of incidence in the denser medium for which the corresponding angle of refraction
in the rarer medium is 900 is called critical angle. It is denoted by C or ic.

80 Oasis School Science - 9 PHYSICS

For example, the critical angle for ordinary glass is about 420 with respect to air. It means that
when the angle of incidence of a ray of light in glass is 420, the angle of refraction in air is 900.

When the angle of incidence is further increased beyond the critical angle, the ray of light
reflects back to the same medium instead of refraction. This phenomenon is called total
internal reflection of light. During this process, no light is refracted or transmitted or absorbed
by the surface of separation. The total incident light is reflected back into the denser medium.

Conditions for total internal reflection
i. The ray of light must pass from a denser medium to a rarer medium.

ii. The angle of incidence in the denser medium must be greater than the critical angle.
Critical angles for some substances with respect to air are given below:

S.N. Substances Refractive Index Critical Angle

1. Ice 1.31 500
2. Water 1.33 490
3. Alcohol 1.36 480
4. Paraffin 1.44 440
5. Turpentine 1.47 430
6. Glycerine 1.47 430
7. Glass 1.5 420
8. Diamond 2.42 240

Relation between the refractive index of a medium and the critical angle
When a ray of light passes from a denser medium (glass medium) to a rarer medium (air
medium), then according to Snell's law,

We have, µ =glass Sin i
air Sin r

For the incident ray moving from a denser medium to a rarer medium, the angle of refraction

is 900 and the refracted ray is parallel to the glass-air interface. Thus, i = ic (critical angle)
and r = 900.

Then, µglass = sin ic
air sin 900

= sin ic  sin 900 =1

But µglass = 1
air µair
sin ic
glass sin 9∴00 glass

µair = sin ic ====1ssiin1n1iaciicrµ1glass

glass = sin ic  sin 900
=Here,aaiirr 1or  µµggllaassss aairir
µair 

µglass is the refractive index of the denser medium (glass) with respect to the rarer

sin ic =mis ekdnioauiwrmµ1n(g,altaihsrse). Thus, if the refractive index of the denser medium with respect to rarer medium
critical angle (iC) can be calculated.

glass 1  PHYSICS Oasis School Science - 9 81
sin ic 
µair = 

Worked out Numerical 4

Calculate the critical angle of water medium with respect to air if the refractive index of
water is 1.33.

Solution: = 1.33
Re fractive index (µ) =?
Critical angle (ic )
= 1
We have, sin ic
µ
= 1
or 1.33 sin ic

or sin ic = 1
1.33
or ic
( )= 1
Sin −1 1.33

= 48045' ≈ 490

The critical angle of water medium with respect to air is 490.

5.10 Total Internal Reflection in a Prism

A prism is made of glass. Since the refractive index of glass is about 1.5, the critical angle of
glass with respect to air is about 420. Therefore, a ray of light travelling from glass (denser
medium) to air (rarer medium) and incident on glass-air interface at an angle greater than 420
suffers total interval reflection.
A prism having an angle of 900 between two refracting surfaces and remaining angles being
equal to 450 is called a total reflecting prism. Such prism is generally used,

i) to deviate a ray of light through 900.

ii) to erect the inverted image without deviation.
iii) to deviate a ray of light through 1800.

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

900 450
450
R
450 450 Q 450 450 450 450 R

Q R
Fig.5.13

prism /ˈprɪzəm/ - a transparent glass or plastic object, in the shape of a triangle, which separates light that passes
through it into the colours of rainbow

82 Oasis School Science - 9 PHYSICS

5.11 Applications of Total Internal Reflection Fig. 5. 14

1. Sparkling of a cut diamond

The critical angle for diamond in air medium is very low, i.e.
about 240. The faces of diamond are cut in such a way that
once a ray of light enters into it entering angle of incidence is
greater than the critical angle. So, a ray of light passing through
diamond suffers repeated total internal reflection at a large
number of faces. As a result, several faces of the diamond
appear silvery. Therefore, a cut diamond sparkles.

2. Light pipe

A transparent rod made of glass or plastic is called light pipe. It is made of a bundle of
highly transparent and flexible glass fibres called optical fibres. The light ray that falls on
each optical fibre gets internally reflected as shown in the figure.

Fig. 5.15 Light pipe

Uses: Light pipes are used by the doctors to view the internal organs of a human body.
Such device is called endoscope. Light pipes are also used for telecommunication for
transmitting signals. They are also used to transmit the images of the objects.

Reasonable fact-1

Internal parts of human body can be seen with the help of a light pipe,
why?
Light pipe is made up of optical fibres. When light pipe is inserted inside the
human body, the light rays coming from internal parts of body suffer total internal
reflection. As a result, the internal parts of the body can be seen.

3. Mirage

Mirage is an optical illusion of water in which Denser layer of air
a pond-like structure is seen on the surface of Tree Rarer layer of air
coal-tarred roads or in the hot desert. In this
phenomenon, an inverted image of a distant Virtual image
object is seen along with the object itself on a hot Fig. 5.16 Mirage
sunny day. It is caused due to the total internal
reflection of light in upward direction. In mirage,
the inverted image of a distant object appears as if
it were the reflection from the surface of water.

Mirage is formed on a very hot day when the surface of the road or desert is heated and

sparkle /ˈspɑːkl/ - to shine brightly with small flashes of light
mirage /ˈmɪrɑːʒ/ - optical illusion of water that is seen in hot deserts or coal-tarred roads

PHYSICS Oasis School Science - 9 83

the air molecules above it become hot. It results in the formation of different layers of
air of different density. The density increases from surface of the road. The air with high
density acts as a denser medium. So the light travelling down from upper layer of air
bends away from the normal as it goes down from cold layers to hot layers of air. After
coming down through a number of layers, the ray of light suffers total internal reflection
and it starts going in upward direction. As a result, we see the mirage.

4. Shining of air bubble in water

The bubble of air rising up in an aquarium appears silvery due to total internal reflection
of light.

The critical angle for water-air interface is 490. When the ray of light propagating in water
(denser medium) is incident on the surface of the air bubble (rarer medium) at an angle
greater than 490, total internal reflection takes place. As a result, the bubble of air in water
shines brightly.

Air i>c
bubble

Fig. 5.17

5.12 Dispersion of Light

When a beam of light is passed through a prism, it splits into seven different colours. This
process is called dispersion of light. The band of seven colours obtained on the white screen
is called spectrum. The seven colours of the spectrum are: violet (V), indigo (I), blue (B), green
(G), yellow (Y), orange (O) and red (R). It shows that the white light is a mixture of seven
colours. So, the phenomenon of splitting white light into its constituent colours on passing
through a glass prism is called dispersion of light. This phenomenon was discovered by
Newton by passing sunlight through a prism in his dark room rainbow is formed due to
dispersion of light. When the sunlight passes through the rain water, it gets dispersed in seven
colours. As a result rainbow is formed.

White light beam VIBGYOR

Glass prism
Fig. 5.18 Dispersion of light through a glass prism

dispersion /dɪˈspɜːʃn/ - the phenomenon of splitting white light into seven colours on passing through a prism
spectrum /ˈspektrəm/ - a band of coloured lights in order of their wavelengths as seen in a rainbow

84 Oasis School Science - 9 PHYSICS

Cause of Dispersion of Light

Light travels in the form of wave. It has a very short wavelength (average wavelength is
5 × 10-4 mm). Among seven colours, red ray has the longest wavelength (i.e. 7×10-4 mm) whereas
violet ray has the shortest wave length (i.e. 4 × 10-4 mm). The wavelength increases gradually
from violet to red. It is to be noted that all the rays have the same speed in the vacuum. But
they have different speed in a denser medium. The ray having the longest wavelength has the
highest speed whereas the rays with shorter wavelength have slower speed. So, the violet ray
has the lowest speed and red ray has the greatest speed. Therefore, the dispersion of white
light occurs because the angle of refraction or angle of bending of different colours is different
when passing through the glass prism. While passing out from the opposite face of the prism,
they do not suffer any dispersion but suffer refraction. As a result, they separate out further
and form a band of colours on a white screen which is called the spectrum.

Activity 2

• Take a right angled prism. Incident a light ray normally on its one face in a dark
room. Observe how the ray travels.

(a) (b)

Fig. 5.19

When the light ray strikes the second face of the right angle prism, the angle will be

45°, which is more than 42°(critical angle of glass). Therefore, the ray suffers total in-
ternal reflection. Due to the reflection of all the rays of light there is no change of the
intensity. This is the reason why prisms are used in binoculars and periscope instead

of mirrors.

The light shows the dispersion when it passes through the prism but not through the glass
slab, why?

A prism has a triangular shape. When a ray of light strikes a surface it bends towards the base
of the prism and gets dispersed. Furthermore they refract in the same direction when they
emerge out from another surface of the prism.

AD E
White spot

White light beam spWehicttre luigmht Coloured

Glass prism Beam of C light F
white light
Fig. 5.20 (a) B

(b)

PHYSICS Oasis School Science - 9 85

A glass slab can be considered as the combination of two prisms as shown in the fig. 5.20 (b).
When one prism ABC splits the rays of light, another prism DEF combines the seven colours
of spectrum to give back white light on emerging out of the second prism. In the same manner,
the dispersed rays of light recombine by forming white light while coming out of the parallel
face of the glass slab. So, there is no dispersion of light through the glass slab.

5.13 Electromagnetic Waves

The waves which travel by making electric field and magnetic field are called electromagnetic
waves. Electromagnetic waves can travel through vacuum. The solar radiation can travel
through vacuum. So, it is a type of electromagnetic wave. Gamma rays, X-rays, UV-rays,
visible light, infrared ray, radio wave, etc. are the electromagnetic waves. Electromagnetic
waves of a short wave length can carry more energy. So, they can easily penetrate a body of
a certain thickness. These rays are harmful to our body. They have higher frequency since
frequency is inversely proportional to the wavelength of the wave. It is due to the reason that
electromagnetic waves have the same speed in the vacuum.
The speed of the electromagnetic waves can be calculated by the given wave equation.
Speed (v) = frequency (f) × wavelength (λ)

∴ v=f×λ

5.14 Electromagnetic Spectrum

The orderly classification of electromagnetic waves according to their wave length or frequency
is called electromagnetic spectrum. The complete electromagnetic spectrum in the increasing
order of their wave length is i) Gamma rays ii) X-rays iii) Ultra-violet radiations iv) Visible
rays v) Infrared radiation vi) Microwaves vii) Radio waves.

The electromagnetic spectrum with the wavelength, source and nature of electromagnetic
waves is given below:

Fig. 5.21 Electromagnetic spectrum
electromagnetic /ɪˌlektrə(ʊ)maɡˈnetɪk/ - having both electrical and magnetic characteristics

86 Oasis School Science - 9 PHYSICS

Fig. 5.22 Electromagnetic waves

Properties of Electromagnetic Waves
1. Electromagnetic waves travel at the same speed in vacuum ( i.e. 3 × 108 m/s).
2. They are transverse waves.
3. They do not need any material medium for propagation.
4. They obey laws of reflection and refraction of light.
5. They are not affected by any electric and magnetic field.

Uses of electromagnetic waves
1. Gamma-rays are used in radiography, treating cancer and measuring the thickness of

objects.

2. X-rays are used in radiography, treating cancer tumors (radiotherapy), and CAT scan (to
detect gold, silver, revolver, etc. concealed in bags).

3. Ultra-violet radiations are used for sterilizing food, detecting invisible writing and finger prints.

4. Visible light is used in photography, photosynthesis, etc.
5. Infra-red radiations are used for heating, in night vision devices, treating muscular

strain, etc.
6. Microwaves are used in microwave ovens for cooking/warming food, communication in

RADAR, etc.

7. Radio waves are used for communications like in radio, television, telephone transmission.

conceal /kənˈsiːl/ - to hide sth/sb
sterilize /ˈsterɪlaɪz/ - to kill the bacteria in or on sth

PHYSICS Oasis School Science - 9 87

Comparative study of infra-red radiation, visible light and ultra-violet radiation

Infra-red radiation Visible light Ultra-violet radiation

1. It is invisible 1. It is visible. 1. It is invisible.

2. It produces maximum 2. It produces heating effect. 2. It does not produce any
heating effect. heating effect.

3. It has no effect on photo- 3. It darkens the photo- 3. It darkens the photo-
graphic plate.
graphic plate. graphic plate.

4. It does not affect human 4. It does not affect human 4. It causes health hazards
like skin cancer.
health. health.

5.15 X-rays and Ultra-violet rays

X-rays: The wavelength of x-ray ranges from 0.01 nm to 10 nm. X-rays can penetrate skin and
muscles but not bones. Over exposure to x-ray may cause cancer. The major uses of x-rays are
as follows:

i) X-rays are used in radiography to examine fractured bones.

ii) X-rays are used in radiotherapy to treat cancer tumours.

iii) X-rays are used in CAT scan (to detect gold, silver, revolver, etc. concealed in bags) in
airport, custom offices and other sensitive zones.

iv) X-rays are used to study molecular structure of crystals.

Ultra-violet rays : The wavelength of ultra-violet rays ranges from 10 nm to 400 nm. The
exposure of human body to the ultra-violet rays (UV-rays) for a long time is dangerous. It
causes skin cancer, eye-disease like cataract, etc. The major uses of ultra-violet rays are as
follows:
i) Ultra-violet rays are used to find out the purity of ornaments, egg, ghee, etc.
ii) Ulta-violet rays are used in sterilization of medical equipment.
iii) Ultra-violet rays are used to produce vitamin D in food of animals and plants.

88 Oasis School Science - 9 PHYSICS

SUMMARY

• The process of bending of light when it passes from one medium to another is called
refraction of light.

• The ratio of the sine of angle of incidence to the sine of angle of refraction for the given
pair of media is constant which is called refractive index.

• The actual depth of a pond is called its real depth. But the virtual depth at which a
body is seen is called apparent depth.

• Critical angle is the angle of incidence in the denser medium for which the corre-
sponding angle of refraction in the rarer medium is 900.

• Mirage is an optical illusion which can be observed in hot deserts and
coal-tarred roads due to the total internal reflection of light in upward direction.

• When a beam of light is passed through a glass prism, it splits into seven different
colours. This process is called dispersion of light.

• The velocity of different coloured rays of the light is different in different media. Due
to the difference in their speed, dispersion takes place.

• The wave which travels by making electric field and magnetic field is called electro-
magnetic wave.

• The electromagnetic waves having shorter wavelength like X-ray, UV-ray, gamma
ray, etc. are very harmful.

• Infra-red ray causes maximum heating effect but ultra-violet radiation does not pro-
duce any heating effect.

• Electromagnetic wave travels with the speed of light in vacuum in a straight path and
its speed can be calculated by the wave equation, i.e. v= f × λ.

Exercise

Group-A

1. What is light?
2. Name any two sources of light.
3. What is refraction of light?
4. State the laws of refraction of light.
5. Define:
i) Incident ray ii) Normal iii) Reflected ray iv) Angle of incidence
v) Angle of reflection
6. What are refracted ray and angle of refraction?

PHYSICS Oasis School Science - 9 89

7. Define emergent ray and emergent angle.

8. What is the velocity of light in air and glass medium?

9. What is a denser medium and a rarer medium?

10. What is the relationship between velocity of light and refractive index?

11. What is the refractive index of the given media?

i) Water ii) Glass iii) Ice iv) Alcohol

12. What is total internal reflection of light?

13. What are the conditions necessary for total internal reflection of light?

14. What are the limitations of Snell's law?

15. What is light pipe? Why is it used?

16. What is optical fibre?

17. What is the relationship among refractive index, real depth and apparent depth?

18. What is dispersion of light?

19. What are the seven colours that appear due to the dispersion of light?

20. What are electromagnetic waves? Write with examples.

21. Write down any two properties of electromagnetic waves.

22. What is frequency? Write down its SI unit.

23. What is the relationship between wavelength and frequency?

24. What is the speed of electromagnetic wave in a vacuum?

25. What is endoscope? Why is it used?

26. What is the apparent image of water called which is seen in a desert or pitched road?
Group-B

1. Light bends when it passes from one medium to another, why?

2. The depth of a pond appears less than the real depth. Give reason.

3. A coin placed in a beaker appears to be raised when the beaker is filled with water, why?

4. If a spear is jabbed exactly at that place where the fish appears in water, the fish is not killed.
Give reason.

5. Diamond sparkles even in dim light, why?

6. Light pipe is used to observe internal body parts, why?

7. Mirage is seen in coal-tarred road on hot days. Give reason.

8. Why is x-ray called an electromagnetic wave?

9. Write any two differences between denser medium and rarer medium.

10. Differentiate between real depth and apparent depth in any two points.

90 Oasis School Science - 9 PHYSICS

11. Differentiate between refraction of light and dispersion of light.
12. Write any two differences between X-ray and visible ray.

Group-C
1. What are the conditions necessary for total internal reflection of light? Write any two

applications of total internal reflection of light.

2. What is endoscope? Why is it used? What is the relationship among refractive index, real
depth and apparent depth?

3. In which condition is mirage seen? Explain with figure.

4. Write down the importance of X-ray in the field of medical science.

5. Write down the importance of electromagnetic waves.

6. Write any three uses of UV-rays.

7. Write cany three applications of electromagnetic waves.

Group-D

1. Draw a neat diagram showing dispersion of light through a glass prism. Differentiate
between X-ray and Infra-red ray in any two points.

2. Answer the following questions on the basis of given ray Light A
diagram. B

i) Name the rays A and B.

ii) Which ray has more speed (A or B) in the glass Prism
medium?

iii) Which phenomenon is shown in the given diagram?

iv) Ray B is bending more than the ray A. What is the
reason behind it?

3. Study the given diagram and answer the following A N
i
questions: P Q
B
i) Which phenomenon is shown in the given r Glass slab
diagram?
N' M

ii) Name the angle of incidence, angle of refraction R shorter path C
S
and emergent angle in the given diagram. Y

iii) Which medium is denser out of air and glass? e
Explain the reason. lateral shift

M' X D

4. A ray of light is passing through an another medium by making an angle of 450. If the
refractive index of the medium is 1.4, calculate the magnitude of angle of refraction. The
apparent depth of water in a pond appears less than the real depth. Why?

5. The speed of light in air is 3 × 105 km/s and the refractive index of paraffin is 1.44.
Calculate the speed of light in the paraffin. Starts twinkle but planets do not. Justify this
statement.

PHYSICS Oasis School Science - 9 91

6UNIT Estimated teaching periods

Theory 8
Practical 2

sound

Objectives Heinrich Hertz

After completing the study of this unit, students will be able to:

• describe the nature of sound wave.

• find the sources and frequency of infra, audible and ultra sounds.

• describe the reflection and refraction of sound with appropriate figure and
explain its effect in daily life.

• find the speed of sound, intensity and pitch.

• describe the effects of sound in the environment.

• find the effects of noise pollution and ways to control it.

6.1 Introduction

In our everyday life, we hear a variety of sounds around us. At home we hear the sound of
family members. We also hear the sound of telephone ring, mobile phone, television, etc. At
school, we hear the sound of students, teachers, school bell, etc. All these sounds produce
sensation of hearing. It is a form of energy produced by vibrating objects. Thus, sound is a
form of energy which produces the sensation of hearing.

We can hear the sound coming from a distant place through the material medium. Sound
travels through the medium in the form of waves, called sound waves. Actually, there are two
types of waves: longitudinal wave and transverse wave. Sound wave is a longitudinal wave.

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

6.2 Wave Motion

If a piece of stone is thrown into a pond of still water, circular ripples are produced on the
surface of the water. These ripples carry energy from the stone to all parts without actual
movement of the water from one point to another. This type of motion is simply called wave

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

transverse wave /trænzˈvɜːs weɪv/ - a wave that vibrates at 90º to the direction in which it is moving

92 Oasis School Science - 9 PHYSICS