Blooming science-8 part-1- press

Liquids transmit pressure equally in all directions

This law is also called Pascal’s law.

Verification: Liquids transmit pressure equally in all
directions.

Take a hollow rubber ball or polyethene bag. Make a Fig: Liquid transmits
number of small pin holes. Fill the ball with water pressure equally in all
and press it. Water comes out of all the pin holes
simultaneously. This shows that the increase in pressure directions
on the liquid at some point is communicated equally in
all directions.

Pascal’s Law

This law is formulated by the French Scientist Blaise Pascal in 1647 A.D. who was
born in 1623 A.D. in France. This is one of the basic principles of Hydrostatics.
Hydrostatics is that branch of physics which deals with the properties of fluids
(liquids and gas) at rest.

Pascal’s law states that “when pressure is applied on a liquid enclosed in a vessel, it
is transmitted equally in all the directions.” The pressure acts at right angles to the
surface of the vessel exposed to the liquid.

Liquid Maintains its Level

Following activities can be done to verify liquid maintains level.

Scan for practical experiment

1. Take a glass and fill it with water. The water level is
parallel to the ground. Now, tilt the glass and you will see
that the water level is still parallel to the ground. This
proves that liquid maintains its level.

visit: csp.codes/c08e06

Fig: Liquid maintains its level

2. Take a U-shaped tube and fill it with water. You can see

that the liquid level is same in both the legs of the tube.

This also proves that the liquid maintains its level. Fig: Liquid maintains its level

Blooming Science & Environment Book 8 51

3. In figure, you are given a glass vessel which has Fig: Liquid maintains its level
different structures attached. As you go on filling
water in this kind of vessel, you will notice that the
liquid level in each structure is the same.

Atmospheric Pressure

The earth is surrounded by air approximately up to the height of 9600 km from the
surface of the earth. This region of the air, which surrounds the earth, is called the
atmosphere. The atmosphere of the earth is a mixture of different gases.Near the
surface of the earth the air is made up of about 78% of nitrogen, 21% of oxygen,
slightly less than 1% argon, tiny amount of carbon dioxide and traces of other gases.
Because of the gravity, these gases tend to accumulate near the surface of the earth.
Therefore, the air is denser at the sea level and becomes thinner and thinner as we go
up. The air has weight hence, it exerts pressure.

Thus, the pressure exerted by the atmosphere is called the atmospheric pressure.
The atmosphere exerts pressure of about 105 Nm-2 at the sea level. The atmospheric
pressure decreases as we go up from the sea level. Therefore, it is less at the top of
Mt. Everest than in Janakpur and Nepalgunj.

The atmospheric pressure acts not only on the surface of the earth but also on all the
objects including living beings. The surface area of an average sized man is 2m2. The
total force exerted on his body by the atmosphere is given by,

F = P × A

= (105Nm-2) × (2m2)

= 2 × 105N

This is a great force. But we do not feel such great force on our body. This is due to
the fact that blood in our body exerts nearly equal pressure. Thus, the pressure inside
our body is about equal to the atmospheric pressure.

We feel uneasy at very different pressure. At the high altitude the pressure of the air is
less. At such place nose-bleeding may occur due to the greater pressure of the blood.
Similarly, if living organisms are placed in a vacuum, the cells of the organisms will
explode because of their internal pressure.

Due to the difference in pressure at different altitudes, the air blows from place to
place. Aeroplanes and jet planes fly at very high altitudes. The air pressure in these
planes is adjusted to make the passengers comfortable so that they can breathe easily.

52 Blooming Science & Environment Book 8

Activity

To observe atmospheric air exerts pressure.

Material needed: A glass tumbler, water,cardboard paper method:
Method:
1. Take a glass tumbler and fill water in it.
2. Cover it with a cardboard and invert it with the help of your palm.
3. Hold the glass tightly and remove the supporting palm and observe whether
water falls or not.
The water and postcard does not fall down as the atmospheric pressure exerted
by air to the cardboard.

Importance of Atmospheric pressure
Atmospheric pressure is very important in our daily life. Some useful devices used
in our daily life work due to atmospheric pressure.Some of the important uses of
atmospheric pressure are given below :

1. Atmospheric pressure is useful to fill ink into a pen.

2. Medicine & water can be filled in a syringe due to atmospheric pressure.

3. It makes possible to fill air and inflate balls, bicycle tyres, vechiles' tyres, etc, with
the help of pump.

4. A hand pump lifts water due to the effect of atmospheric pressure .

5. We can drink juice, cold drinks, etc by using a straw pipe with the help of
atmospheric pressure.

Blooming Science & Environment Book 8 53

Main Points to Remember

1. A sharp knife exerts more pressure than a blunt knife.

2. The pressure is defined as the force (N) acting in unit surface area (m2).

Pressure = Force (N) , P= F
Area (m2) A

3. A body which has very less weight can even exert more pressure than a heavy

body if the surface area occupied by it is small.

4. Air occupies space. Air has weight and it can exert pressure.

5. The envelope of the air that surrounds the surface of the earth is called
atmosphere.

6. Air is everywhere surrounding us. Air pressure can be measured by using
barometer.

7. Force and pressure are related. Force can bring a body into motion or a body in
motion can be brought to rest.

8. Force can be measured in Newton (N) or dynes. Pressure is measured in
Newton per square metre (N/m2) or Pascal

9. Normal atmospheric pressure is about 760 mm of Hg at sea level.

10. Liquid exerts pressure equally on all sides.

11. The pressure of the liquid increases as the height of the liquid column just
above it increases (P ∝ h).

12. The pressure of the liquid increases as the density of the liquid increases (P ∝ d).

13. The pressure at any point within liquid is the same in all directions.

14. The pressure at a point within liquid does not depends on the volume of the
liquid in the vessel but it depends only on the height of the liquid above the
point.

15. The pressure at a point is independent of the shape of the vessel.

16. Liquid maintains its level. Liquid flows from higher level to lower level.

PRO J ECTWORK

Take a metal can with lid. Put some water in it and heat it by opening its mouth.
Now close the lid tightly after water is boiled and pour some cold from above
and observe. write down the conclusion and discussion in your class.

54 Blooming Science & Environment Book 8

Exercise

1. Fill in the blanks.
a) The use of ski helps a person to glide smoothly over snow because the ski

__________ the area over which his weight acts.
b) Force exerted per unit area is called ______________.
c) The atmospheric pressure is highest at _____________.
d) The larger the area over which the force acts, the _____________ is the

pressure.
e) The pressure exerted by the atmosphere is called _______________.
f) Liquid find its own __________.
g) The atmospheric pressure decreases with _____________.
h) Pressure caused by air in the atmosphere is called ________.
i) Atmospheric pressure changes in the _____________.

2. Put a tick mark ( ) if the statement is correct and cross ( ×) if the statement
is wrong.

a) Only closed containers have air pressure.
b) A bicycle or motor tube bursts out if there is too much air in it.
c) Atmospheric pressure decreases as we go up.
d) Pressure is more when force acts on a large surface area.
e) At the same depth, the pressure exerted by a liquid, depends on the direction.
f) Every square centimetre of your body experiences a force equal to the

weight of 1 kg. due to the atmospheric pressure.
g) Atmospheric pressure is high at high mountains.
h) Sudden fall in atmospheric pressure indicates fair weather ahead.
i) At sea level, the atmospheric pressure is equal to pressure exerted by

column of mercury of height 76 cm.
g) The atmospheric pressure at the sea level is 860 mmHg.
3. What happens when:
a) you press the balloon filled with air?
b) you dip an open bottle in a bucket full of water slowly?
c) you fit the balloon at the nozzle of the tap and open it slowly?
d) you fill the balloon with water and make pin-pricks at the same level?
e) you go on filling air in the balloon? Give the reason.

Blooming Science & Environment Book 8 55

10 cm4. What is pressure? In which unit is it measured?
20 cm5. What do you mean by atmospheric pressure?
6. Where is the pressure greater, at bottom of bucket filled with water or in

the middle of water level?
7. How can you show that liquid exerts pressure? Give an example.
8. Give an activity to show the effect of atmospheric pressure.
9. How do the following devices utilize atmospheric pressure?
a) The syringe b) A drinking straw
10. Give reasons:
a) As the altitude increases, pressure decreases.
b) Pressure exerted by a liquid differs according to its depth.
c) When air inside a metal can is taken out by heating, the lid is closed and

cold water poured on outsides, the can collapses.
d) Why is the wheel of a tractor wider than the wheel of a car?
e) A girl standing on her heels exerts more pressure on the ground than an

elephant standing flat on its feet.
f) A doctor pushes the plunger of a syringe down and then pulls it up to

draw liquid into a syringe.
g) A bucket filled at first floor takes less time than the one at the upper level

tap, why?
h) Why does an air filled balloon on the earth surface burst at a great height?
i) The base (bottom) of dams are made broader than tops, why?
k) Water does not fall when a glass filled with water and covered with a

piece of cardboard paper is inverted.

11. Which one exerts more pressure and why?

5 cm

10 cm
2 cm

5 cm



56 Blooming Science & Environment Book 8

Numerical problems

12. Solve the following numerical problems:

a. A box weight 900 N and its base is a square of side 2m. What pressure

does the block exert on the ground. [Ans. 255 Pa]

b. The weight of a box is 300 kg. The lower surface area is 150 square meter.
What is the pressure exerted by the box? (g = 10m/s2)

[Ans. 20 N/m2 or 20 Pa]

c. A 5m × 4m × 3m tank is filled with water. What pressure is exerted by it

at its buttom.

(Given; density of water = 1000 kg/m3 and g = 100m/s2)

30000(Pa)

d. Calculate the pressure exerted by a liquid at 4m if its density is 700kg/m3.

(a) (b) (c)

Glossary

Pressure : force acting per unit area

Atmospheric Pressure : pressure exerts by the air

Liquid Pressure : pressure exerted by liquid

Pressure Gauge : instrument to measure the air pressure in tyres

Manometer : instrument to measure the atmospheric pressure

Standard Atmospheric Pressure : atmospheric pressure at sea level (760 mm of Hg)

Blooming Science & Environment Book 8 57

Chapter

5 Work, Energy and Power

Learning outcomes Estimated Periods: 3

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

 tell the relationship and differentiate work, energy and power.

 describe and demonstrate transformation of energy.

 write solve the formulae and numerical and problems related to work,

energy and power.

We have to do different kinds of work in our daily life. Some kinds of works are
related with the movement of objects. Energy is necessary to do work. Energy
produces force necessary to do work. Some people, machines, animals can provide
more energy to do work. They can do work faster and so they have higher power than
those who do work slowly. So, work, energy and power are inter-related and they are
working everywhere around us.

Work

The meaning of work in science is a little bit different from its meaning in everyday
use. Force is necessary to move an object. If force applied cannot move an object,
then work is not done.

Work is said to be done by a force only when the force applied on a body moves it in
the direction of the force.

In figure a body is lifting a book from the ground and he is doing work. But in second
case, the man could not move the vehicle, so he can’t do the work.
The amount of work done depends upon (i) amount of force used and (ii) the distance
moved by the force in the direction of the force.

58 Blooming Science & Environment Book 8

Hence,

Work done = force × distance moved

(W) = (F) × (d)

The amount of work done is the product
of force and the distance moved by the
force.

Kinds of Work 10 kg 2m

Whenever a force is applied, it always 10 kg
acts against another force. So, work is

also always done against a force. Work is mostly done against two kinds of force in

everyday life. They are:

a. Work against friction

b. Work against gravity

Work Against Friction:

Whenever a body slides on the other, a work is done against the friction.

Even in a leveled and smooth road, it is necessary to use a force to keep on a bicycle
in motion, otherwise it comes to rest after a while due to friction.

A constant use of a certain force on a body produces acceleration on the body. The
product of acceleration produced on a body and the mass of a body is equal to the
force used.

Therefore, Force (N) = mass (kg) x acceleration (m/s2)

F =m×a

The force required to produce an acceleration of 1 m/s2 in a mass of one kilogram is

called 1N. i.e. 1N = 1 kg x 1 m/s2 = 1 kg m/s2

Again, the magnitude of work done is the product of force and the distance moved
by a body in the direction of force.

Hence,

Work done (J) = force (N) x distance (m)

W =F×d

Now, if the force used in 1 Newton and distance moved is also 1m, then the work
done is equal to 1 joule.

i.e. 1J = 1N x 1m = 1Nm

When the application of 1 Newton of force moves a body to a distance of 1 metre,
the work done is said to be one joule.

Blooming Science & Environment Book 8 59

The SI unit of force is Newton (N) and that of distance is metre (m), so the SI unit
of work is joule.

Solved Numerical Problems

1. A man pushing a car has to overcome a force of 200N for a distance of
10 m. How much work does he perform?

Given,
F = 200 N
d = 10 m
Using the equation, W = F x d = 200N x 10 m = 2000 J.
\ The work done by the man = 2000 J.
2. The total weight of a woman and a bicycle is 100 kg. She pedals the bicycle

with such a force that the acceleration produced is 0.5 m/s2. How much
force did she use? How much work is done by her when she travels a
distance of 8000 metres (8km)?
Given,
a = 0.5 m/s2
m = 100 kg
Using the equation,
F = m x a = 100 kg x 0.5 m/s2 = 50 N
Again,
F = 50 N
d = 8000 m
Using the formula,
W = F x d = 50 N x 8000 m = 4,00,000 J = 4 x 105 J.
\ The force used by the woman = 50 N and work done by her = 4 x 105 J.
Work Against Gravity:
Can you lift a 50 kg mass from the ground? Why? We know that the earth is pulling
every mass on its surface towards its centre and the pulling force is called gravity.
Whenever we are lifting something from the ground, we are doing work against the
gravity. Not only that but also when you riding the steps of a stair case or a mountain,
you are lifting or pushing your body against the gravity.

60 Blooming Science & Environment Book 8

When a body falls to the ground, acceleration is produced in it due to the action of
the force of gravity and that is called acceleration due to gravity. Acceleration due to
gravity is constant for everybody, heavy or light and its value is equal to 9.8 m/s2 (10 m/
s2 approximately). When a body is lifted vertically upward, a work is said to be done
against gravity.

If you need to calculate the force necessary to lift a body from the ground, you can
use 10m/s2 as the acceleration due to gravity.

Therefore, in figure when the man rides the staircase,

Force = mass of his body (m) x acceleration due to gravity (g)

Here the force is the weight of his body.

or, F = mg = 50 x 10 = 500 N

Now, to reach the top of the staircase, he has to
ride 3m against the gravity.

Hence, Stair h =3 m
Work done = F x h = 500 N x 3m = 1500 J.

Here, the width of steps of staircase is not 50 kg
considered because the man is doing the work
against gravity only. Fig: Work is done against gravity
when he rides stair case.

Solved Numerical Problem

If a man lifts a load of 10 kg to a height of 2m, how much work does he perform?
Given,
m = 10 kg
g = 10 m/s2
Using equation, F = mg = 10 kg x 10 m/s2 = 100N
Again,
h = 2m
F = 100 N
Using the formula,
W = F x h = 100 N x 2m = 200 J
\ The amount of work done by the man is 200 J.

Blooming Science & Environment Book 8 61

Energy

Energy is defined as the ability to do work.

Energy produces force to do work. Energy present in our body enables us to move
our organs, to work, to walk etc. The energy in our body is seen as the force of our
muscles.

A battery contains chemicals that react to produce electric current. Electric energy
enables a fan to rotate. It also runs motors, vehicles etc.

Energy has several forms. They are:

1. Mechanical energy Scan for practical experiment
2. Chemical energy
3. Heat energy visit: csp.codes/c08e08
4. Light energy
5. Sound energy
6. Electrical energy
7. Magnetic energy
8. Atomic energy

1. Mechanical Energy

All the moving bodies and body which has capacity to move have a kind of energy
called mechanical energy. Mechanical energy has two forms:
(a) Kinetic Energy
(b) Potential Energy

Kinetic Energy:

Energy found in a moving object by virtue of its motion is called kinetic energy.
Running water of river has kinetic energy and electricity is produced from this
energy. Wind, tides in ocean, waves etc. are also sources of kinetic energy.

Kinetic energy of a moving body depends upon its mass and velocity. Kinetic energy
of a moving body is directly proportional to the mass of the body. What will happen
if a car and a truck with same speed collide with each other? Certainly, the truck will
push the car ahead because it has more mass and has more kinetic energy too.

Kinetic energy of a moving body also depends upon velocity of a body. A small
bullet can easily penetrate our body because it has very high velocity and the energy
is also high.

If a body of mass ‘m’ is moving with a velocity ‘v’, by experiment it is found that
kinetic energy is equal to 21mv2

62 Blooming Science & Environment Book 8

or, Kinetic energy (KE) = 12mv2
The equation shows that if velocity of a body is made double the kinetic energy
increases by four times.

Potential Energy:

Each and every body at a certain height has tendency to fall down to the ground
because it is acted upon by the force of gravity. So, water flows down in a river.
A football thrown upward falls down. Before falling down, these bodies posses a
form of energy called potential energy. Usually, the higher up a body, the greater is
its potential energy content and also the bigger the mass of a body, the greater is its
potential energy.

Similarly, tension is created in a spring or any
other elastic body, when it is pulled, pushed,
compressed or distorted. The potential energy
is stored in such body. This potential energy
created a force in the spring which tends to bring
it to its original position or state. When you lift a
leg to kick off a ball, some muscles of your legs
contract and other relax. Then, these muscles
gain potential energy.

Energy found in a body by virtue of its position or state of tension is called potential
energy.

Now it is clear that when work is done on a body, potential energy is gained by it.
When you lift a stone, or stretch a rubber, work is done on these bodies. The potential
energy gained is equal to work done on the body.

For example, if a body of mass ‘m’ is taken up to a height ‘h’ against the acceleration

due to gravity ‘g’, it has a tendency to fall back to the m
ground again.

Hence,

Work done = F x h = m x g x h

\ Potential energy (P.E.) = m x g x h h

Potential and kinetic energy are inter convertible. The water g
collected in a reservoir has potential energy. When it is
let to fall down, the potential energy changes into kinetic Fig: Body at height gains
energy which turns a grind mill or the turbine of a generator potential energy
to produce electricity. Potential energy stored in a spring of
clock makes it work.

Blooming Science & Environment Book 8 63

Some useful conversion of kinetic energy into work:

a. The wind has enormous kinetic energy. The kinetic energy of the wind can be
used for driving windmills to generate electricity, for lifting water from a well,
etc. Wind also propels sailboats. This means, moving wind has capacity to do
work i.e. it possesses energy.

b. The flowing water of a river possesses kinetic energy which can run water
turbine to grind grains and to produce hydro-electricity. This shows the flowing
water of the river has capacity to do work and thus possesses the energy.

Heat Energy

Heat is a form of energy. If flows from a body at higher temperature to a body at
lower temperature. It gives the sensation of warmth.

The sun is the main source of heat. The heat energy is used to cook food, to dry food,
etc. Heat is produced due to the combustion of inflammable products like petrol,
diesel, kerosene, wood, gas, coal, etc. The heat can be converted into useful mechanical
energy. For example, steam is generated when water is heated in a boiler by the
combustion of coal. The steam runs the engine and hence, the steam engine moves.

Light Energy

The light is a form of energy. It produces the sensation of sight in the eye. If a body is
heated, it is capable to emit light. The sun is the main source of light. Plants prepare
their food by photosynthesis in which sunlight plays a vital role. Solar heater, solar
cooker, solar cells, etc. use solar energy. When we light a candle, we get light. This
is the conversion of chemical energy of the candle into light energy.

Sound Energy

Sound is a form of energy that is possessed by the vibration of molecules in a
sounding body. For example, if we touch a ringing bell, it feels vibrating. Our ear is
not sensitive to all vibrations produced by a vibrating body. Our range of hearing,
i.e., audible range is from 20 Hz to 20,000 Hz frequency of sound.

Sound requires a material medium for its propagation. It cannot travel in vacuum.

Magnetic Energy

The energy stored in the magnet whose effects can be felt in the magnetic field is
called magnetic energy. A magnet can easily pull the pieces of iron nails towards it,
when placed in the magnetic field. The motion of these small pieces of iron towards
the magnet is due to the magnetic energy.

Magnet is used for many purposes. Big electromagnets are used in factories to lift the
heavy weights to separate iron pieces from a heap of waste materials. Besides this, it
is used in radio, telephone, telegraph, dynamo, etc.

64 Blooming Science & Environment Book 8

Electrical Energy

The electrical energy is possessed due to the continuous flow of electrons in a circuit.
Some major sources of electrical energy are hydropower stations, atomic power
station, battery, dynamo, etc.

Electrical energy is used to:
a. run machines in factories, electrical trains, lift, trolley buses, etc.
b. run fans in summer and heaters in winter.
c. run motor to pump out water from a well.

Modern life is not possible without electricity. Electricity is used for many purposes
in our daily life.

Chemical Energy

This energy can be released in different forms. For example, if a matchstick is struck
against a matchbox, it burns and gives us heat and light. Here, the chemical energy
stored in the matchstick is changed into heat and light energy on burning.

The chemical energy of petrol, diesel, etc. is used to run vehicles. When a torch is
switched on, the chemical energy stored in the cell is converted into heat and light
energy. Similarly, food eaten by us has chemical energy, which changes in other
forms during respiration.

Nuclear Energy

The energy which is released by the splitting of heavier atom into simpler atoms
(fission) or forming heavier atom from the fusion of simpler atoms (fusion) is called
nuclear energy. The nuclear energy is used in atomic power of the stations to produce
electrical energy. It can also be used for destructive purpose i.e. for making atom
bomb, hydrogen bomb etc. A large amount of radiation is emitted which has to be
shielded to protect people while producing atomic energy. Only some developed
countries are using nuclear energy.

Transformation of Energy Scan for practical experiment

When we tune our radio, sound is heard. Have we even thought visit: csp.codes/c08e07
where the sound energy comes from? This is transformed
from electrical energy. Thus, the conversion of energy from
one form to other form by using certain devices is called
the transformation of energy. Some examples of the energy
transformation are given below:

1. During burning, the chemical energy stored in coal or wood changes into heat
and light energy.

2. A solar cell changes light energy into electrical energy. These cells are commonly
used in electronic calculators, photographic cameras, etc.

Blooming Science & Environment Book 8 65

3. When two flint stones are struck against each other, sparks of light are produced
along with sound. Thus, the mechanical energy changes into light and sound
energy.

4. In a steam engine or in a diesel engine, the heat energy is converted into
mechanical energy.

5. In trolley buses, electrical motors, fans, etc. the electrical energy changes into
mechanical energy.

Power
The rate of doing work is called power.
Mathematically,

Power = Work done
Time

i.e. P = w
t

In SI unit, work is measured in joule (J) and time in second (s). Hence, unit of power

is Js-1. Js-1 is called watt.

If, Work (w) = 1 Joule
Time (t) = 1 second

Then, Work done 1 joule
Time 1second
Power (P) =

= 1 Js-1 = 1 watt

If one joule of work is done in 1 second, power is said to be 1 watt.

Other common units of power are Kilowatt (KW), Megawatt (MW) and Horse Power
(HP).

\ KW = 1000 watt (103 watt) [approx. 750 watt]
1 MW = 1000000 watt (106 watt)
1 HP = 746 watt

66 Blooming Science & Environment Book 8

Higher the power, faster the rate of doing work. For example, suppose a coolie A
takes one minute to raise a box of mass m through a height of 2 metres and another
one B takes 30 seconds for the same job. Then the coolie B has done work faster than
the coolie A. So the coolie B has more power. Power can be measured in terms of
animal power called horse power. 1 h.p. = 746 watts.

Differences between Work and Power

Work Power

1. Force which can produce displacement 1. The rate of doing work is called

along the direction of force is work. power.

2. It does not depend on time. 2. It depends on time.

3. Its SI unit is Joule. 3. Its SI unit is watt.

Solved Numerical Problems

1. If a crane lifts a load of 75N to a height of 20 m in 100s, what is the power
of the crane?

Solution:

Here,

Weight lifted by the crane = 75 N

Height raised (h) = 20m [ d = h]

Time taken (t) = 100s

Power (P) =?

We have

W = F x d

= 75N x 20m = 1500 Joule

\ P = W
t

= 1500 joule = 15 watt
100 second

Therefore, the power of the crane is 15 watt.

Blooming Science & Environment Book 8 67

2. Calculate the power of a porter, if he can carry 40 bricks at a distance of
75m away in 50 s. The weight of each bricks is 10N?

Solution:

Here, = 40
Number of bricks = 75m
Distance covered (d) = 50 s
Time taken (t) = 10N
Weight of each brick =?
Power (P)
Now,

Total weight of bricks (F) = Number of bricks x Weight of each brick [ W = F]
= 40 × 10 = 400N

Again,

We have

W = F x d

= 400N × 75m

= 30000 Joule

Again

P = W
t

= 30000 J = 600 watt
50 s

Hence, the power of the porter is 600 watt.

3. Find the work done by a person if he uses 20 N force to carry a body 15m
away. If the time taken to do that work is 2 s, find his power in kilowatt.

Solution:

Here,

Force applied (F) = 20 N

Distance covered (d) = 15m

Time taken (t) = 2s

Work done (W) =?

Power (P) =?

We have,

W = F x d

68 Blooming Science & Environment Book 8

= 20 N x 15 m = 300 Joule

Again,

P = W
t
300 J
= 2s = 150 watt

150 kw = 0.15 kw [ 1000 watt = 1 kw]
1000

Therefore, the power of the person is 0.15 KW.

4. How much work is done by a person if he stands carrying a load of 50 kg
at the height of 1m at rest?

Solution:

Here,

As the person is not covering any distance, the work done is zero because

W=Fxd

or, W = F x 0 [ \ d = 0]

or, W = 0J

Electric Power

Electric power is also measured in Watt. Each electric bulb is marked with its power.
If a bulb is marked 60W, it consumes electricity at the rate of 60J/s and converts into
light and heat energy.

Solved Numerical Problem

An electric heater has power 2Kw. In 60 seconds how many Joules of heat is
produced?
Solution:
Here,
Power (P) = 2 Kw = 2 x 1000 = 2000 J/s
In 60 seconds, electric energy is converted into heat energy,
W=P×t
= 2000 × 60
= 12000 Joule
= 12 KJ.

Blooming Science & Environment Book 8 69

Main Points to Remember

1. When certain force is applied through a certain distance, work is done.

2. Work done is equal to the product of force and distance.

W=FxD

3. Work is measured in Newton metre of Joules in MKS units and in ergs in CGS
units.

4. If 1N force is applied through a distance of 1m, the work done is equal to 1 Joule.

5. A unit work is said to be done by a unit force when the body moves through a
unit distance in the direction of force.

6. The force required to bring an acceleration of 1m/s2 on a body of mass of 1 Kg
is called one Newton.

9.8N = 1Kg

7. When 1N force acts on a body of mass 1 Kg through a distance of 1m, the work
done is equal to 1 Joule.

8. Energy is defined as the capacity to do work.

9. Energy possessed on a moving body is called kinetic energy (KE).
1
KE = 2 mv2 Joules

KE = Kinetic energy, m = mass of the body, v = velocity of the body.

10. Energy possessed by a stationery body at a certain height is called potential
energy.

PE = mgh Joules

PE = Potential energy, m = mass, g = acceleration due to gravity, h = height of
the body from ground level.

11. The other forms of energy are: (a) Chemical energy (b) Electrical energy (c) Heat
energy (d) Sound energy (e) Light energy (f) Magnetic energy (g) Nuclear energy.

12. Major sources of energy are: Sun, chemical substances, water, air or wind, etc.

13. Energy can neither be created nor can be destroyed but it can transform from one
form to another. this is called principle of conservation of energy.

14. Power is defined as rate of doing work. Work done per unit time is also called
power.

Power (P) = Work done (joule)
Time (second)

15. 1 KW = 1 Kilowatt = 1000 Watt = 103 W

1 MW = 1 Megawatt = 10,00,000 Watt = 106 W

1 HP = 1 Horse Power = (746 Watt)

70 Blooming Science & Environment Book 8

PRO J ECTWORK

Measure the height of each steps of staircase which
contains several steps. Measure your weight.
(1Kg = 9.8N). Now, stay at the bottom of the
staircase and as soon as you press the button to the
stop watch, run as fast as you can upwards. The
stop watch measures the time. When you reach the
other step (marked last), press button to stop the
watch. Now, you can calculate your power.

Work done (W) = Weight (N) x height (m)
Power (P)
= Work done (w)
Time (t)

Exercise

1. Answer the following questions

(a) What is work done? Classify it.

(b) What do you understand by work against friction?

(c) In plants, there is chemical energy, from where do the plants get this
energy? Give a block diagram.

(d) Write short notes on:

(I) Electrical energy (III) Nuclear energy

(II) Electrical power (IV) Kinetic energy

(e) How does potential energy differ from kinetic energy?

(f) Show the difference between work, energy and power.

(g) What is power? Write its unit.

(h) What is transformation of energy? Give an example.

(i) State principle of conservation of energy.

(j) What is horse power?

2. Define: (b) Energy (c) Power
(a) Work

3. What type of energy is contained in the following?

(a) a kilogram of rice (b) a stretched string

(c) a moving bullet (d) a litre of kerosene

(e) water stored in a dam

Blooming Science & Environment Book 8 71

4. Solve the following numerical problems.

a. If a crane can lift a load of 3000 N through a height of 10 m in 4 seconds, find
its power in Watts, Kilo watts and Horse power. (7500 W, 7.5 kw, 10Hp)

b. What is the power of a man weighing 1000N who can climb a staircase

of 5m in 10 seconds. (500W)

c. If a crane can lift a stone of 720N through a height of 20m in 24 seconds,
what is,

(i) the work done by a crane? (14,400J)

(ii) power of the machine? (600 W)

d. A man can push a car against 300N frictional force through a distance of

10m. What work has he done? (3000 J)

e. Find the power of a machine that transfers 600 joules of energy every

minute. (10 W)

f. An electric motor has a power rating 1.5 KW. How much work is done

by the motor in 20s? (30,000 J)

(62,500 J)

g. If a ball having a mass of 4 kg is rolled along a smooth surface, with a

uniform velocity of 750 m/min, what is the kinetic energy possessed by

a ball? (312.5 J)

h. If a boy can carry 10 bricks each weighting 2N to a distance of 10m in

20 seconds. Calculate the power of the body. (10 W)

Glossary

Work : product of force and distance

Energy : the capacity to do work

Kinetic energy : energy in body due to its motion

Potential energy : energy stored in a body because of its condition or position

Heat energy : energy produced due to heat

Chemical energy : energy stored in coal, oil and gas which is released in the form
of heat and light by chemical reaction.

Light energy : energy obtained by light

Sound energy : form of energy which is obtained due to vibration

Magnetic energy : energy which is obtained by magnet

Electrical energy : energy obtained due to electricity

Nuclear energy : energy due to nuclear reaction

Joule : unit of work energy

Watt : unit of power

Horse power : unit of power equal to 746 W

72 Blooming Science & Environment Book 8

Chapter Heat

6

Lesson Objectives Estimated Periods: 4+1

On the completion of this unit, the students will be able to:
 define heat and temperature with their relation.
 differentiate heat and temperature
 show relation of different temperature units (Celsius and Fahrenheit)
 describe structure and working principle of different types of thermometer.

(simple and clinical)

When we rub our hands together they become warm. A saw, which is used for cutting
wood, becomes warmed during its action. When the fuels like coal, petrol, diesel or
kerosene are burnt heat is produced. We say that the chemical energy of the fuels
gets converted into heat energy. Heat is a form of energy which produces sensation
of hotness or coldness.

In daily life, you often come in contact with hot and cold objects. A cup of steaming
coffee is hot whereas a lump of ice is cold. Can you touch a piece of burning coil?
Of course not. It will be very hot. We say that the hot bodies are at high temperature
and cold bodies are at low temperature. This means the temperature suggests us how
hot or cold a body is.

Matter exists in three states, viz; solid, liquid and gas. The state of material depends
upon temperature. Usually a solid on heating becomes liquid and a liquid substance
on heating becomes gas. Gas on cooling becomes liquid and liquid on cooling
becomes solid.

Solid Heat Liquid Heat Gas
Cool Cool

Matter is made up of molecules. They are in a state of constant motion or vibration.
So, the molecules have kinetic energy. Due to the kinetic energy of the molecules a
body becomes hot. When the molecules vibrate very fast, its kinetic energy increases
and so the body gains more heat. That means the temperature of a body also depends
upon the kinetic energy of its molecules. As the kinetic energy of molecules increase
the temperature of the body also increases.

Blooming Science & Environment Book 8 73

Temperature

Temperature is the degree of hotness or coldness of a body. Heat flows from the body
at high temperature to a body at low temperature till the temperature of both bodies
becomes same. When a body gets heat its temperature rises.

All the bodies are made up of small particles called molecules or atoms. The
molecules inside the body are in the state of vibration. The vibrating molecules
have their kinetic energy. The sum of molecular energy inside a body measures, the
amount of heat contained by the body. When a body is heated, its molecules move
faster. Their average energy is increased and the temperature of that body rises.

Measurement of Temperature

We can get a rough idea of how hot or cold is an object by touching it. But, the sense
of touch is not very accurate. When an object is very hot or very cold, we cannot
touch it. We therefore, measure the temperature with the help of a device is called
thermometer. The first thermometer was invented by Galileo Galilei in 1592AD. It
was known as a thermoscope.

Effects of heat on matter can be used as the basic principle to construct thermometers.
In ordinary thermometers, the principle of expansion of solid, liquid and gas is used
for their construction. Most commonly used thermometers are liquid thermometers.
They use mercury or alcohol as thermometric liquid and work under the principle of
expansion of liquids.

The SI unit of temperature is Kelvin (K). The commonly used units of temperature
are degree Celsius (oC) and degree Fahrenheit (oF).

Differences between Heat and Temperature

S.N. Heat S.N. Temperature

1. Heat is a form of energy which 1. Temperature is the degree of

gives sensation of warmth. hotness and coldness

2. Heat is a cause of hotness or 2. Temperature is the effect of heat.

coldness of body.

3. Heat depends upon the total 3. Temperature is the average kinetic

kinetic energy of all molecules of energy of molecules.

body.

4. Heat is measured in joule or 4. Temperature is measured in degree
calorie. Celcius, Fahrenheit or Kelvin.

74 Blooming Science & Environment Book 8

Thermometer

A thermometer is an instrument or device used to measure the temperature of a body.
Thermometer are of different types but liquid thermometers are more common in use.
These thermometer contains liquid like mercury and alcohol. These thermometer are
constructed on the basis of the fact that liquid expands on heating and contracts on
cooling.

Construction

A thermometer consists of a stem having uniform capillary tube with an expanded
chamber at the upper end and a bulb containing liquid at the other end.

A clean capillary tube with a uniform bore having expanded chamber or funnel at
one end and a bulb at the other end is taken. It is alternately heated and cooled to
remove air in it and to fill the bulb and stem with mercury or alcohol. The bulb is
heated to a temperature slightly higher than the maximum temperature it is supposed
to measure. The expanded chamber is broken and sealed by rotating it over a small
hot blow pipe flame. It is then calibrated.

The bulb has a thinlass wall through which heat flow easily to the liquid contained
in it.

Working

The heat is transferred through the thin bulb to the liquid contained in it. The liquid
in the bulb expands and passes into the capillary tube. The change in temperature is
seen as the change in the level of the liquid in the capillary tube. The liquid does not
wet (stick) the capillary tube.

Liquids used in Thermometers

Mercury and the alcohol are the most commonly used liquids in the thermometers.
Mercury thermometer is used to measure temperatures of hot places. Because
it freezes at -39oC and boils at 357oC. Alcohol thermometer is used to measure
temperature of cold places. Because it freezes at -115oC and boils at 78oC.

Reasons of using mercury in the thermometer

a) It is a good conductor of heat. It quickly transmits heat through it, hence the
temperature remains uniform throughout.

b) It does not stick to the inner wall of the glass tube. It is available in pure and dry
form.

Blooming Science & Environment Book 8 75

Reasons for preferring mercury over alcohol as thermometric liquid

a) Its boiling point is 357oC and freezing points is -39oC. Hence, it is used for
measuring a very wide range of temperature.

b) It is opaque and shinning and is thus easily visible in the stem of the thermometer.

Reasons for preferring alcohol to mercury as a thermometric liquid for recording
low temperature in cold places

a) It freezes at -115oC while mercury at -39oC. Thus, alcohol Scan for practical experiment
can measure much lower temperature than mercury.

b) Alcohol is more sensitive in expansion than mercury. So visit: csp.codes/c08e09
it is used in measuring low temperature.

Calibration of Thermometer

There are two fixed points on the scale of the thermometer.
They are:

(a) Lower fixed point: The temperature at which pure ice melts under standard
atmospheric pressure of 760 mm of Hg is taken as the lower fixed point of the
scale. It is 0oC.

(b) Upper fixed point: The temperature at which pure water boils under standard
atmospheric pressure of 760mm of Hg is taken as the upper fixed point of the
scale. It is 100oC.

Determination of Lower Fixed point

Activity

To Determine lower fixed point in a thermameter

Materials required:

Funnel, Funnel stand, breaker, ice cubes etc.

Method : Ice point
Melting ice
This activity has to be performed at sea level .Some
ice cubes are kept in a funnel and it is fitted with the Funnel
help of stand above a beaker.The thermometer to be
calibrated is inserted in the icecubes. The level of
mercury goes down and remains constant at certain
point. It is marked as lower fixed point or 0oC at sea
level.

76 Blooming Science & Environment Book 8

Determination of Upper Fixed point
Activity

To Determine upper fixed point in Stem point
thermometer BoilingSwtFealaatemsrk

Materials required. Beaker stand heating
apparatus,work fitted with delivery tube and
thermometer to be calibrated.

Method :

This activity also has to be performed at sea
level. A thermometer whose lower fixed
point is marked is fixed with a cork in a
beaker with some water. The beaker is kept
on tripod stand and fixed with stand and
clamp. The water heated with ther burner. The mercury level in the capillary
tube increase and stops at certain point. The point is marked as upper fixed point
or 100oC at sea level.
After marking upper and lower fixed points, it is divided into 100 equal parts
and scale is made in degree Celsius scale.

Types of Thermometer

Thermometer are of different shapes and sizes. They are used for different purposes.
Three types of commonly used thermometers are described below:

Clinical Thermometer

Clinical thermometer is a special kind of mercury thermometer. It is used to measure

the temperature of the body. It is available in both Celsius and Fahrenheit scales. It is

smaller in size with a short range of temperature from 35oC to 42oC (94oF to 108oF).

It is so graduated because the temperature of a normal human body is 37oC (98.6oF)

which is indicated on the thermometer by a special mark which is usually a small

arrow head in red. Average body temperature

Mercury Constriction

The clinical thermometer has a narrow constriction in the capillary tube near the
bulb. It stops the mercury thread running back into the bulb, when the thermometer
is taken out from the body. Hence, the temperature of a person can be read even long
after it has been taken out. To use the thermometer again, it is given a few smart and
vigorous jerks to force the mercury in the capillary tube through the constriction,
back into the bulb.

Blooming Science & Environment Book 8 77

The body of the thermometer is not cylindrical but prismatic in shape. This space
magnifies very thin thread of mercury in the thermometer. This makes the reading
easy and correct. The thick prismatic shape body of the thermometer also prevents
the transfer of heat of user’s hand to the mercury in the capillary tube.

Each degree is divided into five equal parts so that temperature corresponds to one
fifth of a degree can be read on it.

Laboratory or General Thermometer

General thermometer is an ordinary thermometer used to measure the temperature of
different substances in the laboratory. It is generally long, round and cylindrical in
shape with thin wall. It has marking from -10oC to 110oC. Its capillary tube is very
thin. The part of the capillary tube above the mercury is vacuum. This makes the
mercury raise easily in the tube. It has no constriction.

Bulb Thread

Mercury

Temperature Scales

All thermometer scales have two fixed points. The two fixed points in a thermometers
are the lower fixed point and the upper fixed point. The temperature at which pure
water freezes at sea level is called lower fixed point. The temperature at which pure
water boils at sea level is called the upper fixed point.

In general three types of scales are used to measure temperature. They are as follows:

The Celsius Scale

In this scale, upper fixe point is taken as 100oC and lower fixed point is 0oC. The
interval between the fixed points is divided into 100 equal parts. Each part of division
represents one degree Celsius.

The Fahrenheit Scale

In this scale, the upper fixed point is 212oF and the lower fixed point is 32oF. The
interval between these two fixed points is divided into 180 equal parts. Each part of
division represents 1 degree Fahrenheit.

The Kelvin Scale

In this scale, upper fixed point is 373k and the lower fixed point is 273k. The interval
between these two fixed points is divided into 100 equal parts and each part of the
division represents are kelvin.

All the temperature scales have two fixed points but the number of intervals between

78 Blooming Science & Environment Book 8

these two points are different. The 0 of Celsius corresponds to 32 of Fahrenheit and
273 of Kelvin. They are lower fixed points. Therefore, the relation between Celsius
Fahrenheit and Kelvin scales can be expressed in the following equation.

C-0 = F - 32 = K - 273
100 180 100

C F - 32 K - 273
or, 100 = 180 = 100

or, C = (F - 32) x 100 = 5 (F - 32)
180 9

To convert oF to oC, formula used is

C = 5 (F - 32) C = K - 273 or K = C + 273
9

Similarly, to convert oC to oF, formula used is

F = ( 9C ) + 32
5

1. If a thermometer shows the temperature 37oC, what is its equivalent
temperature on the Fahrenheit scale?

Solution:

Here, Temperature in Celsius scale (C) = 37oC

Temperature in Fahrenheit scale (F) = ?

According to the formula,

C-0 = F - 32
100 180
or,
or, 37 - 0 = F - 32
100 100

F - 32 = 37 × 180
100

F = 37 × 180 + 32
100

= 66.6 + 32

= 98.6 oF

37oC = 98.6 oF

Blooming Science & Environment Book 8 79

2. At what temperature are the readings of Celsius and Fahrenheit scale the
same?

Solution:

Here, Let, C = F = X

According to the formula:

C-0 = F - 32
100 180

Substituting X for C and F, we get

or, X10-00 = X - 32
180

or, X = X - 32
100 180

or, 180X = 100X - 3200

or, 180X - 100X = -3200

or, 80X = -3200

or, X = -3200 = -40
80

The common temperature is -40oC.

i.e., - 40oC = -40oF

Hence, at -40o, the readings of Celsius and Fahrenheit scales are same.

3. What is -20oC equivalent to in Fahrenheit?

Solution:
Here, Temperature in Fahrenheit scale (F) = ?
Temperature in Celsius scale (C) = -20oC

According to the formula;

or, C10-00 = F - 32
180

or, -20 - 0 = F - 32
100 180

or, F - 32 = - 20
180 100

or, F - 32 = - 20 × 180
100

80 Blooming Science & Environment Book 8

or, F = -36 + 32

or, F = -4oF

\ -20oC = -4oF

Main Points to Remember

1. Heat is the kinetic energy of all molecules of a substance.

2. Temperature is a measure of how hot or cold a body is. Temperature is usually
measured in the Celsius scale (oC) or Fahrenheit scale (oF).

3. A thermometer is a device used to measure the temperature of a body.

4. There are three temperature scales in use on thermometers - the Celsius scale,
the Fahrenheit scale and the Kelvin or absolute scale.

5. Celsius thermometers and Fahrenheit thermometers are commonly used to

measure temperature. The relationship between the readings on the three kinds

of thermometers, is given by the relation.

C-0 = F - 32 = K - 273
100 180 100

6. Mercury is commonly used as a thermometric substance, because it:

a) is a shiny liquid

b) does not stick to the wall of the glass-tube

c) expands uniformly

d) can be used for a wide range of temperature

7. A mercury thermometer is used to measure the temperature of extremely hot
place. Mercury thermometer can measure temperature between - 39oC to 357oC.

8. An alcohol thermometer is used to measure the temperature of extremely cold
place. Alcohol thermometer can measure temperature between -115oC to
78.3oC.

9. Normal human body temperature is 98.6oF or 36.9oC (37oC).

10. A thermometer used to measure the temperature of human body is called
clinical thermometer.

11. A clinical thermometer has a constriction near the bulb in it. Due to this, the
mercury thread does not fall on removing the bulb from the mouth of the
patient.

12. The temperature at which ice melts under normal conditions, is called the
melting point of ice and that at which water boils under normal conditions is

Blooming Science & Environment Book 8 81

called the boiling point of water. They are 0oC and 100oC in Celsius scale and
32oF and 212oF in Fahrenheit scale. They are also called lower fixed point and
upper fixed point.

13. Alcohol thermometer is used to measure temperatures at very cold places
whereas the mercury thermometer is used to measure temperatures at very hot
places.

PRO J ECTWORK

1. Use a clinical thermometer and measure your body temperature.
2. With the help of laboratory thermometer measure the temperature of boiling

water.
3. Take same ice cubes and measure its melting point

Exercise

1. Fill in the blanks.

a) Our body temperature is _______ oF.

b) -40oC is _______ on Fahrenheit scale.

c) 0oC is equal to _____ K.

d) A thermometer is an instrument used to measure ________.

e) On Kelvin scale, the temperature of pure melting ice is ________ an that
of steam is _______.

2. State whether the following statements are 'True' or 'False'.

a) A mercury thermometer is more sensitive than an alcohol thermometer.

b) A mercury thermometer is more accurate than an alcohol thermometer.

c) Alcohol expands more than an equal volume of mercury for an equal rise
in temperature.

d) A temperature of 1oC is the same as a temperature of 1K.

e) Mercury wets glass.

3. Select the most correct alternatives.

a) The temperature at which Celsius and Fahrenheit scales will show same
readings is

i) 0oC ii) -40o

iii) 32o iv) -32o

b) The number of divisions between the upper and the lower fixed points in
the Kelvin scale is

i) 273 ii) 373

iii) 100 iv) 212

82 Blooming Science & Environment Book 8

c) The temperature of a body changes by 25oC. The corresponding change
on the Fahrenheit scale is

i) 25oF ii) 45oF

iii) 60oF iv) 77oF



d) A Celsius and a Fahrenheit thermometer is placed in a water bath. The reading of
the Celsius thermometer is one-third that of the Fahrenheit thermometer,
what is the temperature of bath.

i) 20oC ii) 30oC

iii) 50oC iv) 80oC

3 3

4. Answer the following questions.

a. Write any two differences between heat and temperature.

b. Name two common scales of temperature.

c. What is the temperature of pure melting ice and that of steam on the
Celsius scale?

d. What are the fixed points on Fahrenheit scale?

e. What is the normal temperature of healthy human body in oC and oF?

f. Give two reasons why mercury is preferred to alcohol as a thermometric
liquid.

g. What are the advantages of using alcohol instead of mercury in
thermometers? Write.

h. Draw a labeled diagram of a clinical thermometer and explain its
working.

l. What do you mean by ‘fixed points’ of thermometer? How many fixed
points are there?

j. Give four differences between clinical thermometer and simple
thermometer.

5. Give reasons for the following.

a) We cannot use a mercury thermometer to measure a temperature of
-80oC.

b) An alcohol thermometer cannot be used to measure the temperature of
boiling water.

c) Water is not suitable as a thermometric liquid.

d) The bulb of a clinical thermometer made of thin glass.

e) A clinical thermometer has a constriction near the bulb.

Blooming Science & Environment Book 8 83

6. Solve the following numerical problems.

a. The maximum temperature recorded in Nepal on a summer day is 45oC.

Find its value in Fahrenheit. [Ans. 112oC]

b. The average temperature of a healthy person is 98.6oF. What is it in Celsius

scale? [Ans. 36.9oC]

c. The temperature of a normal human body is 37oC. What is the corresponding
temperaturon (a) Fahrenheit scale and (b) Kelvin scale? [Ans. 98.6oF, 310K]

d. At what temperature on the Fahrenheit scale will the numerical value of the
Celsius scale be one-third of the Fahrenheit scale? [Ans. 80oF]

e. A Celsius and a Fahrenheit alcohol thermometer give the same reading at a
particular temperature. What is that temperature? [Ans. -40oC, -40oC]

9. Convert the following temperature as

(a) 30oC into Fahrenheit

(b) 200oF into Celsius

(c) 200oC into Kelvin

(d) 100oK into Celsius

[Ans. (a) 86oF (b) 93.3oC (c) 473K (d) -173oC (e) 310 K (f) 310.7K (g)
98.6oF]

Glossary

Heat : from of energy obtained by molecular vibration

Temperature : the degree of hotness or coldness of a body

Thermometer : device to measure temperature of body

Celsius : scale of thermometer having two fixed point 0oC and
100oC

Fahrenheit : scale having lower fixed point is 32oC and upper point is
212oF

Kelvin : SI unit of temperature

Lower fixed point : the temperature of pure melting ice

Upper fixed point : the temperature when pure water turns to vapour at sea
level

Clinical thermometer : thermometer used to measure body temperature

Maximum and

minimum thermometer : a thermometer used to record and measure the maximum
and minimum temperature of a body

Radiator : object emits heat by radiation

Absorbers : object which absorbs hea

84 Blooming Science & Environment Book 8

Chapter Light

7

Learning Outcomes Estimated Periods: 4+1

On the completion of this unit, the students will be able to:
 introduce mirror and describe its types, properties and uses.
 introduce mirror and demonstrate reflection of light by spherical mirrors
 draw the ray diagrams and image formed by spherical mirrors
 tell the uses of concave and convex mirrors.
 define refraction of light and state its laws.

The branch of science which deals with the study of light and its phenomenon is
called Optics. Light is a form of energy, which produces the sensation of sight in the
eyes. We can see any object only when the light from the object enters into our eyes.
This is how reading a book is possible. The objects, kept in a dark room, cannot be
seen because no light from these objects enters into our eyes. If an iron rod is heated,
it becomes red hot and emits light. This shows that heat energy is converted into
light. Thus, light is a form of energy.

Ray and Beam of Light

Light travels in a straight line. The term ‘ray’ refers to a single narrow path of light.
It is represented by a straight line with an arrowhead as shown in the figure. The
arrowhead shows the direction in which the light is travelling.

A collection of rays of light is called the beam of light. It is of three types: parallel,
convergent and divergent beam.

(a) Parallel beam (b) Convergent beam (c) Divergent beam

Fig: Light beams

The beam of light, in which all the rays are parallel to each other, it is called the

Blooming Science & Environment Book 8 85

parallel beam. The beam of light coming from distant sources such as the sun and
stars is the parallel beam.

The beam of light, in which the rays of light meet at a point is called convergent
beam. The beam of light reflected by a concave mirror and converged by a hand lens
or convex lens is the convergent beam.

The beam of light, in which the rays of light scatter from a point, is called divergent
beam. The light reflected from a convex mirror or the beam of the light from electric
bulb, tube light, candle, torch, etc. is a divergent beam.

Mirror

We can see our face while combing hair by using a Scan for practical experiment
mirror. Similarly to see the vehicles coming from
back side mirrors are used in vehicles. A mirror is an
instrument which reflects light falling on it and forms
image of the object. A mirror is made from glass whose
one side is smooth and other side is polished. An image
is formed when an object is placed in front of it by
reflecting light coming from a source.

There are two types mirror used in our daily life. They are;

1. Plane Mirror

2. Spherical Mirror

1. Plane Mirror visit: csp.codes/c08e10

A Mirror with a flat and smooth reflecting surface is called a plane mirror . A plane
mirror forms an erect, virtual and laterally inverted image. These are usually used
as looking glass at home.

Fig : A plane mirror showing reflection of light

2. Spherical mirror
A mirror with a curved reflecting surface is called a spherical mirror . These mirrors
do not have plane reflecting surface. Their reflecting surface is raised or depressed.
The spherical mirror is a part of the sphere with inner or outer reflecting surface.
There are two types of spherical mirrors. 1. Concave mirror, and 2. Convex mirror

86 Blooming Science & Environment Book 8

1. Concave Mirror P

In a concave mirror, reflection takes place on the C F
inner surface of the mirror.

A concave mirror is also called a converging
mirror since a parallel beam of light incident on it
converges to a point after reflection.

2. Convex Mirror C

A spherical mirror whose outer surface is reflecting surface,
is called a convex mirror. In a convex mirror, reflection takes
place on the outer surface of the mirror.

A convex mirror is also called a diverging mirror since a
parallel beam of light incident on it, appears to be diverging
from a point after reflection.

A shining stainless steel table-spoon is a good example of
both concave and convex mirrors. Its inner surface is like
a concave mirror and outer surface is like that of a convex
mirror.

Concave surface
Convex surface

Terms used in Spherical Mirrors

1. Pole: It is a geometrical centre of the surface of mirror. It is represented by a
point P.

2. Centre of Curvature: It is the centre of the hollow sphere of which mirror is
a part. It is represented by C.

3. Radius of Curvature: It is the radius of the hollow sphere of which mirror is
a part. It is denoted by R.

R R
X CRP
XC R PY Y
R R
Polished
Polished surface
surface
Convex Mirror
Concave Mirror

Blooming Science & Environment Book 8 87

4. Principal Axis: The line joining the pole of the

mirror and the centre of curvature and produced

on both sides, is called Principal axis. In above

diagram, the line XCPY is the principal axis. C F P

5. Principal Focus: The point on principal axis

where a beam of light incident on a spherical Fig: Ray going parallel
mirror parallel to the principal axis converges or to principal axis

appears to be diverging from after reflection, is called Principal focus. It is

represented by the letter F. The principal focus is real in case of concave mirror

and virtual in case of convex mirror.

P

F CF P

6. Focal Length: It is the distance between principal focus and pole of the mirror.
It is denoted by f. In the give diagram PF is the focal length of the mirror. The
focal length is half the radius of curvature.

Rules to draw Ray Diagram from Concave Mirror

The following rays coming from the object are usually used to construct ray diagrams
for locating the image formed by a concave mirror.

(a) A ray of light travelling parallel to the principal

axis after reflection from a concave mirror passes C F P
through its focus (F).

Fig: Ray going parallel
to principal axis

(b) A ray of light passing through the focus (F) of a CF P
concave mirror after reflection goes parallel to
the principal axis.

Fig: Ray passing
through the focus

88 Blooming Science & Environment Book 8

(c) A ray of light passing through the centre of curvature (C) of a concave mirror
returns back along the same path after reflection. This is because the ray passing
through the centre of curvature strikes the mirror at 90o.

C FP CP

Methods for Drawing Ray Diagrams

1. Draw a straight line of length about 8 cm. A M
Mark a point at 4 cm and denote it by C. This B D
point C represents the Centre of Curvature.
CF P
2. Take a compass and draw an arc of 4 cm with E
the help of compass by keeping its pointed B'
end at C. Make this arc a concave mirror as
shown in figure. G A'

N

3. Mark P at the middle of the concave mirror MN. Now CP is the principal axis.
Measure 2 cm from point C and mark F. Even you can take the middle point of
C and P and M F. This point F is the Focus.

4. Measure 2.5 cm from C and mark this point. Draw a vertical line AB of 1 cm
at this point. Make arrow at the top as shown in figure. This line AB represents
object. Note that you can keep object at any place beyond C.

5. Draw a line AD starting from A parallel to the principal axis PC. This line AD
represents incident ray. Join DF. This DF line is the reflected ray.

6. Join A and F and produce it to meet the mirror. This line AF is another incident
ray passing through the focus ‘F’. Draw EG line parallel to the principal axis.
This is also a reflected ray.

7. The two reflected ray DF and EG cut at a point. Mark this point A'. Draw a
perpendicular line from this point of intersection to the principal axis. This line
A'B' is the image of the object AB placed beyond C.

8. Measure the height of the image A'B' and its distance from P (pole of the
mirror). You will find that the image is smaller than the object and the image
is between F and C. Thus, a real, inverted, smaller image can be obtained by
drawing ray diagrams.

Blooming Science & Environment Book 8 89

Nature of Image for Different Positions of Object

Now, let us draw ray diagrams showing formation of images when the object is
placed at different distances from the mirror.

(a) When the object is at infinity C F

Image is real, inverted, small in size and is formed at
principal focus (F) of the mirror.

(b) When object is between infinity and C (i.e. beyond C)

Image is real, inverted, smaller than object and is

formed between principal focus (F) and centre of A B' F
A'
curvature (C) of the mirror. In figure. A'B' is the B C P

image of an object AB.

(c) When object is at C

Image is real, inverted, of same size as object and is A

formed at centre of curvature (C) of the mirror. In figure B F P
AB is the object and A'B' is the image. B' C

A'

B

(d) When object is between F and C Objects

Image is real, inverted, magnified and is formed beyond C. In A' CA F P
Image

figure, A'B' is the image. B'

(e) When object is at F C A P
To infinity B
The rays become parallel after reflection and hence
image is formed at infinity. The image is real, F
inverted and very large. The image is indistinct.

90 Blooming Science & Environment Book 8

(f) When the object is between the pole (P) and the focus (F)

Image is virtual, erect, magnified and is formed on C A A'
the other side of the mirror. (i.e. behind the mirror.) B'

FB P

The position and nature of the image formed by concave mirror can be
summarized as

S. No. Position of object Position and nature of image formed

1. At infinity Image is real, inverted, small in size and forms at F.

2. Beyond C Image is real, inverted and smaller in size, Image is

formed between F and C.

3. At C Image is real, inverted and same in size. Image is
formed at C.

4. Between F and C Image is real, inverted and magnified. Image is formed
beyond C.

5. At F Image is real, inverted and highly magnified. Image is
formed at infinity.

6. Between P and F Image is virtual, erect and magnified. Image is formed
behind the mirror.

Uses of Concave Mirrors:

1. Concave mirror is used as reflector in a large astronomical telescope, radio,
telescope to view stars and planets.

2. Concave mirrors are used as reflectors in projectors, headlight of cars, motor
cycles and bicycles, searchlight, torches, etc. to reflect the light of the bulb.

3. Concave mirrors are used by dentists to focus light on the tooth to be examined
and by the surgeons to focus a sharp narrow beam of light into the ear, nose,
throat, eyes etc. and to observe these organs.

4. Concave mirrors are used as shaving mirrors and as make-up mirrors to see the
enlarged erect image of the face.

5. Concave mirrors are used as radiation collector in solar heating devices such as
solar cooker.

Blooming Science & Environment Book 8 91

Formation of Images by a Convex Mirror

The following points should be kept in mind while drawing ray diagram for convex
mirror.

1. A convex mirror is also a part of the sphere. The C Focus P
centre of curvature (C) and the focus (F) of convex Centre of curvature
mirror lie behind the mirror.

2. A ray of light coming along the principal Convex mirror
focus travels parallel to the principal axis after
reflection. The light ray reflected by the mirror Convex mirror
seem to appear from the principal focus.
CF P

3. A ray of light moving towards the centre of curvature

retraces back its path after reflection. Similarly, rays P
coming along centre of curvature retrace back their C

path after reflection as shown in figure. The reflected Convex
mirror
rays follow the same path.

The image formed by a concave mirror can easily be located but in case of convex
mirror, the rays of light parallel to the principal axis when incident on convex mirror
diverge after reflection and appear to come from a point F behind the mirror. This
focus is a virtual point. Hence, a convex mirror forms only a virtual image. In fig, the
formation of image of object AB due to convex mirror is shown. Only one type of
image is formed by a convex mirror.

B B' C

A P A' F
Object Image

Fig: Image formed by a convex mirror
92 Blooming Science & Environment Book 8

When the object is at infinity, the image appears to be FC
at the focus (F) which is virtual. As the object is moved
from infinity towards the convex mirror, the image is
always virtual, erect, smaller in size and on the other
side of the mirror. The image formed by a convex
mirror is always a virtual and erect image, smaller than
the object and forms between P and F behind the mirror.

Uses of Convex Mirror

Convex mirror is used for the following purposes:

1. Convex mirrors are used as a rear-view mirrors or side mirrors (also called
driver’s mirror) on automobiles such as cars, trucks, and buses to see back part.

2. Convex mirrors are used as vigilance-mirrors in big shops and stores.

3. Convex mirrors are used as lamp reflector in street lamps to diverge light over
a larger area.

A convex mirror is preferred due to use as a driver’s mirror because of the following
reasons:

(i) A Convex mirror always form an erect (right side up) image of an object
whatever may be its distance from the mirror. This makes the identification of
the object easy.

(ii) A convex mirror forms an image which is much smaller than the object. As a
result, image of a large number of objects can be seen in the mirror at the same
time. Hence, a convex mirror gives a wider field of view.

Image of car

Car

Sign Convention for Spherical Mirror

Following sign convention is used for measuring various distance in the ray diagrams
of spherical mirrors.

(i) All distances are measured from the pole of the mirror.

(ii) Distances measured in the direction of the incident ray are positive.

(iii) Distances measured in the direction opposite to that of the incident rays are
negative.

(iv) Distances measured above the principal axis are positive, e.g. height of an
object and of an erect image are positive.

Blooming Science & Environment Book 8 93

(v) Distances measured below the principal axis are negative e.g. height of a real
inverted image is negative.

Differences between Concave Mirror and Convex Mirror

Concave Mirror Convex Mirror

1. It has the inner surface reflecting 1. It has the outer surface reflecting and

and outer surface polished. inner surface polished.

2. It may form an enlarged, diminished 2. It always forms diminished image of

or equal-sized image of an object. an object.

3. It forms real and inverted image of 3. It always forms erect and virtual

an object except when an object is image.

placed between F and P.

4. It is used for make-up or in lamp 4. It is used in vehicles as side mirror.
posts.

Refraction of Light A Angle of Air Normal
Incident incidence
When a ray of light AO in
air is incident on a glass ray M
surface, it bends along OB
in the glass as shown in the P i Q
figure. MN is the normal to Glass
the glass surface at O. The O
ray OB then emerges along r Refracted ray
BC in air.

We notice that the light ray

AO has bent along OB N B R
while it is travelling from S Angle of refraction B'

air would have travelled in a Emergent ray C
straight line OB' in air. The

bending of the light as it passes through one optical medium to the another is called

refraction of light.

The optical medium, i.e. the medium through which light Scan for practical experiment
travels, is classified as rarer and denser medium. The medium
through which light passes easily or in which the velocity of visit: csp.codes/c08e11
light is more, is called rarer medium. The medium in which
the velocity of light is less, is called denser medium. For
example, in the case of air and water, air is rarer medium and
water is denser medium because the velocity of the light in

94 Blooming Science & Environment Book 8

water is less than that in air. For water and glass ,water is rarer medium and glass is
denser medium. Thus, we cannot list the different media as rarer and denser but we
can list any two media as the denser and the rarer by comparing the velocity of light
in these media. Thus, the rarer and denser medium are relative terms.

Note: The denser and rarer media cannot be classified on the basis of density. For
example, steam has less density than dry air, therefore, it rises up. But, steam is
denser medium and dry air is rarer medium of light.

The velocity of light in air, water and glass is 3 x 108 m/s, 2.2 x 108 m/s and 2 x 108

m/s respectively. The velocity of light in air (or in a vacuum) is 3 x 108 m/s. The light

travels more slowly when it A M Normal
enters into different medium like
glass and water. Because of ∠i Incident ray
change in velocity of the light in Angle of incidence
different media, it bends when it O
travels from one optical medium Air
∠r
Glass

Angle of refraction

to other medium. Therefore, the Refracted ray
change in speed of light when it
passes from one medium to B
another is the causes of the
N

Fig: Refraction of light

refraction of light.

Laws of Refraction
The following are the laws of refraction of light:

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

In the figure, the incident ray AO, the refracted ray OB and the normal MN at
the point of incidence O, all lie on the plane of the paper.

2. The ratio of ‘Sine’ of the angle of incidence to the ‘Sine’ of the angle of
refraction is constant for any two given media. This is called Snell’s law.

Mathematically,
SSiinnri = m [Constant]

In the figure, i the angle of incidence in a medium (say air) and r is the angle of

Blooming Science & Environment Book 8 95

refraction in the other medium (say glass) in which the light enters. The ‘constant’
is called the refractive index (R.I.) of the medium (glass) where the light enters. It is
denoted by m. For air-water media, the refractive index of water is 1.33. For air-glass
media, the refractive index of the glass is 1.5.

Differences between Real and Virtual Image

Real Image Virtual Image
1) It can be obtained on the screen. 1) It cannot be obtained on the screen.

2) The rays of light forming the image 2) The rays of light forming the image

actually pass through the position do not actually pass through the

where the image is formed. position of the formation of image

but they only appear to pass.

3) It is inverted. 3) It is erect.

4) In case of mirrors, it is formed in 4) In case of mirrors, it is formed

front of the mirror i.e. on the same behind the mirror.

side of the mirror as the object.

Apparent Depth of Pond

When we look at the pond of water, the

rays of light coming from the bottom of Air Eye
the pond are bent on emerging from the r B
surface of the water, and so, the pond
appears to be shallow. A
Apparent deothO R
If a rod is partially immersed in a trough, Real depth
r Water
I

it behaves same as the stick put in the i
water of pond. In this case the rod or M

the stick which is partially immersed in

water looks bent at the point of separation of two media, i.e.water and air.

Apparent Bending of Rod

Rays of light coming from the various points of portion Rod Air (r) Eye
of rod in water refract away from the normal as they
emerge from water into air. Each point on the dipped A1
part of the rod, therefore, has a corresponding image Water (d)
point. The rod, therefore, appears bent at the surfa
A

96 Blooming Science & Environment Book 8

Main Points to Remember

1. Light is a form of energy and it produces the sensation of sight.

2. The bodies, which provide the light naturally are called natural sources of light.

3. The bodies, which provide the light artificially are called artificial sources of
light.

4. Light travels in a straight line in a medium.

5. Ray is a single narrow path of light that is represented by a straight line with an
arrow head.

6. A collection of rays of light is called beam of light. It is of three types: parallel
beam, divergent beam and convergent beam.

7. Mirror is the surface which reflects a large fraction of incident light and form
image of an object. In general, mirrors are of two types; plane mirror and
spherical mirror.

8. Spherical mirrors are the part of a sphere of which inner or outer surface is
polished.

9. An image that can be obtained on a screen is called real image. It is formed by
the actual intersection of reflected or refracted rays.

10. An image that cannot be obtained on a screen is called virtual image. It is
formed by the apparent intersection of rays.

11. When a beam of light is incident parallel to the principal axis of a small aperture
spherical mirror after reflection, it passes or appears to pass through a point on
the principal axis. The point is called focus.

12. A concave mirror changes a parallel beam of light into a converging beam.
Hence, it is called a converging mirror.

13. A convex mirror changes a parallel beam of light into a diverging beam of
light. Hence, it is called diverging mirror.

14. The bending of light when it passes through one optical medium to another is
called refraction of light.

15. The rarer and the denser mediums are the relative terms.

PRO J ECTWORK

Take a concave mirror of focal length 15cm.
a. Look at your face in it, placing it about 40 cm away from you. How do you see

your own face? Is your face small or magnified? Is the image of your face real
or virtual?
b. Look at your face in it, placing it about 25cm away from you. What difference
do you find before and now?
c. Look at your face in it, placing about 10 cm away from you. What is the
position and nature of the image of your face?

Blooming Science & Environment Book 8 97

Exercise

1. Define the following terms.

a. light b. incident ray c. refracted ray

d. mirror e. refraction f. refractive index

2. Differentiate between.

a. Concave mirror and Convex mirror b. Real image and Virtual image

c. Ray and beam of light

3. Give reasons:

a. A pencil appears bent when dipped partially in a beaker containing water.

b. A concave mirror is used as a reflector in a torch light.

c. A convex mirror is used in automobiles to have a clear view of the traffic
behind.

d. We use a plane mirror to see our face in our daily life.

e. A pond appears shallower than it actually is.

4. Write any two uses of.

a. Plane mirror b. Convex mirror c. Concave mirror

5. Answer the following questions.

a. What do you mean by focus?

b. What is refraction of light?

c. What are the laws of refraction?

d. A person is standing 5m away in front of a plane mirror. Where is the
image formed? What is the distance between him and the image?

6. Explain the refraction of light on the glass slab.

7. How is the direction of a ray of light changed when it travels from an
optically denser to an optically rarer medium?

8. A student, 4 feet tall went for swimming in a pool. He saw the depth of the
water in the pool less than 4 feet, will he sink? Write with reason.

9. What type of image is formed by convex mirror? Write with diagram.

10. Draw ray diagrams to show the position of image formed in each of the
cases by the concave mirror. Describe the nature of the image.

a. When an object is placed beyond C.

98 Blooming Science & Environment Book 8

b. When an object is placed at the point C.
c. When an object is placed between F and C.
d. When an object is placed at the point F.

11. Complete the following ray diagrams. Air

Air Air

Glass Glass

Glass

(i) (ii) (iii)

12. At what position when an object placed near a concave mirror.

a. form magnified and inverted image?

b. form magnified and erect image?

13. When the angle of incidence of a ray is 45o, the corresponding angle of
refraction will be 30o. If the mediums were air and water, write with
diagram from which media the light emerged and in which medium it
entered.

14. If a man shoots the spear at a place where he sees the fish inside the water,
is it possible that it will hit the fish? Write with reason.

15. The image formed by a concave mirror is real, inverted and diminished.
Locate the position of the object. Draw the ray diagram.

16. Study the give figure and answer these questions. Q
a. Name the type of mirror shown in the figure.

b. Name the points C, F and O. A

c. Write the relationship between OC and OF. O
d. What is the distance OF called? Write its symbol. C B F

e. Write the name of the line that passes through O and C.

f. Complete the given diagram. P

Blooming Science & Environment Book 8 99

Glossary

Reflection : process of returning back of light in the same medium when
it strikes on the surface of body

Concave mirror : a spherical mirror in which the inner surface reflects light

Convex mirror : spherical mirror in which the outer surface reflects light

Centre of curvature : the centre of the sphere of which the curved mirror forms a
part

Radius of curvature : the radius of the sphere which a curved mirror forms a part

Pole: the centre of a curved mirror

Principal axis : the line passing through the pole and centre of curvature of
mirror

Principal focus : the point on a principal axis a mirror at which a parallel beam
of light is converged by mirror or diverged

Focal length : the distance between the pole and focus of mirror

Real image : the image producible on the screen

Virtual image : the image not producible on the screen

Magnified image : bigger image

diminished image : smaller image

Refraction : the bending of light it goes from one medium to another

Lateral inversion : the sideways inversion of object in its image

100 Blooming Science & Environment Book 8