Science 8

Solved Numerical Problem 4.2
$ WDQN RI GLPHQVLRQV P ð P ð P LV ÀOOHG ZLWK ZDWHU &DOFXODWH WKH
pressure exerted by the water at the bottom.
Given,

Depth of water (h) = 1m.
Density of water (d) = 1000kg/m3
Acceleration due to gravity (g) = 9.8m/s2
Liquid pressure (P) = ?
We have,
P = dgh Or, P = 1000 × 9.8×1 Or, P = 9800 Pascal
Therefore, the pressure exerted is 9800 Pascal.

Solved Numerical Problem 4.3
$ WDQN LV ÀOOHG ZLWK D OLTXLG XSWR D KHLJKW RI P &DOFXODWH WKH GHQVLW\
of the liquid if it exerts a pressure of 500 Pascal at the bottom.
Given,

Depth of the liquid (h) = 2m
Density of the liquid (d) = ?
Pressure at the bottom (P) = 500 Pascal
Acceleration due to gravity (g) = 9.8m/s2
We have,
P = dgh or, 500 = d × 9.8 × 2 Or, d = 25.51 kg/m3
Therefore, density of the liquid is 25.51kg/m3.

Properties of liquid pressure

a) Liquid pressure increases with depth.
ĐƟǀŝƚLJ ϰ͘ϯ To show the variation of liquid pressure with depth
Materials required:
Plastic bottle, a nail, table, water, etc.
Procedure:
1. Take a plastic bottle and make three holes of the

same size at different heights from the base.
2. Close the holes with corks.
3. Fill the bottle with water and keep it on a table.
4. Open all the holes at the same time. What do you

observe?

47 Times' Crucial Science & Environment Book - 8

Observation:
Water from the lowest hole reaches up to the maximum distance and that
from the uppermost hole reaches up to the minimum distance.
Conclusion:
'XH WR PRUH SUHVVXUH DW WKH ORZHU KROHV WKH ZDWHU ÁRZV ZLWK PRUH IRUFH DQG
covers longer distance. This proves that liquid pressure increases with depth.

Think and Solve
1. Water tanks are kept at the top of the buildings. Why?
:DWHU ÁRZV ZLWK JUHDWHU IRUFH DW WKH WDS RI WKH JURXQG ÁRRU WKDQ WKDW

DW WKH XSSHU ÁRRUV :K\"

b) Liquid pressure increases when density of the liquid increases

ĐƟǀŝƚLJ ϰ͘ϰ To show the relation between density and liquid pressure.

Materials required:

A glass tube open at both ends, a piece of balloon, water and salt.

Procedure: Glass tube Glass tube
^Ăůƚ ƐŽůƵƟŽŶ
1. Take a glass tube open at both
ends.
Water

2. Close one end of the tube with a Balloon Balloon
piece of balloon. And tie it properly.

3. Fill the glass tube with water.
Observe what happens.

1RZ UHPRYH WKH ZDWHU IURP WKH JODVV WXEH DQG ÀOO LW ZLWK FRQFHQWUDWHG
salt solution.

5. Observe, what happens.

Observation:

:KHQ WKH JODVV WXEH LV ÀOOHG ZLWK ZDWHU WKH EDOORRQ LV EXOJHG RXW D OLWWOH GXH
WR WKH SUHVVXUH H[HUWHG E\ WKH ZDWHU :KHQ WKH JODVV WXEH LV ÀOOHG ZLWK VDOW
solution, the balloon is bulged out more than in case of water.

Conclusion:

The more bulging out of the balloon in case of salt solution than in water is due
to more liquid pressure exerted by the salt solution. The salt solution has more
density than water and exerts more pressure. This activity proves that liquid
pressure increases with the increase in density.

Times' Crucial Science & Environment Book - 8 48

c) Liquid transmits pressure equally in all directions.
ĐƟǀŝƚLJ ϰ͘ϱ To observe liquid pressure in different directions.
Materials required:
A balloon, water, a needle, etc.
Procedure:
1. Take a balloon and make many small holes

on it with the needle.
2. Fill the balloon with water then press it.
3. What do you observe?
Observation:
The water comes out of the holes of the balloon
with equal force simultaneously.
Conclusion:
The pressure applied to a liquid transmits equally in all directions.

d) Liquid maintains its own level.
ĐƟǀŝƚLJ ϰ͘ϲ 7R VKRZ WKDW D OLTXLG ÀQGV LWV RZQ OHYHO
Materials required:
Pascal’s tube and water
Procedure:
1. Take a Pascal tube.
2. Pour water into it from one tube. What do you

observe ?
Observation:
The liquid in all tubes remains in the same level.
Conclusion:
The liquid maintains its own level. It does not depend upon the shape of the
vessel.

Density

'HQVLW\ RI DQ REMHFW LV GHÀQHG DV WKH PDVV SHU XQLW YROXPH

Density = Mass (kg) Or, d = m
Volume (m3) v
The SI unit of mass is kilogram and that of volume is m3. Therefore, SI unit of

density is kg/m3. Its CGS unit is g/cc. The density shows heaviness or lightness

of an object. The object having more density is heavier and the object having

less density is lighter. Density of iron is more than the density of wood. It

means if we take a piece of iron and wood each of equal volume, the mass

49 Times' Crucial Science & Environment Book - 8

of iron is more than that of the wood. Density of a material does not depend
XSRQ VL]H DQG VKDSH ,W UHPDLQV À[HG IRU D SDUWLFXODU PDWHULDO )RU H[DPSOH
density of iron remains unchanged even though its shape and size changes.

ĐƟǀŝƚLJ ϰ͘ϳ 7R ÀQG WKH GHQVLW\ RI DQ REMHFW

Materials required:

Beam balance, measuring 500
cylinder, a piece of stone, 400
etc.

Procedure : 300

7DNH D VWRQH DQG ÀQG 200
its mass by using a 100
beam balance.

7DNH D PHDVXULQJ F\OLQGHU DQG ÀOO LW ZLWK ZDWHU XSWR D FHUWDLQ OHYHO

3XW WKH VWRQH LQ WKH ZDWHU RI PHDVXULQJ F\OLQGHU DQG ÀQG WKH LQFUHDVHG
volume of the water. The difference between the initial volume and
increased volume of the water gives the volume of iron.

4. Find the density of the stone by using formula

Density = Mass (kg)
Volume (m3)

For convenience, you can measure the mass of stone in gram and the volume
in cubic centimetre (cc).

Relative density

When density of a substance is compared with the density of water at 4°C, it
is called relative density of the substance.

It is calculated by the formula,

Relative density = Density of the substance
Density of water at 4°C

Relative density does not have any unit since it is the ratio of similar physical
quantities.

Numerical example: If the density of iron is 7.9 g/cc and density of water at
ƒ& LV J FF ÀQG WKH UHODWLYH GHQVLW\ RI LURQ

Relative density = Density of the Iron = 7.9 = 7.9
Density of water at 4°C 1

It means that the density of iron is 7.9 times more than the density of water.

Times' Crucial Science & Environment Book - 8 50

Density of some substances

SN. Substance Density (Kg/m3) Density (g/cc)
1. Water 1000 1.0
2. Kerosene 800 0.8
3. Ice 920 0.92
4. Wood 800 0.8
5. Iron 7800 7.8
6. Mercury 13600 13.6

'HQVLW\ RI D ERG\ DQG ÁRDWDWLRQ

Floatation of an object depends upon its density. If the density of an object
LV OHVV WKDQ WKDW RI OLTXLG WKH REMHFW ÁRDWV RQ WKH OLTXLG :KHQ DQ REMHFW KDV
density more than that of a liquid, the object sinks in the liquid. When density
of an object is equal to that of a liquid, the object sinks at the surface of the
liquid.

1g/cc 0.9g/cc

1.5g/cc

&ŝŐ ΖĂΖ &ŝŐ ΖďΖ &ŝŐ ΖĐΖ

,Q WKH DERYH ÀJXUHV REMHFWV KDYLQJ GLIIHUHQW GHQVLWLHV DUH NHSW LQ WKH ZDWHU RI
GHQVLW\ J FF ,Q ÀJ
D
WKH GHQVLW\ RI REMHFW LV J FF ZKLFK LV PRUH WKDQ WKH
density of the water. Therefore, it sinks in the water.

,Q ÀJ
E
WKH GHQVLW\ RI WKH REMHFW DQG GHQVLW\ RI WKH ZDWHU DUH HTXDO L H
1 g/cc. Therefore, the surface of the object is at the same level as the surface
RI WKH ZDWHU ,Q ÀJ
F
WKH GHQVLW\ RI WKH REMHFW LV OHVV WKDQ WKDW RI WKH ZDWHU
7KHUHIRUH LW ÁRDWV RQ WKH ZDWHU VXUIDFH

Solved Numerical Problem 4.4

A body has a mass of 500kg and volume of 0.5m3. Find its density.

Given,

Mass (M) = 500 kg

Volume (V) = 0.5m3

Density (D) = ?

We have,

D = M
V
500 5000
= 0.5 = 5 =1000 kg/m3

Therefore, density of the body is 1000kg/m3.

51 Times' Crucial Science & Environment Book - 8

Solved Numerical Problem 4.5

Find the density of the given object.

Given,

Volume of the object (V) = 4 × 5 × 5 cm3 0.5 kg 5cm
4cm
= 100 cm3

Mass (M) = 0.5kg = 0.5 × 1000g

= 500 g 5cm

Density (D) = ?

We have,

D = M = 500 = 5 g/cc
V 100

Therefore, density of the objective 5 g/cc.

Solved Numerical Problem 4.6

Density of a substance is 2.5 g/cc. If its mass is 5kg, calculate its volume.

Given,

Density (D) = 2.5 g/cc

Mass (M) = 5 kg = 5 × 1000 g = 5000 g

Volume (V) = ?

We have,

V = M = 5000 = 50000 = 2000cc
D 2.5 25

Therefore, volume of the substance = 2000 cc.

ĂůƟƚƵĚĞ : height

vacuum ͗ ƐƉĂĐĞ ĞŶƟƌĞůLJ ĨƌĞĞ ĨƌŽŵ ŵĂƩĞƌ

ǁĞĂƚŚĞƌ ĨŽƌĞĐĂƐƟŶŐ ͗ ƚŽ ƉƌĞĚŝĐƚ ƚŚĞ ĐŽŶĚŝƟŽŶƐ ŽĨ ǁĞĂƚŚĞƌ

1. Pressure is defined as the force acting per unit area.
2. The pressure exerted by the atmosphere is called atmospheric pressure.
3. Atmospheric pressure decreases when altitude increases.
4. Manometer is a device which is used to measure pressure exerted by lungs.
5. Barometer is an instrument which is used to measure atmospheric pressure.
6. Pressure gauge is an instrument which is used to measure air pressure exerted by

air contained in the tube of vehicles, balls, etc.

Times' Crucial Science & Environment Book - 8 52

7. The pressure exerted by a liquid due to its weight is called liquid pressure.
8. Liquid pressure increases with depth.
9. Liquid pressure increases when density of the liquid increases.
10. Liquid transmits pressure equally in all directions.
11. Liquid maintains its own level.

Exercise

A. Fill in the blanks.
1. The pressure exerted by atmosphere is called …………… .
2. Atmospheric pressure increases with increase in …………… .
3. …………… is used to measure pressure exerted by lungs.
4. …………… barometer does not use any liquid.
5. …………… is the formula for the calculation of liquid pressure.
6. Liquid maintains its …………… .
««««« GHÀQHG DV WKH IRUFH DFWLQJ SHU XQLW DUHD

B. Answer these questions in short.
1. What is pressure ? Mention its unit and formula.
2. What are the factors that affect pressure?
3. What is atmospheric pressure ? In which unit is it measured?
4. What is manometer ? How does it work?
'HÀQH L %DURPHWHU LL 3UHVVXUH JDXJH
6. What is liquid pressure ? What is the formula for liquid pressure ?
7. Derive: P = dgh
8. Mention some uses of atmospheric pressure.
9. Mention some properties of liquid pressure.

C. Give reasons: (Take g = 9.8 m/s2)
1. It is easier to cut with a sharp knife than with a blunt one.
2. Nose bleeding occurs when we move to higher altitudes.
3. We don’t feel enormous pressure of atmosphere.
4. The liquid pressure varies with the depth.

53 Times' Crucial Science & Environment Book - 8

$ EXFNHW LV ÀOOHG IDVWHU DW WKH ORZHU VWDLUV WKDQ DW WKH XSSHU VWDLUV
6. The bottom of the dam is made thicker.
7. A balloon gets burst if you pump much air into it.

D. Numerical problems. (Take value of g = 10 m/s2)
1. A box has a mass of 4kg and surface area 4m2. Calculate the
SUHVVXUH H[HUWHG E\ WKH ER[ RQ WKH ÁRRU
2. Calculate the force applied if 200 Pascal pressure is exerted over
the area of 0.2m2.
3. Calculate the surface area of a box whose mass is 200kg and exerts
D SUHVVXUH RI 3DVFDO RQ WKH ÁRRU
4. What is the pressure exerted by a liquid on the bottom of a vessel if
the height of the level of liquid is 2m and density is 13600kg/m3?

5. Calculate the pressure exerted by the liquid at the 4m
bottom of the container if its density is 1000 kg/m3.

6. Calculate the pressure exerted by the water when
D WDQN KDYLQJ GLPHQVLRQV Pð Pð P LV KDOI ÀOOHG
with it.

Answers 2. 40N 3. 20m2
5. 4000Pa 6. 5000Pa
D. 1. 10N/m2
4. 272000Pa

Collect some materials such as stone, brick, cork, cooking oil, kerosene oil,
wood, iron, etc. Measure their mass and volume using beam balance (or
physical balance) and a measuring cylinder. Calculate the density of each of
them and compare your values with those given in the book.

Times' Crucial Science & Environment Book - 8 54

5CHAPTER Work, Energy
and Power

James Watt
He is known for the discovery of steam engine,

separate condenser, parallel motion, etc.

Estimated Periods: 6
Objectives: At the end of the chapter, the students will be able to:

'HILQH ZRUN ZULWH LWV ZD\ RI PHDVXUHPHQW DQG WHOO LWV XQLW
'HILQH HQHUJ\ DQG LQWURGXFH GLIIHUHQW IRUPV RI HQHUJ\
([SODLQ WKH WUDQVIRUPDWLRQ RI HQHUJ\
'LVWLQJXLVK EHWZHHQ ZRUN HQHUJ\ DQG SRZHU
6ROYH VLPSOH QXPHULFDO SUREOHPV UHODWHG WR ZRUN HQHUJ\ DQG SRZHU

How are we able to do work?
What is work? How is work different from energy?
Do the energy and power mean the same? Discuss.

Introduction

Force is necessary to do work. The work means the use of force to move an
object through some distance. Hence, the amount of work done depends upon
force. On the other hand, stored energy in a body enables it to apply force.
7KXV ZH EHFRPH DEOH WR ZRUN ZKHQ ZH KDYH VXIÀFLHQW HQHUJ\ WR H[HUW IRUFH
for moving an object.

Work

We understand any of our tasks as work in our daily life. For example, reading,
cooking food, caring children, looking after a house, etc are also regarded as work.
But in physics, they are not work. Then, what are these if not work? All these
activities are the jobs of people. Hence, it is the job (but not work) of a student to
read and write, it is the job of a cook to prepare foods, it is the job of a watchman to
look after a building, etc. Interestingly, all jobs involve work in one way or the other.
If you want to perform work, you need to use force to move something. However,
if the force applied by you is unable to move the object, no work is done. You
can do work only when an object that you are pushing or pulling actually

55 Times' Crucial Science & Environment Book - 8

moves. If you lift a book out of your bag, you exert a force and do work. But, if
you simply hold the book (without moving) in front of you, will you do work?
No matter your muscles get tired due to holding the book still, you are not
doing work because you are not moving the book. Work is done only if a force
applied upon a body moves it through some distance.
Thus, work is said to be done if a force applied upon a body moves the body
through some distance in the direction of force. The distance through which a
body moves in a straight line is called its displacement. So, work is expressed
mathematically as:

Work = Force × Distance
Or, W = F × d
The SI unit of work is Nm or Joule (J). Its CGS unit is erg. Here, from the
GHÀQLWLRQ RI ZRUN ZH KDYH

W=F×d
If a force of 1N moves a body through 1 metre distance, then

W = 1N × 1m
1J = 1N × 1m
Hence, the work done is said to be one joule if a force of 1 Newton moves a body
through 1 metre distance in the direction of force.
1 Joule = 1 Newton × 1 metre
Sometimes, we use the multiples of joule to measure heavy work. These units
are kilojoule (KJ) and mega joule (MJ).
1KJ = 103 Joules
1MJ = 106 Joules

Special cases of work

Work can be positive, negative or zero based on the direction of applied force
and the movement of the body.
Case I: Work done is positive if the body has displacement in the direction

of the applied force.
Example: When you push a stone away from you in a straight line, you apply force
away from you and the stone also moves away from you. In such case, both force
and displacement act in the same direction. Hence, the work done is positive.
Case II: Work done is negative if the body has displacement in the opposite

direction of the force considered.

Times' Crucial Science & Environment Book - 8 56

Example: When you lift a stone straight up from the ground, the work done by
the force of gravity is negative. This is because the force of gravity pulls the
stone vertically downward but its displacement is in the upward direction.
Case III: Work done is zero if the applied force cannot move an object.
Example: If you push the trunk of a big tree, you cannot move it no matter you
apply all of your strength. In such case, although you applied force, no work is done.

Conditions for the work done

:RUN LV GRQH RQO\ LI WKH IROORZLQJ HVVHQWLDO FRQGLWLRQV DUH IXOÀOOHG
a. Force must be applied upon a body.
b. The force must displace the body in the direction of force.

Types of work done

The amount of work done is affected by two forces, i.e. gravity and friction.
Thus, work done can be divided into two types:

1. Work done against gravity

If we try to lift a sack of rice from 'ƌĂǀŝƚLJ
WKH ÁRRU ZH IHHO GLIÀFXOW EHFDXVH
the sack is pulled downward
by the gravity of the earth. The
force with which the earth pulls
every object towards its centre is
called gravity.If we lift the sack
up through some height, force is
applied against gravity and the
work is done against the gravity.

Thus, if the force is applied in opposite direction of gravity while doing
work, it is called work done against gravity.

2. Work done against friction

When we try to pull a
heavy wooden log along
the ground, we experience
GLIÀFXOW\ ,W LV GXH WR DQ
opposing force produced
between the two surfaces
in contact. The contact
&ƌŝĐƟŽŶ

between the lower surface of wooden log and the top surface of ground

57 Times' Crucial Science & Environment Book - 8

produces the opposing force. This force which opposes the motion of one
body on the surface of another is called frictional force. The frictional
force acts in opposite direction of the motion of the body.
Hence, if the force is applied in the opposite direction of friction while
doing work, it is called work done against friction.

Energy

We must apply force to do work. But, we can apply force only if we have
VXIÀFLHQW HQHUJ\ LQ RXU ERG\ )RU H[DPSOH FKLOGUHQ DQG HOGHUO\ SHRSOH FDQQRW
carry heavy loads and run very fast. However, young people can perform such
activities easily. These examples show that people having more energy can
exert more force and perform greater amount of work. The same principle
applies to machines; the capacity of doing work by a machine depends on the
amount of energy supplied. Thus, energy is directly related to our work.

(QHUJ\ LV GHÀQHG DV WKH FDSDFLW\ RI D ERG\ WR GR ZRUN The SI unit of energy is
joule. The work and energy have the same units. One joule energy is that amount
of energy which can move a body of weight 1N through 1 metre.

Forms of energy

Energy exists in different forms. Some of the major forms of energy are
explained below:

1. Mechanical energy

It is the energy of a body due to its state of rest or of uniform motion.
Objects have mechanical energy if they are in motion or if they are in a
higher position relative to other objects. The mechanical energy can be
divided into two types- potential energy and kinetic energy. It means that
mechanical energy is the sum of kinetic and potential energy of a body.

a) Potential energy

The energy of a body due to its position or
FRQÀJXUDWLRQ LV FDOOHG SRWHQWLDO HQHUJ\ For
example, a stretched rubber in a catapult,
a brick held at a vertical position above the
ground, water stored in a dam, a compressed
spring, a drawn bow, etc have potential energy.

The potential energy of a body of mass ‘m’ WŽƚĞŶƟĂů ĞŶĞƌŐLJ
when it is raised above the ground at height (of a stretched rubber)
‘h’ is given by: PE = m×g×h

Where, g = acceleration due to gravity and, g= 9.8m/s2.

Times' Crucial Science & Environment Book - 8 58

Solved Numerical Problem 5.1
If a brick of mass 2 kg is raised above the ground at the height of 3 m,
what is its potential energy?
Given,
Mass of brick (m) = 2 kg.
Height above the ground (h) = 3 m
Acceleration due to gravity (g) = 9.8 m/s2 (known value)
Potential energy (PE) = ?
Now, we have: PE = m×g×h = 2 × 9.8 × 3 = 58.8 J.

b) Kinetic energy

The energy of a body due to
its state of motion is called
kinetic energy. For example,
a moving car possesses kinetic
energy due to its motion.
Similarly, energy of a bullet
<ŝŶĞƟĐ ĞŶĞƌŐLJ

ÀUHG IURP D JXQ HQHUJ\ RI (of a bullet)

ÁRZLQJ ZDWHU HQHUJ\ RI ZLQG HQHUJ\ RI D VWRQH UROOLQJ GRZQ D FOLII
etc have kinetic energy. The kinetic energy of a body depends on its
mass and velocity. Objects having more mass, velocity or both have
more kinetic energy.

The kinetic energy (KE) of a body of mass ‘m’ moving with a velocity
‘v’ is given by:

KE = 1 mv2
2
The equation shows that kinetic energy of a body is equal to half of
the product of mass of the body and the square of its velocity.

Solved Numerical Problem 5.2

A car of mass 500 kg is moving on a plane road with a velocity of 15
m/s. Calculate the kinetic energy of the car.

Given,

Mass of car (m) = 500 kg.

Velocity of car (v) = 15 m/s

Kinetic energy of car (KE) = ? 1
1 2
Using the formula: KE = 2 mv2 = × 500 × (15)2 = 56,250 J.

59 Times' Crucial Science & Environment Book - 8

2. Heat energy

In general sense, heat is the form of energy that gives
the sensation of warmth. It arises due to the motion
of molecules in a body. Hence, KHDW HQHUJ\ LV GHÀQHG
as the total kinetic energy of the molecules contained
in a body.

Heat energy is used for numerous purposes in our
daily life. We need heat energy to keep our body warm,
,ĞĂƚ ĞŶĞƌŐLJ

to cook our food, to dry our clothes, to run vehicles, to run machines in
industries, to melt metals, etc. The sun is the main source of heat energy
IRU WKH HDUWK 7KH RWKHU VRXUFHV RI KHDW DUH ÀUH HOHFWULF KHDWHU EXUQLQJ
coal, petroleum products, etc.

3. Light energy

Light is the form of energy which makes the things visible. Generally,
light is given off by the hot bodies. The sun, glowing electric lamps,
lantern, burning candle, tuki, etc are the hot sources of light. On the
RWKHU KDQG WKH PRRQ DQG ÀUHÁ\ DUH WKH FROG VRXUFHV RI OLJKW

ĂŶĚůĞ Ƶůď >ĂŶƚĞƌŶ ŝLJŽ ^ƵŶ DŽŽŶ

Light is very useful for all the living beings. We need light to perform
almost all of our activities. Animals need light to walk around and search
their food. Plants need light to prepare their food by photosynthesis.

4. Electrical energy

7KH HQHUJ\ RI D ERG\ GXH WR WKH ÁRZ RI HOHFWULFLW\ LV FDOOHG HOHFWULFDO HQHUJ\
Actually, the electrical energy is caused due to the electrons present in
a conductor. The electricity is of two types on the basis of motion or rest
state of the electrons. They are current electricity and static electricity.
7KH HOHFWULFLW\ GXH WR WKH ÁRZ RI HOHFWURQV LQ D FRQGXFWRU LV FDOOHG FXUUHQW
electricity. Similarly, the electricity due to the deposition of electrons in
a body is called static electricity. Mainly, the current electricity is used
in our daily life for different purposes.

The electrical energy is obtained from different sources, such as
hydroelectricity plant, generator, dynamo, battery, photoelectric cell, etc.

Times' Crucial Science & Environment Book - 8 60

The current electricity produced in the source is brought to our home using
the conducting wires. Current electricity is used to run different devices
such as radio, television, computer, electric heater, electric iron, etc.

Television Iron Heater Computer

5. Chemical energy

The energy stored in chemical compounds is known as chemical energy.
The energy contained in our foods such as rice, Roti, cake, bread, pulses,
fats, oil, etc is regarded as chemical energy. Similarly, the fuels such as
diesel, petrol, coal, etc and the electric cells also have chemical energy.

Chemical energy is obtained as a result of chemical reaction.In our body,
the digested food reaches the cells and burns to produce heat energy. We
use such energy to keep our body warm and perform different activities. An
electrical cell changes its chemical energy into electrical energy. A gas stove
changes the chemical energy of cooking gas into heat and light energy.

'ĂƐ ĐLJůŝŶĚĞƌ ƌĞĂĚ &ŽŽĚ &ŝƌĞ

6. Nuclear energy

The nucleus of an atom possesses a large amount of energy. This energy
can be released when a large nucleus breaks down into smaller ones or
when smaller nuclei combine to form a heavier nucleus. Such processes
are called nuclear reactions. The energy which is obtained from the
nuclear reactions is called nuclear energy. The nuclear energy can be
used to generate electricity.
The process of breaking down of a heavy nucleus into lighter ones is
FDOOHG QXFOHDU ÀVVLRQ Similarly, the process of combining lighter nuclei
into heavy nucleus is called nuclear fusion. Nuclear fusion occurs in the
VXQ 7KH QXFOHDU ÀVVLRQ DQG IXVLRQ SURGXFH KXJH DPRXQW RI HQHUJ\

61 Times' Crucial Science & Environment Book - 8

7. Magnetic energy

The energy possessed by a magnet is called magnetic
energy. The magnetic energy is used in iron industries
and mines to lift heavy loads. Magnetic energy is also
used to produce electricity. Magnetic energy is used
in several devices such as radio, television, computer,
microphone, electric toys, etc.

8. Sound energy

The energy produced due to the vibration of bodies is called sound
energy. We hear different kinds of sounds with our ears. We can produce
different sounds by the vocal cords present inside our neck. Similarly, a
ringing bell, guitar, Madal, radio, etc are the sources of sound.

Guitar Madal Radio Bell

Sound travels through a medium by creating disturbance in the medium
molecules. It can travel through all three states of matter, i.e. solid, liquid
and gas. Sound can exert force on the surface where it falls. Hence, sound
is taken as the form of energy.

Transformation of energy

In fact, energy can neither be created nor be destroyed; just one form of
energy can be changed into the other form. This is known as the principle of
conservation of energy. In other words, we cannot create energy according to
our need. So, we have to change the available form of energy into the required
form using a device. For example, wax has chemical energy. We turn it into a
candle to use it according to our need. When we burn a candle, the chemical
energy of the candle is transformed into light and heat energy. The process of
change of one form of energy into another is called transformation of energy.
Transformation of energy occurs inside our body also. In our body, the chemical
energy of the digested food is turned into heat energy during respiration. This
energy keeps us warm and helps us to do different types of work.
There are different devices to convert one form of energy into another. For
example, an electric cell converts chemical energy to electrical energy, a bulb
converts electrical energy into heat and light energy, a radio converts electrical

Times' Crucial Science & Environment Book - 8 62

energy into sound energy, a microphone changes sound energy into electrical
energy, an electric bell transforms electrical energy into sound energy, etc.

Bulb Radio Microphone Electromagnet

Power

3RZHU LV GHÀQHG DV WKH UDWH RI GRLQJ ZRUN ,W PD\ DOVR EH GHÀQHG DV WKH UDWH
of transformation of energy. Mathematically,

Power = Work done
Time taken
W
? P = t

The SI unit of power is Watt (W). The unit Watt is derived from the units of
work and time. Since Joule (J) is the SI unit of work and second (s) is the SI
unit of time, the SI unit of power is J/s. The unit J/s is known as Watt.

The bigger units of power are Kilowatt and Megawatt. Kilowatt and Megawatt
are the multiples of Watt.

1000 watt = 1 kilowatt (1kw)

1000 kilowatt = 1 megawatt (1Mw)

Electrical power is expressed in the units of KW and MW. In domestic use,
the consumed electricity is measured in terms of ‘unit’. One unit is equal
to 1 kilowatt hour (KWH). If one kilowatt power of electricity is used by an
equipment for one hour, the consumed electricity is 1 KWH or 1 unit.

Power is also measured in the unit horse power (HP).
1 H. P. = 746 Watt = 750 Watt (approximately)

Here, W
t
P =

If a work of 1 Joule is done in the time of 1 second, then

P = 1J/1s ? 1W = 1J/1s

Thus, power of a body is said to be one Watt if it performs the work of one
Joule in one second.

63 Times' Crucial Science & Environment Book - 8

You must have seen electrical lamps labelled with 25W, 40W, 60W, 100W,
etc. If a bulb is labelled with 100 watt, it means that the bulb can convert 100
joules of electrical energy into light and heat energy in one second.

Solved Numerical Problem 5.3
If a force of 200N throws a brick 20 metres away, what is the work done?
Given,

Force (F) = 200N
Distance (d) = 20m
Work (w) = ?
Using the formula,
W = F × d = 200N × 20m = 4000Joule
Therefore, the work done is 4000 Joule.

Solved Numerical Problem 5.4

Krishnagopal’s mass is 50 kg. He carries a rice sack of mass 20 kg to
the distance of 300 m in 1 minute. What is his power?

Given,

Force (F) = Weight = m × g = (50 kg + 20 kg) × 10 = 700 N [ J § P V2]

Distance (d) = 300m

Time (t) = 1 min = 60 seconds

Work (W) = ?

Using the formula,

W = F × d =700 × 300 = 2,10,000 J = 2.1 × 104 J.

Again, =thWet p=ow21e06r 00o0f 0K=ris3h,5n0a0goWp.al

P is 3,500 W.
Hence,

Solved Numerical Problem 5.5
,I D EXOOHW ÀUHG IURP D JXQ KDV D PDVV RI JUDPV DQG NLQHWLF HQHUJ\
20 Joule, calculate the velocity of the bullet.
Given,
Mass of bullet (m) = 100g = 0.1 kg
Kinetic energy (KE) = 20J
Velocity (v) = ?
We know that,

Times' Crucial Science & Environment Book - 8 64

KE = 1 mv2
2
1
or, 20 = 2 × 0.1 × v2

or, 40 = 0.1 × v2

or, v2 = 40 0.1 or, v2 = 400 or, v = 400 v = 20 m/s

Hence, the velocity of the bullet is 20m/s.

Relation between work, energy and power

Energy is the capacity of a body to do work. When a person performs some
work upon a body, the person loses some energy. The body gains some energy
if work is done upon it. According to the principle of energy, the amount of
energy lost by the person is equal to the amount of energy increased in the
body. For example, work is done if a person pushes a log of wood (load) over
a plane surface through some distance. While doing so, the person loses some
energy whereas the load gains that energy and comes to motion.

The rate of change of one form of energy into another is called power. It is the
measure of how fast a person or a machine can change one form of energy in
another. If we transform one form of energy into the other very fast, our power
ZLOO EH PRUH ,Q WKH VDPH ZD\ LI PRUH HQHUJ\ LV XVHG WR GR D ZRUN LQ D À[HG
time, the power will be more. Thus, we can say that work, energy and power
are interrelated.

ĨƌŝĐƟŽŶ ͗ ŽƉƉŽƐŝŶŐ ĨŽƌĐĞ ƚŽ ŵŽƟŽŶ ŽĨ ĂŶ ŽďũĞĐƚ
catapult ͗ Ă ďĂůůŝƐƟĐ ĚĞǀŝĐĞ ƵƐĞĚ ƚŽ ƚŚƌŽǁ ƐƚŽŶĞ
lantern ͗ Ă ůĂŵƉ ǁŝƚŚ Ă ƚƌĂŶƐƉĂƌĞŶƚ ĐĂƐĞ ƉƌŽƚĞĐƟŶŐ ƚŚĞ ŇĂŵĞ

1. Work is said to be done if a force applied upon a body moves it through some
distance in the direction of force.

2. Work done is said to be 1 Joule if a force of 1 Newton moves a body through 1
metre in the direction of force.

3. The capacity of a body to do work is called energy.
4. Energy exists in several forms. They are: mechanical energy, heat energy, light

energy, electrical energy, chemical energy, nuclear energy, magnetic energy, sound
energy, etc.
5. The rate of doing work by a body is called its power.
6. The process of change of one form of energy into another is called transformation
of energy.

65 Times' Crucial Science & Environment Book - 8

Exercise

A. Answer these questions in very short.
1. What enables us to do work?
2. What is 1 Joule work?
3. What is energy?
'HÀQH :DWW SRZHU
:KDW LV QXFOHDU ÀVVLRQ"
'HÀQH SRZHU DQG ZULWH LWV XQLW
7. The power of a bulb is 100W. What does it mean?
,V ZRUN GRQH RQ D ERRN LI IDOOV GRZQ WR ÁRRU IURP D GHVN" ,I VR ZKDW
force is involved?

% 'HÀQH 2. 1 Joule energy 3. Kinetic energy
1. Work 5. Nuclear fusion 6. Transformation of energy
4. Potential energy

C. Name the form of energy possessed by the following:

1. Roti 2. Electric cells 3. Madal

4. Flowing river 5. Firewood 6. Flying bird

7. Stretched rubber 8. Wind

D. What kind of energy transformation occurs in the following?

1. Speaker 2. Electric bulb 3. Electric fan

4. Microphone 5. Radio 6. Mobile phone

E. Write down differences between:

1. Kinetic and potential energy 2. Work and energy

1XFOHDU ÀVVLRQ DQG QXFOHDU IXVLRQ :RUN DQG SRZHU

F. Answer these questions:
1. What is work? Write down the conditions necessary for doing work.
2. What factors affect work? Explain.
3. What is current electricity? Write down its use in our daily life.

Times' Crucial Science & Environment Book - 8 66

4. Lakpa and Pasa each pushed an empty wheelbarrow. Although
Lakpa used twice the force, they did the same amount of work. How
is it possible?

5. When a footballer kicks a ball, work is done on the ball. Is the
footballer still doing work on the ball as it rolls across the ground?
Explain.

6. You pushed the wall of your classroom very hard but it did not
move. Did you do work? Explain.

7. What is transformation of energy? Explain with examples.

G. Solve the following numerical problems. (Take value of g = 10 m/s2)
1. Deepta moves a load of 30 kg through 10 metres. What is the work
done by him?
2. Dristika's weight is 30 kg. If she climbs up a ladder of height 6
metres carrying a load of 20 kg, what is the work done by her? If she
takes 30 seconds for doing this work, what is her power?
3. Suyog throws a stone of weight 2 kg through a distance of 30 meters
in 2 seconds. Calculate his power.
4. Calculate the potential energy of a stone of mass 70 kg which is
lifted above the ground by 5 metres.
5. If a moving body of mass 100g has kinetic energy 45 Joules, what
will be its velocity?

Answers 2. 3000 Joules, 100 W 3. 300W
5. 30m/s
G. 1. 3000 Joule
4. 3430 Joules

Observe the working of different devices such as electrical cell, radio, television,
microphone, torch light, electric heater, bell, speaker, electric motor, ceiling
fan, etc. What kind of energy transformation occurs in each of these devices?
:ULWH \RXU REVHUYDWLRQ DQG ÀQGLQJV

67 Times' Crucial Science & Environment Book - 8

6CHAPTER Heat

Daniel Gabriel Fahrenheit

'DQLHO *DEULHO )DKUHQKHLW ZDV D 'XWFK *HUPDQ 3ROLVK
SK\VLFLVW HQJLQHHU DQG JODVV EORZHU ZKR LV EHVW NQRZQ
IRU LQYHQWLQJ WKH PHUFXU\ LQ JODVV WKHUPRPHWHU DQG IRU

developing a temperature scale now named after him.

Estimated Periods : 6
Objectives: At the end of the chapter, the students will be able to:

GHILQH KHDW DQG WHPSHUDWXUH
VKRZ WKH UHODWLRQ EHWZHHQ KHDW DQG WHPSHUDWXUH
PHQWLRQ YDULRXV XQLWV RI WHPSHUDWXUH DQG VKRZ WKH UHODWLRQVKLS DPRQJ WKHP
H[SODLQ WKH VWUXFWXUH DQG IXQFWLRQV RI ODERUDWRU\ DQG FOLQLFDO WKHUPRPHWHU

Why is ice colder than wood?
Are heat and temperature the same?
What instrument do you use to measure your body temperature?
In which units do you measure your body temperature?
Which instrument is used to measure maximum and minimum temperature of a
day? Discuss.

Introduction

When we touch a cup of hot tea or hot iron, we feel hot. Here, energy transfers from
those hot bodies to our body and we feel hot. This energy is called heat energy.

+HDW LV GHÀQHG DV D IRUP RI HQHUJ\ ZKLFK EULQJV VHQVDWLRQ RI ZDUPWK

,WV 6, XQLW LV -RXOH ,W LV DOVR PHDVXUHG LQ FDORULH +HDW DOZD\V ÁRZV IURP
hotter objects to colder objects.

Every substance is made up of molecules. These molecules are in vibrating
condition. Due to the vibration of the molecules, every molecule contains
kinetic energy. The total sum of kinetic energy of molecules of a body is called
heat energy. The kinetic energy of each molecule depends upon its mass and
speed of vibration. The kinetic energy of a moving object is:

KE = 1 mv2
2

Thus, total heat energy contained in a substance depends upon the total mass
of the molecules and average speed of vibration of molecules.

Times' Crucial Science & Environment Book - 8 68

The more the speed of vibration of the molecules, the more the heat energy
contained in the object. The more the total mass of molecules, the more the
heat energy contained in the object.

Temperature

When we touch warm water, we feel hot. But we feel hotter when we touch
boiling water. The warm water and boiling water are not equally hot. We
need numerical quantity to express the degree of hotness or coldness. This
quantity is called temperature. 7HPSHUDWXUH LV GHÀQHG DV D TXDQWLW\ ZKLFK
measures the degree of hotness or coldness of a body. The hotter objects have
more temperature than colder objects.
When a body is heated, the vibration of the molecules contained in it increases
then the temperature too. The increase in vibration of molecules causes the
increase in temperature. Thus, temperature measures the average speed of
vibration of molecules.
The SI unit of temperature is Kelvin (K). It is also measured in degree
Centigrade (°C) and degree Fahrenheit (°F).

Heat and temperature

Heat contained in an object is the sum of kinetic energy of all the molecules
contained in it. Kinetic energy of a molecule depends upon its mass and average
speed. The objects having more heat are hotter and the objects having less
heat are colder. But, heat is contained in all types of objects either hotter or
colder. Ice also has heat because its molecules contain kinetic energy. A body
becomes colder or hotter due to the difference in temperature. Hotter bodies
have more temperature. The molecules contained in hotter bodies vibrate with
higher speed. Thus, heat and temperature are very much related. A hotter
body can have more temperature than the colder one but may not have more
heat.
Activity 6.1 To study the difference between heat and temperature
Materials required:
Beaker, a kettle, stand, burner, water,
thermometer, etc.
Procedure:
D 7DNH D NHWWOH DQG ÀOO KDOI RI LW ZLWK ZDWHU
b. Put the kettle on a tripod stand and heat

with a burner so that the water becomes
warm.

69 Times' Crucial Science & Environment Book - 8

F 7DNH D EHDNHU DQG ÀOO LW ZLWK ZDWHU XS WR KDOI OHYHO
d. Heat the water of the beaker up to boiling point.
e. Measure the temperature of the water of kettle and beaker with

thermometer. What do you observe?
Observation:
The temperature of water of beaker is more than that of kettle. But total heat
energy of the water of kettle is more than that of beaker.
Explanation:
Temperature of a body depends only upon the speed of vibration of the molecules
but not on the mass of the molecules. But, heat depends upon both the mass and
speed of vibration of molecules. Due to more speed of vibration of molecules
in the water of beaker, it has more temperature. As the kettle contains more
water, the water of kettle has more heat due to more mass of the molecules even
though the speed of vibration of molecules is less.

Differences between heat and temperature

Heat Temperature

1. It is a form of energy which 1. It is the degree of hotness or
produces sensation of warmth. coldness of an object.

2. Heat contained in an object 2. Temperature of a substance
depends upon the mass of total depends upon the speed of the
molecules and the speed of vibration of molecules but not on
vibration of the molecules. the mass of the molecules.

3. Its SI unit is Joule. 3. Its SI unit is Kelvin (K).

4. It is measured by calorimeter. 4. It is measured by a thermometer.

Thermometer

It is a device which is used to measure the temperature of a substance.

It works on the principle that “the liquid expands on heating and contracts on
FRROLQJµ 7KH EXOE RI D WKHUPRPHWHU LV ÀOOHG ZLWK D OLTXLG FDOOHG WKHUPRPHWULF
liquid. Mercury or alcohol is used as thermometric liquid in a thermometer.

Thermometer is of two types:

a) Laboratory thermometer b) Clinical thermometer

Characteristics of thermometric substance

a. Mercury
1. It is a silver coloured shiny liquid. Therefore, it can be seen clearly
in the glass.

Times' Crucial Science & Environment Book - 8 70

2. It is good a conductor of heat. Therefore, it easily passes heat from
the object.

3. It does not wet the wall of glass tube. Thus, rise and fall of mercury
in the tube is clear.

,W IUHH]HV DW ï ƒ& DQG ERLOV DW ƒ& 7KHUHIRUH LW PHDVXUHV WKH
WHPSHUDWXUH IURP ï ƒ& WR ƒ&

5. It has uniform rate of expansion and contraction.

b. Alcohol
1. It is a transparent colourless liquid. Therefore, it is to be coloured
before use.
2. Its expansion rate is six times more than that of mercury.
,W IUHH]HV DW ï ƒ& DQG ERLOV DW ƒ& 7KHUHIRUH DOFRKRO
thermometer is used to measure very low temperature. But, it
cannot measure the temperature higher than 78°C.

Think and Solve

Mercury thermometer is used to measure the temperature of boiling
water. But the mountaineers carry alcohol thermometers. Why?

a. Laboratory thermometer

It is a thermometer which is used to measure the temperature of
substances. Generally, it is used in laboratory. It measures the
temperature from ï ƒ& to 110°C.

It consists of a capillary tube with a narrow bore. The capillary tube is
swollen to a bulb at one end and is sealed at the other end. The bulb
LV ÀOOHG ZLWK PHUFXU\ RU DOFRKRO 7KHUH LV D FRYHU RI JODVV RXWVLGH WKH
FDSLOODU\ WXEH 7KH RXWVLGH FRYHU LV JUDGXDWHG LQ GHJUHH IURP ï ƒ& WR
110°C. When the bulb comes in contact with a body whose temperature
is to be measured, there may be rise or fall of the level of mercury in the
FDSLOODU\ WXEH $IWHU VRPHWLPH WKH OHYHO RI PHUFXU\ UHPDLQV À[HG 7KH
level of mercury in the tube is the temperature of the object.

DĞƌĐƵƌLJ ĂƉŝůůĂƌLJ ƚƵďĞ Glass tube

Bulb Temperature scale
>ĂďŽƌĂƚŽƌLJ ƚŚĞƌŵŽŵĞƚĞƌ

b. Clinical thermometer
It is a thermometer which is used to measure the temperature of a
human body. A clinical thermometer consists of a capillary tube with a

71 Times' Crucial Science & Environment Book - 8

ÀQH ERUH 2QH HQG RI WKH FDSLOODU\ WXEH LV VZHOOHG WR D EXOE 7KH EXOE LV
ÀOOHG ZLWK PHUFXU\ RU DOFRKRO $QRWKHU HQG RI WKH WXEH LV VHDOHG 7KHUH LV
a narrow constriction in the capillary tube near the bulb. The capillary
tube is covered by a glass tube. It is graduated in degree from 35° to 42°C
(or 94°F to 108°F).

Alcohol ĂƉŝůůĂƌLJ ƚƵďĞ Glass tube

ŽŶƐƚƌŝĐƟŽŶ ůŝŶŝĐĂů ƚŚĞƌŵŽŵĞƚĞƌ Temperature scale
Bulb

When we have to measure the temperature of a human body, we keep the
bulb of thermometer in the armpit or mouth. The body heat results in rise
in the level of mercury and reading is obtained. When the thermometer
is taken out from the body, the temperature decreases. But, the level
of mercury does not fall to the bulb due to the presence of constriction
present in the capillary tube. This helps to take correct reading. After its
use, the thermometer is jerked to return the mercury to the bulb.
Activity 6.2
Take a clinical and a laboratory thermometer. Observe the bulb, capillary tube,
thermometric liquid used in them: alcohol or mercury. Find the temperature of
your body by using clinical thermometer. Using the laboratory thermometer,
ÀQG WKH WHPSHUDWXUH RI ZDWHU LFH HWF

Calibration of thermometer

The process of marking temperature scales in the thermometer is called
calibration. 7R FDOLEUDWH D WKHUPRPHWHU ZH KDYH WR ÀQG RXW WKH XSSHU DQG
ORZHU À[HG SRLQWV

/RZHU À[HG SRLQW

It is the temperature of pure melting ice at normal atmospheric pressure. Its
value is 0°C.

Activity 6.3 7R ÀQG ORZHU À[HG SRLQW Stand
Materials required:

A beaker, stand, uncalibrated thermometer, Funnel Thermometer
ice cubes and a funnel. Melting ice

Procedure:

1. Take some ice cubes in a funnel and Beaker
hold the funnel with a stand as shown Water
LQ WKH ÀJXUH

Times' Crucial Science & Environment Book - 8 72

2. Put a beaker just below the stem of the funnel.
3. Insert the bulb of a thermometer into the ice pieces.
4. Observe the level of the mercury in the tube.
Observation
You will see the falling of mercury level in the capillary tube. After sometime,
WKH OHYHO RI WKH PHUFXU\ UHPDLQV À[HG 7KLV SRLQW LV WKH ORZHU À[HG SRLQW RI WKH
WKHUPRPHWHU ,WV YDOXH LV ƒ& 0DUN WKH OHYHO RI PHUFXU\ DV ORZHU À[HG SRLQW

8SSHU À[HG SRLQW

It is the temperature of pure boiling water at normal atmospheric pressure.
Its value is 100°C.

Activity 6.4 7R ÀQG XSSHU À[HG SRLQW RI D WKHUPRPHWHU
Materials required:
$ URXQG ERWWRPHG ÁDVN FRUN ZLWK WZR KROHV WULSRG VWDQG D UHWRUW VWDQG
L-shaped glass tube, a wire gauge, a burner, etc.
Procedure:
7DNH D URXQG ERWWRPHG ÁDVN DQG SXW VRPH ZDWHU LQ LW
)LW D WKHUPRPHWHU DQG / VKDSHG JODVV WXEH RQ WKH PRXWK RI WKH ÁDVN

ZLWK WKH KHOS RI D FRUN DV VKRZQ LQ WKH ÀJXUH
.HHS WKH ÁDVN RQ WKH WULSRG VWDQG DQG À[ WKH VWHP RI WKH ÁDVN ZLWK D

retort stand.

+HDW WKH ZDWHU RI WKH ÁDVN ZLWK D EXUQHU

Thermo me ter L-shaped g lass tube

Stand Ro und bo ttom flask
Pure w ater
Wire g auze
Burner Tripod stand

Precaution:
The bulb of the thermometer should be little above the level of water.
Observation:
You will see the rising of the mercury level. When the water boils, the level of
WKH PHUFXU\ ZLOO EH WKH PD[LPXP DQG UHPDLQ FRQVWDQW ,W LV WKH XSSHU À[HG
point of thermometer. Its value is 100°C. Mark the level of mercury as the
XSSHU À[HG SRLQW $IWHU À[LQJ WKH ORZHU DQG XSSHU À[HG SRLQWV GLYLGH WKH
distance between them into hundred equal parts. Each part is 1°C. In this
way, calibration of the thermometer is done.

73 Times' Crucial Science & Environment Book - 8

0D[LPXP PLQLPXP WKHUPRPHWHU

The thermometer which is used to measure maximum and minimum
temperature reached within twenty four hours in a particular place is called
maximum- minimum thermometer.
,W FRQVLVWV RI D 8 VKDSHG JODVV WXEH SDUWO\ ÀOOHG A B
with mercury and partly with alcohol. Its one limb minimum maximum

vacuum

¶;· LV FRQQHFWHG ZLWK D ODUJH F\OLQGULFDO EXOE ¶$·
ZKLFK LV FRPSOHWHO\ ÀOOHG ZLWK DOFRKRO 7KH OLPE °C
‘Y’ is connected with a small bulb ‘B’ which is -25 alcohol °C
-20 40
SDUWO\ ÀOOHG ZLWK DOFRKRO 7KHUH LV OLWWOH YDFXXP -15 35
30

in the bulb ‘B’. -10 X metal indices Y 25
The limb Y shows the maximum temperature and -5 20
WKH OLPE ¶;· VKRZV PLQLPXP WHPSHUDWXUH 0 15
5 10
10 5

A small steel index is kept over the surface of 15 ŵĞƌĐƵƌLJ 0
mercury in each limb. When temperature of the 20 -5
surrounding atmosphere increases, the expansion 25 -10
of alcohol in bulb ‘A’ pushes the mercury. Thus 30 -15
the index of limb ‘Y’ moves upwards and points 35 -20
40 -25

maximum temperature. When the atmospheric
temperature decreases, the contraction of alcohol in bulb ‘A’ causes the index
RI OLPE ¶;· PRYH XSZDUGV WR UHFRUG WKH PLQLPXP WHPSHUDWXUH

The index cannot fall itself. So, it records maximum and minimum temperature.
Magnet is used to draw the indices to the surface of mercury for resetting the
thermometer for next use.

Temperature scales

There are mainly three scales for the measurement of temperature. They are:
(i) Celsius scale (ii) Kelvin scale (iii) Fahrenheit scale

a) Celsius scale (Centigrade Scale)
,Q WKLV VFDOH ƒ& LV WKH ORZHU À[HG SRLQW DQG ƒ& LV WKH XSSHU À[HG
point. The interval between these two points is divided into 100 equal
parts or divisions. Each part is called one degree centigrade or degree
Celsius. It is denoted by °C.

b) Fahrenheit scale
,Q WKLV VFDOH WKH ORZHU À[HG SRLQW LV ƒ) DQG WKH XSSHU À[HG SRLQW
LV ƒ) 7KH LQWHUYDO EHWZHHQ WKHVH À[HG SRLQWV LV GLYLGHG LQWR
equal parts or divisions. Each part is called one degree Fahrenheit. It is
denoted by °F.

Times' Crucial Science & Environment Book - 8 74

c) Kelvin scale
,Q WKLV VFDOH WKH ORZHU À[HG SRLQW LV . DQG WKH XSSHU À[HG SRLQW LV .
The interval between these two points is divided into 100 equal parts or
divisions. Each division or part is called one Kelvin. It is denoted by K.

Relation among the temperature scales

,Q DOO WHPSHUDWXUH VFDOHV WKH ORZHU À[HG SRLQW LV WKH WHPSHUDWXUH RI PHOWLQJ
LFH DQG WKH XSSHU À[HG SRLQW LV WKH WHPSHUDWXUH RI ERLOLQJ ZDWHU 7KH LQWHUYDOV
between these two points are divided into different parts in different scales.

Substance Celsius Fahrenheit Kelvin
Melting point of Ice 0°C 32°F 273K
Boiling point of water 100°C 212°C 373K

If ‘C’ is the reading in Celsius scale, ‘F’ is the reading in Fahrenheit scale and ‘K’ is
the reading in Kelvin Scale, the relation can be shown by the following equations.

C–0 = F–32 = F–273 Or, C = F–32 = F–273
100 180 100 5 9 5

Solved Numerical Problem 6.1
Convert 37°C into °F.

Given, C = 37

F=?

We have, C–0 = F – 32
or, 100 180

or, 37 – 0 = F – 32
100 180

RU ) ï

or, 100F = 9860

or, F = 98.6

? 37°C = 98.6°F

Solved Numerical Problem 6.2

Convert 37°C into K.

Given, C = 37

K=?

We have,

or, C–0 = K – 273
100 100

75 Times' Crucial Science & Environment Book - 8

or, 37 – 0 = K – 273
100 100

or, 37 = K - 273

or, K = 37 + 273

or, K = 310

? 37°C = 310 K

Solved Numerical Problem 6.3

Calculate the temperature at which both Fahrenheit Scale and Celsius
Scale show the same reading.

Let the temperature be x.

or, C–0 = F – 32
100 180

or, x–0 = x – 32
100 180

RU [ [ ï

RU [ ï

or, [x = ï– 38 2000
?

,W PHDQV ï ƒ& ï ƒ)
At ï40° both Fahrenheit scale and Celsius Scale have the same reading.

ŬĞƩůĞ : a container in which water is boiled

ĐŽŶƐƚƌŝĐƟŽŶ : narrow place

ĐĂƉŝůůĂƌLJ ƚƵďĞ ͗ Ă ƚƵďĞ ǁŝƚŚ Ă ǀĞƌLJ ƐŵĂůů ŚŽůĞ

1. Heat is a form of energy which brings sensation of warmth.

2. Temperature is the degree of hotness or coldness of a body.

3. Thermometer is a device which is used to measure the temperature of a body.

4. Clinical thermometer is the thermometer which is used to measure the temperature
of a human body.

5. Maximum- minimum thermometer is the thermometer which is used to measure the
maximum and minimum temperature reached within 24 hours in a particular place.

Times' Crucial Science & Environment Book - 8 76

6. Celsius scale, Fahrenheit scale and Kelvin scale are there different temperature
scales.

7. Mercury can measure temperature from –39°C to 357°C.
8. Alcohol can measure temperature from –117°C to 78°C.

Exercise

A. Answer these questions.
'HÀQH
i. Heat ii. Temperature iii. Thermometer
2. How is heat produced? What are the factors upon which the amount
of heat contained in a body depends?
3. What is laboratory thermometer? Draw its diagram and label
important parts.
4. What is clinical thermometer? Draw a diagram of clinical
thermometer and label its parts.
5. Mention the advantages of mercury over alcohol when they are
used as thermometric liquids.
6. What is maximum thermometer? Draw its diagram and label
important parts.
7. Write down the formula to relate celsius and fahrenheit scales.

B. Differentiate between:
1. Heat and temperature
2. Laboratory thermometer and clinical thermometer
3. Celsius and Fahrenheit scale
/RZHU DQG XSSHU À[HG SRLQW

C. Answer in detail.
:KDW LV WKH ORZHU À[HG SRLQW" +RZ LV WKH ORZHU À[HG SRLQW RI D
thermometer determined?
:KDW LV WKH XSSHU À[HG SRLQW" +RZ LV WKH XSSHU À[HG SRLQW RI D
thermometer determined ?
3. How are heat and temperature related?

77 Times' Crucial Science & Environment Book - 8

D. Give reasons:
1. Constriction is kept near the bulb in a clinical thermometer.
2. Alcohol thermometer is used in very cold regions.
3. Clinical thermometer has gradation from 35°C to 43°C.
4. Alcohol is coloured while using in thermometer.
5. Alcohol thermometer cannot measure the temperature of boiling
water.
6. The glass cover of the bulb of thermometer is very thin.

E. Numerical problems
1. Convert 42°C into °F.
2. Convert 90°F into °C.
3. Convert 274K into °C.

4. The temperature of the human body is 37°C. What will be the
temperature in Fahrenheit scale?

5. Calculate the temperature at which both Celsius and Fahrenheit
scales show the same reading.

7KH WHPSHUDWXUH ï ƒ& LV NQRZQ DV DEVROXWH ]HUR :KDW ZLOO EH
its value in Kelvin scale?

Answers 2. 32.22°C 3. 1°C
ï 6. 0K
E. 1. 107.6°F
4. 98.6°F

Times' Crucial Science & Environment Book - 8 78

7CHAPTER Light

Willebord Snell
+H LV NQRZQ IRU WKH 6QHOO·V /DZ

Estimated Periods: 10
Objectives: At the end of the chapter, the students will be able to:

GHILQH PLUURU WHOO LWV W\SHV DQG H[SODLQ WKH SURFHVV RI UHIOHFWLRQ IURP PLUURU
H[SODLQ UHDO DQG YLUWXDO LPDJH
GUDZ UD\ GLDJUDPV IRU WKH UHIOHFWLRQ WKURXJK WKH VSKHULFDO PLUURUV
H[SODLQ WKH XVHV RI VSKHULFDO PLUURUV

How are we able to see?
Do you know what mirror is?
How many types of mirrors do you know? What are they? Discuss.

Introduction

Light is invisible itself but it makes the things visible on which it falls. It
PD\ EH GHÀQHG DV WKH IRUP RI HQHUJ\ ZKLFK SURGXFHV WKH VHQVDWLRQ RI YLVLRQ
In physics, there is a separate branch to study light and related phenomena.
The branch of physics which deals with the study of light and its phenomena
is called optics.

5HÁHFWLRQ RI OLJKW

When light falls on the surface of an opaque object, some part of the light is
absorbed by the body, very less part of it is transmitted through the object and
PRVW RI WKH OLJKW LV UHÁHFWHG The process of returning of light into the same
PHGLXP DIWHU VWULNLQJ D VXUIDFH LV FDOOHG UHÁHFWLRQ RI OLJKW We see the things
GXH WR UHÁHFWLRQ RI OLJKW

79 Times' Crucial Science & Environment Book - 8

Incident light N Reflected light
A B

Absorbed light O Y
;

Transmitted light

$W ÀUVW WKH OLJKW IURP WKH VRXUFH IDOOV XSRQ WKH REMHFWV 7KHVH REMHFWV UHÁHFW
PRVW SDUW RI WKH OLJKW :KHQ WKH UHÁHFWHG OLJKW IDOOV RQ RXU H\HV ZH VHH WKH
WKLQJV ZKLFK KDYH UHÁHFWHG WKH OLJKW 6PRRWK DQG RSDTXH VXUIDFH DFWV DV D
JRRG UHÁHFWRU RI OLJKW

5HÁHFWLRQ IURP D SODQH PLUURU

A mirror is a regular piece of glass which is polished on one side and can
return the rays of light into the same medium.
6XSSRVH D UD\ RI OLJKW $2 LV LQFLGHQW RQ D SODQH PLUURU ;< DV VKRZQ LQ WKH
ÀJXUH /HW WKH UD\ RI OLJKW $2 LV VHQW EDFN LQWR WKH VDPH PHGLXP DORQJ 2%
after striking the mirror. Let ON be the normal at the point of incidence.

N
AB

ir

XOY
ZĞŇĞĐƟŽŶ ŽĨ ůŝŐŚƚ

Incident ray
7KH UD\ RI OLJKW WKDW FRPHV IURP D VRXUFH DQG VWULNHV RQ WKH UHÁHFWLQJ VXUIDFH
is called incident ray. ,Q WKH ÀJXUH $2 IDOOV RQ WKH VXUIDFH ;< 6R WKH UD\ $2
is an incident ray.
5HÁHFWHG UD\
The ray along which the incident ray returns into the same medium after
VWULNLQJ WKH VXUIDFH LV FDOOHG UHÁHFWHG UD\ ,Q WKH ÀJXUH 2% LV WKH UHÁHFWHG
ray.
Point of Incidence
7KH SRLQW DW ZKLFK WKH LQFLGHQW UD\ VWULNHV RQ D UHÁHFWLQJ VXUIDFH LV FDOOHG
point of incidence. ,Q WKH ÀJXUH 2 LV WKH SRLQW RI LQFLGHQFH

Times' Crucial Science & Environment Book - 8 80

Normal

An imaginary perpendicular line which is drawn at the point of incidence is
called normal. ,Q WKH ÀJXUH 21 LV WKH QRUPDO

Angle of Incidence
The angle between the incident ray and the normal is called angle of incidence.
,Q WKH ÀJXUH ‘AON = ‘i is the angle of incidence. It is also called incident angle.

$QJOH RI UHÁHFWLRQ
7KH DQJOH EHWZHHQ WKH UHÁHFWHG UD\ DQG QRUPDO LV FDOOHG DQJOH RI UHÁHFWLRQ
,Q WKH ÀJXUH ‘%21 LV WKH DQJOH RI UHÁHFWLRQ ,W LV DOVR FDOOHG UHÁHFWHG DQJOH

/DZV RI UHÁHFWLRQ

:KHQHYHU D UD\ RI OLJKW JHWV UHÁHFWHG LW REH\V WKH IROORZLQJ ODZV

7KH LQFLGHQW UD\ WKH UHÁHFWHG UD\ DQG WKH QRUPDO DW WKH SRLQW RI LQFLGHQFH
all lie in the same plane.

7KH DQJOH RI LQFLGHQFH LV DOZD\V HTXDO WR WKH DQJOH RI UHÁHFWLRQ

ĐƟǀŝƚLJ ϳ͘ϭ 7R YHULI\ WKH ODZV RI UHÁHFWLRQ

Materials required:

A plane mirror, a drawing board, a sheet of X O Y
white paper, mirror holders, A1 B1 i r B2
thumb pins, pins, pencil, etc. A2

Procedure: A B
N

1. Spread a sheet of paper uniformly over a &ŝŐ͗͘ sĞƌŝĮĐĂƟŽŶ ŽĨ ůĂǁƐ ŽĨ ƌĞŇĞĐƟŽŶ
GUDZLQJ ERDUG DQG À[ LW ZLWK VRPH WKXPE SLQV

'UDZ D VWUDLJKW OLQH ;< LQ WKH PLGGOH RI WKH SDSHU DQG GUDZ D QRUPDO
ON at the middle of the line. (The normal is drawn by making 90° on one
side of the line).

'UDZ D VODQWHG OLQH $2 DV VKRZQ LQ WKH ÀJXUH DQG SODFH D SODQH PLUURU
RYHU WKH OLQH ;< DQG HUHFW LW ZLWK WKH KHOS RI PLUURU VWDQG

4. Erect two pins tAh1eapnldanBe1 vertically at certain distance along the slanted
line in front of mirror.

5. Observe the image of the pins from the other side of normal and erect
two pAin1 sanBd2 and A2such a way that the pins overlap the images of the
pins B1.

6. Remove the pins and mark the positions of the pins A1, B1, B2 and A2 with
a pencil.

7. Draw a line OB which meets the points B2 and A2.
8. Measure the angles ‘AON and ‘BON.

81 Times' Crucial Science & Environment Book - 8

Observation:
You will observe that ‘AON =‘BON. Also AO, OB and ON all lie at the same
plane.
Conclusion:
From the above activity, it can be concluded that:
D 7KH DQJOH RI LQFLGHQFH LV HTXDO WR WKH DQJOH RI UHÁHFWLRQ
E 7KH LQFLGHQW UD\ WKH UHÁHFWHG UD\ DQG QRUPDO DW WKH SRLQW RI LQFLGHQFH

all lie in the same plane.
7KXV WKH ODZV RI UHÁHFWLRQ DUH YHULÀHG

Image

If you place a ball on a table in front of a plane mirror, you will see another ball
inside the mirror. If you remove the ball from the table, no ball will be seen in the
mirror. In this activity, the ball that appears in the mirror LV WKH ɇimage’ of the ball
placed on the table. A plane mirror forms the images of the objects similar to that
of the ball.

Mirror

Object Image

/ŵĂŐĞ ĨŽƌŵĞĚ ďLJ ƉůĂŶĞ ŵŝƌƌŽƌ

7KH VWUXFWXUH RI DQ REMHFW IRUPHG E\ WKH UHÁHFWLQJ VXUIDFH GXH WR UHÁHFWLRQ RI
light is called image. :KHQ WKH UHÁHFWHG UD\V IURP D PLUURU RU DQ\ UHÁHFWLQJ
surface) meet or appear to meet, an image of the object is formed. An image
may be real or virtual.
A real image LV IRUPHG E\ WKH DFWXDO LQWHUVHFWLRQ PHHWLQJ RI WKH UHÁHFWHG
rays whereas a virtual image LV IRUPHG ZKHQ WKH UHÁHFWHG UD\V DSSHDU WR
intersect at a point. A real image can be obtained on a screen but a virtual
image cannot be obtained on a screen.

Differences between a real and a virtual image

Real Image Virtual Image

1. It is formed at the point where the 1. It is formed at the point where the
UHÁHFWHG UD\V PHHW UHÁHFWHG UD\V DSSHDU WR PHHW DIWHU
diverting.

2. It can be obtained on the screen. 2. It cannot be obtained on the screen.

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

Times' Crucial Science & Environment Book - 8 82

4. It is always inverted. 4. It is always erect.

5. Its size depends on the distance of 5. Its size depends on the nature of
object from the mirror. mirror.

Mirror

$ PLUURU LV D VPRRWK VXUIDFH ZKLFK UHÁHFWV D ODUJH
fraction of incident light and forms the image of an ZĞŇĞĐƟŶŐ
object. A mirror is made of glass in which one of Surface Polished
WKH VXUIDFHV LV SROLVKHG 0LUURUV FDQ EH FODVVLÀHG Surface

into two types plane mirrors and curved mirrors. ƉůĂŶĞ ŵŝƌƌŽƌ

Plane mirrors

7KH PLUURU ZKRVH UHÁHFWLQJ VXUIDFH LV ÁDW LV FDOOHG SODQH PLUURU One of its
VXUIDFHV LV SROLVKHG 7KH QRQ SROLVKHG VXUIDFH KHOSV WR UHÁHFW UD\V RI OLJKW
coming from a source.

Characteristics of the image formed by a plane mirror

An image formed by a plane mirror has the following characteristics:

1. The image formed by a plane mirror is erect and virtual.
P
Suppose an object AB is placed Plane mirror B'
in front of the plane mirror B
DV VKRZQ LQ WKH ÀJXUH /HW·V
suppose two points P and O on Object
the mirror. A ray of light from Image

point B incidents on the mirror A O A'

normally at point P and returns
along PB. Another ray BO
R /ŵĂŐĞ ĨŽƌŵĞĚ ďLJ ƉůĂŶĞ ŵŝƌƌŽƌ

FRPLQJ IURP WKH VDPH SRLQW % LV UHÁHFWHG DORQJ 25 7KHVH WZR UHÁHFWHG
rays when produced backwards meet at B’ behind the mirror and form
DQ LPDJH 6LQFH WKH UHÁHFWHG UD\V 3% DQG 25 GR QRW PHHW DFWXDOO\ EXW
DSSHDU WR PHHW EHKLQG WKH PLUURU WKH LPDJH IRUPHG LV YLUWXDO 7KH ÀJXUH
also shows that the image formed is upright or erect.

2. The image formed is of the same size as the object.

,I \RX PHDVXUH WKH KHLJKW RI WKH REMHFW DQG WKH LPDJH LQ WKH ÀJXUH \RX
ZLOO ÀQG WKDW WKH REMHFW DQG LPDJH KDYH WKH VDPH KHLJKW

3. The distances of object and image from the mirror are equal.

,I \RX PHDVXUH WKH GLVWDQFHV 2$ DQG 2$· RU %3 DQG 3%· \RX ZLOO ÀQG WKH
equal distances. It means that the object distance is equal to the image
distance.

83 Times' Crucial Science & Environment Book - 8

The distance between the object and the mirror is called object distance
whereas the distance between the image and the mirror is called image
distance.

4. The image formed by a plane mirror is laterally inverted.

Write down a letter B on a white sheet of
paper and erect it in front of a plane mirror.
What will you observe? B
You will observe that the image of the letter
E BB
B appears as . Similarly, you will see E as
and so on. It is due to the fact that right part
of object appears left in the image and the left
WůĂŶĞ ŵŝƌƌŽƌ

part of object appears right in the image. The
side ways inversion in the image of an object (i.e. left appearing right and
right appearing left) in a plane mirror is called lateral inversion.

Uses of plane mirror

1. It is used widely as a looking glass.

2. It is used in the construction of periscope and kaleidoscope.

3. It is used in hair-cutting salon, shops and dressing tables.

4. It is used for different purposes in physics laboratory.

Curved mirrors

7KH PLUURUV ZKRVH UHÁHFWLQJ VXUIDFHV DUH FXUYHG
are called curved mirrors. Parabolic mirrors and
spherical mirrors are the examples of curved mirrors.

Spherical mirrors are regarded as the parts of hollow
sphere, which are silvered according to need.

ƵƌǀĞĚ ĂŶĚ ƉĂƌĂďŽůŝĐ ŵŝƌƌŽƌƐ

ĐƟǀŝƚLJ ϳ͘Ϯ To prepare spherical mirrors

Materials required:
A hollow sphere of glass, glass-cutting tool, enamel or silver gel, brush, etc.
Procedure:
7DNH D KROORZ VSKHUH RI JODVV DQG FXW SDUWV DV VKRZQ LQ WKH ÀJXUH

Concave Convex
mirror mirror

Times' Crucial Science & Environment Book - 8 84

2. Take one of the cut portions and make its outer surface opaque by coating
VLOYHU RU HQDPHO 1RZ UHÁHFWLRQ WDNHV SODFH IURP WKH LQQHU VXUIDFH RI
this cut portion of the glass. Such mirror is called concave mirror.

3. Take the another cut portion and make its inner surface opaque so that
UHÁHFWLRQ RFFXUV IURP LWV RXWHU VXUIDFH 7KH PLUURU WKXV IRUPHG LV FDOOHG
convex mirror.

Observation:
You have constructed concave and convex mirrors.
Conclusion:
Concave and convex mirrors are the parts of a hollow sphere of glass. So, they
are called spherical mirrors.

Spherical mirror

A spherical mirror is a part of hollow glass sphere in which one of the surfaces is
polished. The spherical mirror is of two types concave mirror and convex mirror.

Concave mirror

$ PLUURU ZKRVH LQQHU VXUIDFH LV WKH UHÁHFWLQJ VXUIDFH DQG RXWHU VXUIDFH LV
silvered is called concave mirror. A concave mirror is also known as converging
PLUURU EHFDXVH LW FRQYHUJHV WKH OLJKW UD\V IDOOLQJ RQ LW DW D SRLQW DIWHU UHÁHFWLRQ

silvered
surface

ƌĞŇĞĐƟŶŐ
surface

ŽŶĐĂǀĞ ŵŝƌƌŽƌ

Think and Solve

,Q D WRUFK OLJKW D FRQFDYH PLUURU LV XVHG WR UHÁHFW DQG D EXOE LV SODFHG DW
LWV IRFXV :KDW LV LWV EHQHÀW LQ WKH XVH RI WRUFK OLJKW"

&RQYH[ PLUURU ƌĞŇĞĐƟŶŐ
surface
A mirror whose outer
VXUIDFH LV WKH UHÁHFWLQJ ŽŶǀĞdž ŵŝƌƌŽƌ
surface and the inner
surface is silvered is called silvered
convex mirror. It is also surface
known as diverging mirror
because it diverges the
OLJKW UD\V LQFLGHQW RQ LW DIWHU UHÁHFWLRQ

85 Times' Crucial Science & Environment Book - 8

Think and Solve
Drivers use convex mirror as side mirror to have a view of back objects.
Why?

Terms related to spherical mirrors

&HUWDLQ WHUPV DUH LQYROYHG ZKLOH GHVFULELQJ WKH UHÁHFWLRQ RI OLJKW IURP WKH
spherical mirrors. They are as follows:
a Pole of the mirror (P or O)

The central point of the spherical mirror is called pole of the mirror. It is
UHSUVHQWHG E\ 3 RU 2 ,Q WKH ÀJXUH 3 LV WKH SROH RI WKH PLUURU
All the distances such as object distance, image distance, focal length, etc
are measured from the pole of the mirror.
b. Centre of curvature (C)
The centre of the sphere of which the mirror forms a part is called centre
of curvature. It is denoted by C.

P CF P

CF

c. Radius of curvature (R)
The radius of the sphere of which the mirror forms a part is called radius
of curvature. ,Q WKH ÀJXUH 3& LV WKH UDGLXV RI FXUYDWXUH
In other words, the distance between centre of curvature and the pole of
the mirror is called radius of curvature.

G 3ULQFLSDO D[LV
The straight line that passes through the pole and the centre of curvature
of a mirror is called principal axis. ,Q WKH ÀJXUH WKH OLQH 3& LV WKH
principal axis.

e. Aperture
7KH VXUIDFH RI D PLUURU IURP ZKLFK UHÁHFWLRQ WDNHV SODFH LV FDOOHG LWV
aperture.

Times' Crucial Science & Environment Book - 8 86

f. Focus (Principal focus): F

The point at which all rays parallel to the principal axis striking a mirror
PHHW RU DSSHDU WR PHHW DIWHU UHÁHFWLRQ LV FDOOHG IRFXV It is also known
as principal focus. It is denoted by F. In a mirror, C lies at the double
distance of PF. So, C is also denoted by 2F.

,Q D FRQYH[ PLUURU WKH UHÁHFWHG UD\V GR QRW PHHW DFWXDOO\ EXW WKH\ DSSHDU
to meet at focus. So, it is called virtual focus. But in a concave mirror, the
UHÁHFWHG UD\V DFWXDOO\ PHHW DW IRFXV VR LW LV FDOOHG D UHDO IRFXV

g. Focal length (f)

The distance between principal focus and pole of the mirror is called focal
length. It is equal to half of the radius of curvature.

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

? f = R
2

Rules for drawing ray diagrams in a concave mirror

7KH OLQH GLDJUDPV WKDW VKRZ WKH LQIRUPDWLRQ RI DQ LPDJH IRUPHG E\ UHÁHFWLQJ
surface are called ray diagrams. We should obey the following rules while
constructing ray diagrams:

1. A ray of light incident parallel to the principal axis passes through the
SULQFLSDO IRFXV DIWHU UHÁHFWLRQ

CF P CF P C P
F

;ĂͿ ;ďͿ ;ĐͿ

2 $ UD\ RI OLJKW SDVVLQJ WKURXJK WKH SULQFLSDO IRFXV UHÁHFWV SDUDOOHO WR WKH
principal axis.

$ UD\ RI OLJKW WKDW SDVVHV WKURXJK WKH FHQWUH RI FXUYDWXUH UHÁHFWV DORQJ
the same path. It is perpendicular to the surface of the mirror.
Keeping the above rules in mind, we have to follow the following
procedures to draw ray diagrams:
D 'UDZ DQ DUF ;< ZLWK & DV D FHQWUH 6KDGH WKH RXWHU VXUIDFH RI ;<
with a pencil.

87 Times' Crucial Science & Environment Book - 8

E :ULWH D GRW RQ WKH PLGGOH SRLQW RI ;< DQG ODEHO LW 2 7KHQ GUDZ D
straight line through C and O.

c. Write a dot on the middle point of CO and label it F.

d. Draw a straight line with arrowhead pointing upwards at any point
on the principal axis. Label it AB, which acts as an object.

e. Now, follow the above mentioned rules to show the image of the
object.

Y
B

Object C F O
PA Image

X

,Q WKH ÀJXUH ;< DFWV DV DSHUWXUH RI WKH PLUURU & LV WKH FHQWUH RI
curvature, O is the pole of the mirror, PO is the principal axis, F is the
focus, OF is focal length and CO is the radius of curvature of the mirror.

Images formed by concave mirror

A concave mirror forms images of different natures based on the position
of object in the principal axis. They are shown below with the help of ray
diagrams.

2EMHFW SODFHG DW LQÀQLW\ Y

:KHQ DQ REMHFW LV SODFHG DW WKH LQÀQLW\ LQIURQW
of concave mirror, parallel beam of light falls on
the mirror. Hence, the image is formed at focus 2F F O

DIWHU UHÁHFWLRQ RI SDUDOOHO EHDP 7KH LPDJH LV real,
inverted and highly diminished (very small).

X

2. Object beyond C (2F) Y

When an object is placed beyond C of B

a concave mirror, the image is formed Object C A' F O
between C and F. The image formed is: P A Image
a. real
B'

b. inverted X

c. diminished (smaller then object).

Times' Crucial Science & Environment Book - 8 88

3. Object at C or 2F Y

When an object is placed at C of a concave B F
mirror, the image is formed at C. The image X
formed is: Object
a. real
P AC O
A'
Image

b. inverted B'

c. same size as that of object.

4. Object between C and F Y

When an object is placed between C and F B
of a concave mirror, the image is formed
beyond C. The image formed is:
a. real Object F
b. inverted
P A' O
CA

Image

F PDJQLÀHG ODUJHU WKDQ WKH REMHFW B' X

5. Object at F

When an object is placed at the focus of a concave Y

PLUURU WKH LPDJH LV IRUPHG DW LQÀQLW\ L H DW BA
very far distance from the mirror. The image
formed is: C
P F O

a. real DĞĞƚ Ăƚ ь

b. inverted X
F KLJKO\ PDJQLÀHG

6. Object between F and pole of the mirror B'
Image
When an object is placed Y O A'
between focus and pole of a
concave mirror, the image is B
formed beyond the mirror. The C Object
image formed. is:
FA

a. virtual X
b. erect

F PDJQLÀHG

89 Times' Crucial Science & Environment Book - 8

Summary of images formed by a concave mirror

S.N. Position of Object Position of Image 1DWXUH DQG PDJQLÀFDWLRQ

1. ,QÀQLW\ Focus Real, inverted & highly
diminished

2. Beyond C Between F and C Real, inverted & diminished

3. At C (2F) At C (2F) Real, inverted & same size

4. Between C and F Beyond C 5HDO LQYHUWHG PDJQLÀHG

5. At F ,QÀQLW\ Real, inverted & highly
PDJQLÀHG

6. Between F and O Behind the mirror 9LUWXDO HUHFW DQG PDJQLÀHG

Uses of a concave mirror

1. It is used in searchlight, torch light and headlight of cars to focus light.
2. It is used in the construction of astronomical telescopes and radio

WHOHVFRSHV DV D UHÁHFWRU
3. It can be used as a shaving or make up mirror. When a person adjusts

his face between focus and pole of a concave mirror, an erect, virtual
DQG PDJQLÀHG LPDJH LV IRUPHG 7KLV KHOSV LQ ÀQH VKDYLQJ RI EHDUG RU
accurate make up.
4. It is used by doctors to observe the interior parts of body such as internal
parts of the ear, nose, throat, etc.
5. It is used in solar cooker to converge the solar radiation.
Rules for drawing ray diagrams in a convex mirror
We should obey the following rules while drawing ray diagrams in a convex
mirror:
1. A ray incident parallel to the principal axis appears to diverge from the
principal focus.
$ UD\ RI OLJKW WUDYHOOLQJ WKURXJK WKH FHQWUH RI FXUYDWXUH LV UHÁHFWHG EDFN
along the same path.

C FO C
OF F

Times' Crucial Science & Environment Book - 8 90

3. An incident ray which strikes the pole of the mirror at a certain angle,
UHÁHFWV DW WKH VDPH DQJOH

tO C

t F

,PDJH IRUPHG E\ D FRQYH[ PLUURU

Wherever the object is placed in front of a convex mirror, the image is always
formed behind the mirror. The image is always virtual, erect and diminished.

B
B'

OC
A A' F

/ŵĂŐĞ ĨŽƌŵĞĚ ďLJ Ă ĐŽŶǀĞdž ŵŝƌƌŽƌ

8VHV RI D FRQYH[ PLUURU

A convex mirror has the following uses:

1. A convex mirror is used in street lights to scatter light in the wider area.

2. It is used as a back view mirror in automobiles because it gives a clear
DQG ZLGH YLHZ RI WUDIÀF EHKLQG 7KH ZLGHU ÀHOG RI YLHZ LV GXH WR WKH
fact that a convex mirror always forms an erect, virtual and diminished
image of an object. The small images of big objects help to see several
vehicles in the mirror at a time.

3. A combination of concave and convex mirrors is used in fairs, fun parks
etc to provide entertainment. It is because it gives strange image of an
object.

'LIIHUHQFHV EHWZHHQ D FRQFDYH DQG FRQYH[ PLUURU

Concave mirror Convex mirror

1. Its outer surface is polished. So, 1. Its inner surface is polished. So, the
LQQHU VXUIDFH LV XVHG DV UHÁHFWLQJ RXWHU VXUIDFH LV XVHG DV UHÁHFWLQJ
surface. surface.

91 Times' Crucial Science & Environment Book - 8

2. It mainly forms real and inverted 2. It always forms virtual and erect
image of an object. image of an object.

3. The size of the image depends up 3. It always forms diminished image of
on the distance of object from the an object.
pole of the mirror.

4. Image is mainly formed in front of 4. Image is always formed behind the
the mirror. mirror.

Refraction of light

Light travels with different velocities in different media. It travels with maximum
velocity in vacuum or air. But it has less velocity in water as compared to air.
Again it has less velocity in glass as compared to water. The velocity of light in
air, water and glass is 3×108m/s, 2.2×108m/s and 2×108 m/s respectively.

Light travels in a straight line as long as it is travelling in the same medium.
But when light passes from one medium to another, it bends at the junction
of two media. This bending of light is due to the change in its velocity. The
process of bending of light while passing from one optical medium to next is
called refraction of light.

,Q WKH ÀJXUH

AO = Incident ray, OB = Refracted ray A M O Air
MN = Normal at air-glass interface P Glass
M'N' = Normal at glass-air interface i Q

Nr

‘AOM = i = Angle of incidence

‘BON = r = Angle of refraction M'
BC =Emergent ray
CBN' = e = Angle of emergence S B B' R
PQRS = Glass slab
e

N'

ZĞĨƌĂĐƟŽŶ ŽĨ ůŝŐŚƚ C

OB’ = Original path of the incident ray.

The medium through which light travels is called optical medium. The optical
PHGLD FDQ EH FODVVLÀHG LQWR WZR W\SHV UDUHU PHGLXP DQG GHQVHU PHGLXP 7KH
medium in which the velocity of light is more is called rarer medium. On the other
hand, the medium in which the velocity of light is less is called denser medium.
The terms rarer and denser are the relative terms and the same substance
may act as rarer as well as denser medium. For example, in the pair of air
and water, air is rarer and water is denser medium. It is because the velocity
of light in air is more than that in water. But, for the pair of water and glass,
water is rarer and glass is denser medium

Times' Crucial Science & Environment Book - 8 92

Cause of refraction

As stated earlier, light travels with different velocities in different media. Its
velocity is more in optically rarer medium and less in denser medium. When
light passes from optically rarer to denser medium, it travels with slower
velocity as compared to that in rarer medium. The light bends due to the
change in velocity while entering from one optical medium to next. Hence, the
change in velocity of light while passing from one optical medium to next is the
cause of refraction of light.

Laws of refraction

The refraction of light obeys the following laws:

1. The incident ray, refracted ray and normal all lie at the same plane.

2. The ratio of 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 (a constant, and is read as myu)
Sin r

The constant μ is known as refractive index of the medium. This law is
popularly known as Snell’s law.

The Snell’s law can also be stated in the following ways:

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

AM B
air r
i o air
r glass i glass
N A

B
(a) (b) (c)

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

c. The ray of light does not bend if it passes through the normal. It
means if a ray of light incidents on a surface perpendicularly, it
does not bend.

Effects of refraction of light

We experience several events in our daily life due to the refraction of light.
Some of the examples are given below:

93 Times' Crucial Science & Environment Book - 8

1. A straight stick appears bent if it is partially immersed in water.

When a stick is partly immersed in LJĞ

water, it appears bent due to refraction
of light. The rays of light coming from
the immersed portion of the stick pass ^ƟĐŬ Image

from water to air by bending away from
the normal. Thus, the immersed portion
of stick seems to be raised up (as in the tĂƚĞƌ

ÀJXUH %XW WKH OLJKW UD\V IURP WKH
portion of stick outside the water fall on
the observer’s eyes without bending. Hence, the stick appears bent at
the air-water interface.

2. A pond looks shallower than its actual depth.

A pond looks shallower than its actual Real depth
depth due to refraction of light. The rays Apparent depth
of light coming from the objects at the
bottom of the pond bend away from the Image
normal at water-air interface. When the ĮƐŚ
refracted rays are produced further, they
meet at shallower height. Thus, it seems
that the light rays are coming from the
shallower height (as in frgure) and the
pond looks shallower.

$ FRLQ SODFHG DW WKH ERWWRP RI D YHVVHO ÀOOHG ZLWK ZDWHU DSSHDUV WR EH
raised up due to the same reason (as stated above).

Think and Solve

,V LW SRVVLEOH WR NLOO D ÀVK LQVLGH SRQG ZLWK D VSHDU"
2. Rocks and stones at the bottom of river seem to be elevaed. Why?

opaque : an object through which light cannot pass
real image : an image which can be obtained on screen
virtual image : an image which can not be obtained on screen

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

2. The branch of physics that deals with the study of light and its phenomena is called
optics.

Times' Crucial Science & Environment Book - 8 94

3. A collection of rays of light is called a beam of light.

4. The process of returning of light into the same medium after striking a surface is
called reflection of light.

5. The reflection of light obeys the following laws:

a. The incident ray, reflected ray and normal at the point of incidence all lie at
the same plane.

b. The angle of incidence is always equal to the angle of reflection.

6. A mirror is a smooth surface which reflects a large fraction of incident light and
forms the image of an object.

7. The mirrors are of two types - plane mirrors and curved mirrors.

8. The curved mirrors are of two types spherical mirrors and parabolic mirrors.

9. A spherical mirror is a part of a hollow glass sphere in which one of the surfaces is polished.

10. A spherical mirror is of two types concave mirror and convex mirror.

11. A concave mirror is also known as converging mirror because it converges a parallel
beam of light into a single point after reflection.

12. A convex mirror is also known as diverging mirror because it diverges the parallel
beam of light incident on it.

13. The process of bending of light while passing from one optical medium to another
is called refraction of light.

14. The change in the velocity of light while passing from one optical medium to another
is the cause of refraction of light.

15. The laws of refraction are:

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

b. The ratio of 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
Sin r

Exercise

A. Answer these questions in very short.
1. What is optics ?
2. What is the relation between focal length and radius of curvature?
3. What is principal axis?
'HÀQH ODWHUDO LQYHUVLRQ
5. What is the size of image formed by a plane mirror?
6. Name the two types of curved mirrors.
7. What is incident ray?
8. What kind of image can be obtained on a screen ?
9. Can a concave mirror form a virtual image?

95 Times' Crucial Science & Environment Book - 8

% 'HÀQH 2. Mirror
1. Image 4. Denser medium
3. Refractive index

C. Write down the uses of : 2. Concave mirror
1. Plane mirror 5HÁHFWLRQ RI OLJKW
&RQYH[ PLUURU

D. Give reasons:
1. We use plane mirror as a looking glass.
$ FRQFDYH PLUURU LV XVHG DV D UHÁHFWRU LQ D VHDUFK OLJKW
3. A convex mirror is used in automobiles.
4. A concave mirror can be used as a shaving mirror.
5. A stick partially immersed in water appears bent.
6. $ FRLQ SODFHG DW WKH ERWWRP RI D ZDWHU ÀOOHG YHVVHO DSSHDUV WR EH UDLVHG XS
7. A concave mirror is called converging mirror.
8. A convex mirror is called diverging mirror.

E. Write down differences between:
1. Real image and virtual image
2. Concave and convex mirror
3. Rarer and denser medium.
5HÁHFWLRQ DQG UHIUDFWLRQ RI OLJKW

F. Answer these questions:
:KDW LV UHÁHFWLRQ " ([SODLQ WKH SURFHVV RI UHÁHFWLRQ ZLWK WKH KHOS
of a diagram.
:ULWH GRZQ WKH ODZV RI UHÁHFWLRQ RI OLJKW
3. Explain the refraction of light through a glass slab.
4. Explain with diagrams the position and nature of image formed
when the object is placed between C and F in a concave mirror.
5. Explain the case with diagram in which a concave mirror forms a
virtual image.
6. Explain with diagram to show the position and nature of image
formed by a convex mirror.

Times' Crucial Science & Environment Book - 8 96