Green Science 10 Final PDF (2076)

Group ’B’ (Understanding Type Questions) (2 Marks Each)

16. Write two differences between force and pressure.

17. Which one is the salt solution out of X and Y ? Egg
Why ?

18. Write two differences between Pascal’s law and X Egg
Archimedes’ principle. Y

19. A ship can carry heavier load on salty water than on
fresh water. Why ?

20. An iron nail sinks in water but a ship made up of iron floats. Why ?

21. An iron nail sinks in water but floats on mercury. Why ?

22. Write two differences between hydraulic brake and hydraulic lift.

23. Study the given figure and answer the following questions:

i. What is the upthrust acting on the stone

immersed in water ? (Ans: 5N) 20 N

ii. On which principle is the given experiment
based?

iii. What is the weight of the stone in air ? Water
Stone
(Ans: 25 N)

24. Study the given figure and answer the 5N
following questions:

i. What is the mass of stone in air ?

(Ans: 1.8 kg) 10 N

ii. How much upthrust is exerted on the

stone ? (Ans: 8 N) Water
Stone
iii. Which principle is this experiment based
on ? Write the statement of the principle.

25. A ship coming from sea enters the river, will 8N
its hull sink more or less in river water? Give reason.

Group ‘C’ (Application Type Questions) (3 Marks Each)

26. Write any three applications of hydraulic machine.

27. Describe an activity with figure to verify law of flotation.

28. Describe an experiment to demonstrate the existence of atmospheric pressure on the
surface of the earth.

29. How can you demonstrate hydraulic machine is a force multiplier vessel? Explain.

GREEN Science (Physics) Book-10 51

30. Three objects with different densities A, B and C are in water as shown in the figure.

Now answer the following questions:

i. Which object has higher density than the water? A

ii. If the mass of object A is 1 kg. How much kg of water is B

displaced by the object? (Ans: 1 kg)

iii. Which law is applicable for object B? State the law. C

Group ‘D’ (Higher Abilities Type Questions) (4 Marks Each)

31. Explain the role of piston valve and foot valve in water pump.

32. The ice made of water floats on water, why ? Which AB

instrument is shown in figure? If cross-sectional area of Liquid

piston A is 40 cm2 and that of piston B is 4m2, what load

is necessary on the piston ‘B’ to balance 600N force kept

on piston A? (Ans: 6 × 105 N)

33. Explain the working mechanism of a syringe with a neat and labelled figure.

34. The different weight of a piece of stone on weighing in three different media: air,
water and solution of common salt is given in the table below. Answer the following
questions:

Liquid Medium

A 18 N

B 15 N

C 22 N

i. Which should be water and which one the solution of common salt out of three
medium A, B and C ?

ii. If the weight of 1 kg of mass in air is 10N, find out the mass of the piece of

stone? (Ans: 2.2 kg)

iii. Find out the mass of water displaced by the piece of stone. (Ans: 4N)

35. Describe the working principle of hydraulic machine with a neat and labelled figure.

36. Explain the working mechanism of a water pump with neat and labelled figure.

52 GREEN Science (Physics) Book-10

UNIT Energy

3

Weighting Distribution Theory : 5 Practical: 1

Before You Begin

We do different types of work in our day to day life. We need energy
to do these works. Energy is the ability of a body to do work. Energy
is required to perform various metabolic activities. We need energy
to operate automobiles and various equipment in industries. We use
different forms of energy like light energy, heat energy, sound energy,
mechanical energy, magnetic energy, electrical energy, etc. The needs
of energy sources is increasing day by day to meet the demand of
increasing population. There are different sources of energy. All the
sources of energy are broadly classified into renewable and non-
renewable sources of energy. Among all these sources of energy, the
sun is considered as the main source of energy. In this unit, we will
study energy, its sources, energy crisis, alternative sources of energy
and measures to conserve energy in brief.

Learning Objectives Syllabus

After completing the study of this unit, students will be able to: • Introduction to energy
• Sources of energy
i. define energy and explain it with examples.
- Non-renewable sources of
ii. explain the energy sources and their utilities. energy

iii. describe the sun as the ultimate source of energy. - Renewable sources of
energy
iv. define energy crisis and explain the methods of
solving energy crisis. • Sun as the ultimate source of
energy
v. explain alternative sources of energy and utilize
them in daily life. • Fossil fuel and mineral oil
• Hydro-power
vi. explain measures of conservation of energy with • Alternative sources of energy
examples. • Energy crisis
• Present status of energy use
vii. describe the present status of energy use.
and its conservation

Glossary: A dictionary of scientific/technical terms

energy : the capacity of a body to do work
renewable : which can be obtained again and again after its use
non-renewable : which gets exhausted after use and cannot be obtained again and again
alternative : different from the usual or conventional
ultimate : greatest or most extreme
geothermal : of, relating to, or using the natural heat produced inside the earth

GREEN Science (Physics) Book-10 53

Sources of energy

The objects from which we get energy are called the sources of energy. For example, the
sun, running water, coal, petrol, diesel, wind, etc. All the sources of energy are broadly
classified into two types. They are as follows:

1. Renewable sources of energy

2. Non-renewable sources of energy

1. Renewable sources of energy

The sources of energy which never get exhausted and can be used again and again
are called renewable sources of energy. Solar energy, biomass energy, wind energy,
hydropower, tidal energy, biogas, geothermal energy, etc. are the examples of renewable
sources of energy. These sources of energy are produced continuously in nature. So they
will never get exhausted after their use.

2. Non-renewable sources of energy

The sources of energy which get exhausted after use and cannot be obtained again and
again are called non-renewable sources of energy. Coal, petrol, diesel, kerosene, natural
gas, etc. are non-renewable sources of energy. These sources of energy have a limited
stock in nature. They are formed in nature after millions of years. Therefore, they cannot
be replaced in a short period of time after their use. Fossil fuels like coal, petrol, diesel, etc.
are the dead remains of plants and animals . We get heat energy by burning them.

Differences between renewable and non-renewable sources of energy

Renewable sources of energy Non-renewable sources of energy

1. Renewable sources of energy can be 1. Non-renewable sources of energy

obtained continuously and can be cannot be used again and again as they

used again and again. have limited stock in nature.

2. These sources of energy never get 2. These sources of energy get depleted

depleted and can be replaced within a and take millions of years to be

short period of time. replaced.

Sun as the ultimate source of energy Fig.

Energy can be obtained from different sources like 3.1
the sun, blowing wind, petrol, diesel, coal, running
water, biomass, hydroelectricity, etc. Among them, Sun
the sun is considered as the main source of energy.
All other sources of energy depend on the solar
energy directly or indirectly. Green plants depend on
solar energy to prepare food during photosynthesis.
All animals depend on plants directly or indirectly
for food. So all living beings depend on solar-energy
for food.

54 GREEN Science (Physics) Book-10

Water cycle cannot operate in the absence Do You Know
of heat of the sun. Without water cycle,
hydroelectricity cannot be generated. The process of making food by green
Therefore, the energy obtained from plants in the presence of sunlight is called
running water and hydroelectricity is also photosynthesis. Plants prepare their food
the outcome of solar energy. in leaves.

Cloud formation

Snow Precipitation Condensing water vapour

Surface runoff Evaporation ocean
contributes about
Lakes 80% of total water
Ground water vapour in air

Fig. Salt water Ocean
Fig. intrusion
Impervious layer

3.2

Water cycle

Similarly, petrol, diesel, coal, LP gas, etc.

are also the outcome of solar energy. We

use solar energy to operate solar cooker,

solar water heater, solar calculator, traffic

lights, vehicles, etc. The heat energy of the

sun warms up the air and causes air to

blow. Blowing wind is used to rotate wind

mills, turbines, etc. Various forms of energy

like fossil fuel, bio fuel, geothermal energy,

tidal energy, wind energy are the modified 3.3

form of solar energy. All the sources of Solar panel

energy available on the earth are operated

with the help of solar energy. Because of these reasons, the sun is considered as the main

source of energy or the ultimate source of energy.

The sun is an extremely hot ball of gas. It is made up of 70% of hydrogen gas and about
28% of helium gas. The sun is the main source of energy for all living beings. Some facts
about the sun are given below:

GREEN Science (Physics) Book-10 55

The sun : The ultimate source of energy for the earth

Mass : 2 × 1030 kg (about 3.33 × 105 times more than that of
the earth)

Diameter : 1.4 × 106 km (about 10 times more than that of the
earth)

Temperature of the core : 1.5×107°C

Surface temperature : 5700°C

Solar energy radiation : 4×1026 J/s

Average solar energy on the earth : 1.4kW/m2

Sources of Energy and Utilities

1. Fossil fuel energy

The energy obtained from the fossils that remain buried under the earth crust is called
fossil fuel energy. Fossils are the dead remains of plants and animals buried under
the earth crust. Coal, mineral oil (petrol, diesel, kerosene, etc.) and natural gas are the
examples of fossil fuels. Coal, mineral oil and natural gas are formed from fossils of plants
and animals in nature. So they are called fossil fuels.

Fossil fuel are formed from fossils of plants and animals of millions years ago that buired
deep in the earth’s crust due to light temperature and pressure.

Coal

Coal is a hard black mineral found in nature. It is the fossil fuel which is found in solid
state. Coal is widely used in industries, factories and trains as the main source of heat
energy. Coal is the main source of energy in iron and steel industries, brick factories,
cement factories, etc. Coal gets exhausted after use and it takes millions of years for the
formation of coal. Coal cannot be used again and again after use. Therefore, coal is called
a non-renewable source of energy.

Fig.

3.4 Peat Lignite Bituminous
Anthracite

Coal is burnt to produce heat in industries and factories. Anthracite, peat, lignite,
bituminous, etc. are different types of coal. The percentage of carbon in these coals is
different. Among them, anthracite is considered as the coal of high quality. It consists
about 90% of carbon. The mine of high quality coal, i.e. anthracite has not been found in
Nepal so far. However the mines of low quality coal have been found in Dang district.
Bituminous is the most common coal which produces more heat than peat. Bituminous is
also known as household coal.

56 GREEN Science (Physics) Book-10

Mineral oil

Mineral oil includes liquid fossil fuels like petrol, diesel, kerosene, etc. Mineral oil is used
to run automobiles like car, truck, bus, motorcycle, tractor, aeroplane, helicopter, etc.
Mineral oil is obtained from the mines in the earth's crust by drilling. The mineral oil which
is obtained by drilling the earth's crust is called crude oil. Then the crude oil is purified
by fractional distillation to get natural gas, petrol, diesel, kerosene, mobil, paraffin, etc.
The residue of mineral oil is called tar (asphalt) which is separated and used for pitching
roads. The raw materials for making plastics are also obtained while purifying crude oil.
Similarly, LP gas (liquefied petroleum gas) or cooking gas is also obtained from mineral
oil. It takes millions of years to form mineral oil in nature. Mineral oil gets depleted after
use. So it is called a non-renewable source of energy. Now-a-days, mineral oil is widely
used in automobiles and factories.

Fig.3.5PetrolDieselMobil
Fig. LP gas

Advantages of fossil fuel
The main advantages of fossil fuel (mineral oil and coal) are mentioned below:

1. Fossil fuel is cheaper as compared to other fuels.

2. It is a multipurpose fuel as it can be used in automobiles, industries, etc.

3. It can be transported easily.

4. It can be used to generate electricity.

5. It can be used in those industries that
require maximum heat.

6. It can be used for cooking foods, heating
rooms, boiling water, etc.

In spite of above mentioned advantages,

fossil fuels produce harmful gases and 3.6
smoke while burning and hence cause
Air pollution
environmental pollution. Similarly, they are

non-renewable sources of energy and cannot be obtained again and again after their use.

2. Hydropower

Hydropower is the form of energy which is produced by rotating turbines with the help
of flowing water. It is the most useful source of energy in present day world. It is used to
operate modern electronic and electric equipment like computer, television, radio, printer,

GREEN Science (Physics) Book-10 57

mobile, fax, telephone, etc. Similarly, it is used to operate different types of machines in
industries. The schematic diagram of a hydropower station is shown below:

Electricity can be obtained by

various methods but hydropower Electricity
is pollution free and cheap

source of energy. Though it

is very expensive to generate Reservior Transformer
hydropower in initial phase, it is
very cheap in long term basis as it Fig. Generator
is a renewable source of energy. |Turbine

Our country Nepal is the second

richest country in hydroelectric 3.7

power potential. According to Hydropower plant
researches, it has been estimated

that the hydroelectric power generation capacity of Nepal is about 83000MW. Now-a-

days, the generation of hydropower is increasing throughout the world.

Advantages of hydropower
1. It is a pollution free source of energy.
2. It never gets depleted as it is a renewable source of energy.
3. It is cheap source of energy in a long term basis.
4. It is easy to transmit and use.
5. It can be used to operate various electric and electronic equipment.
6. It is used to get heat and light.
7. It is used to operate various means of information and communication.

Alternative Sources of Energy

Now-a-days, coal, mineral oil and natural gas are widely used throughout the world.
These sources of energy are non-renewable. So these sources of energy cannot be used
again and again. As a result, energy crisis occurs in the world. Therefore, alternative
sources of energy should be developed and used. The energy sources which are used in
place of conventional sources of energy are called alternative sources of energy. Biofuel,
tidal energy, wind energy, solar energy, etc. are the examples of alternative sources of
energy. In present day world, it is very necessary to develop and use alternative sources
of energy to solve energy crisis. In this unit, we will discuss following alternative sources
of energy.

1. Bio fuel Do You Know

The fuel which is obtained from different Natural gases are odourless. So it is
parts of plants and animal dung is called bio difficult to find out the leakage of those
fuel. Biomass energy is the energy obtained gases. Therefore, ethyl mercaptan is mixed
from straw, saw dust, etc. Nepal is an in natural gases to get the smell if there is
agricultural country. So, the raw materials leakage of these gases.
for biomass energy are easily available in

58 GREEN Science (Physics) Book-10

different parts of the country. Therefore, bio fuel is one of the major alternative sources of
energy. In rural areas, cattle dung is used for making dung cake and biogas or gobar gas.
Biogas is the cooking gas which is produced by decomposition of animal dung. Biogas
consists of methane, carbon dioxide, hydrogen and hydrogen sulphide.

In rural areas, biogas (gobar gas) is used as a fuel for cooking food. It is also used to get
light and to run engines. Now-a-days, farmers are encouraged to install biogas plants
and the government of Nepal also provides subsidy to the farmers of rural areas to install
biogas plant.

Advantages of bio fuel
1. Animal dung is used to produce biogas. Biogas is used for cooking food and lighting

bulb.
2. The residue produced while preparing biogas can be used as organic fertilizer.
3. It produces less smoke while burning. So it does not affect our health.
4. It produces more heat.
5. It is cheaper and can be produced easily.
6. It can be used to run engines.

2. Nuclear energy Fig.

The energy which is obtained from 3.8
nuclear fusion and nuclear fission
reaction is called nuclear energy. It is Nuclear power plant
the energy which remains stored in
the nucleus of an atom. The enormous
amount of energy is released by nuclear
reactions.

Nuclear energy is of two types.

a. Nuclear fusion reaction

The nuclear reaction in which lighter nuclei combine to form a heavy nucleus by releasing
a large amount of energy is called nuclear fusion reaction.

The formation of helium atom by the fusion of hydrogen atoms is an example of nuclear
fusion reaction. During this process, a large amount of energy is released. Nuclear fusion
reaction continuously occurs in the sun and stars. Nuclear fusion reaction has not been
controlled so far on the earth. Nuclear bomb is made on the basis of nuclear fusion reaction.
This reaction produces a tremendous amount of energy in the form of heat and light.

b. Nuclear fission reaction Do You Know

The nuclear reaction in which a Nuclear fusion reaction occurs only in the sun
heavy nucleus breaks down into two and stars.
lighter nuclei by releasing a large
amount of energy is called nuclear Nuclear fusion reaction produces more energy
fission reaction. than nuclear fission reaction.

GREEN Science (Physics) Book-10 59

Heavy nuclei of radioactive elements like uranium, plutonium, etc. split into two lighter
nuclei when they are hit with free neutrons. A large amount of energy is also produced
during nuclear fission reaction.

When a heavy unstable and radioactive atom is hit with free neutron, the atom splits into
two lighter nuclei. During this process, some mass gets lost which produces tremendous
amount of heat energy.

The amount energy produced during nuclear fission reaction can be calculated by the
Eienstein's mass-energy relation which is given below.

E = mc2

Where, E = Energy generated

m = Loss in mass

c = Speed of light in vacuum, i.e. 3 × 108m/s

Nuclear fission reaction is carried out in nuclear power plants to produce heat. The heat is
used to produce steam for operating steam engines. Steam engines are used to rotate turbines
for generating electricity. Electricity of thousands of megawatt (MW) can be generated in
developed countries by controlled nuclear fission reaction in nuclear power plants.

Though nuclear fusion reaction produces more energy than nuclear fission reaction,
nuclear fusion reaction is not possible on the earth as it requires very high temperature
and pressure like in the sun and stars.

Differences between nuclear fusion and nuclear fission reaction

Nuclear fusion reaction Nuclear fission reaction

1. Two light nuclei combine to form 1. One heavy nucleus splits into two
a heavy nucleus in nuclear fusion lighter nuclei in nuclear fission
reaction. reaction.

2. It produces much more energy than 2. It produces less energy than nuclear
nuclear fission reaction. fusion reaction.

3. It cannot be controlled on the earth. 3. It can be controlled on the earth.

4. Hydrogen bomb is made on the basis 4. Atom bomb is made on the basis of
of nuclear fusion reaction. nuclear fission reaction.

3. Wind energy Fig.

The energy which is obtained from fast 3.9
blowing wind is called wind energy. It is a
renewable source of energy which is being
used from ancient time. Wind energy can
be used to run sea boat, wind mill, etc.
It is also used for generating electricity,
pumping water, etc.

Wind mills are used to generate
electricity.

60 GREEN Science (Physics) Book-10

Strong wind blows in the hilly and Himalayan regions of Nepal. The blowing wind is used
to rotate turbines for generating electricity. Wind energy is being used in some parts of our
country. In the context of Nepal, wind energy can be a good alternative source of energy.

4. Tidal energy Barrage
Level of the high tide
Now-a-days, electricity can be generated
by rotating turbines with the help of Sluice gates
water current of tides. The energy which
is obtained from tides of sea is called tidal Tidal basin
energy. It is a renewable source of energy.
Fig. Fig. Turbine
Water of tides is collected in large dams.
The water collected in dams is sent through 3.10 Ocean Estuary floor
pipes to rotate turbines for generating
electricity. However, there is no possibility Tidal energy is used to generate electricity.
of tidal energy in Nepal as Nepal is a land-
locked country.

5. Solar energy

Solar energy is the best alternative source
of energy as it is available in different parts
of the earth. The energy obtained from the
sun is called solar energy. The solar cell
is a device which converts solar energy
directly into electrical energy with the help
of photo cells.

Solar energy can be trapped by solar

panel to charge the battery. The electricity 3.11
generated from solar energy can be used
Solar panel
to get light, heat water and operate various

electric as well as electronic equipment. In context of Nepal, solar energy is one of the

major alternative sources of energy to solve energy crisis.

Solar energy is used to operate artificial satellites and communication devices in remote

areas. Solar energy does not cause pollution. Solar energy is a very good alternative source

of energy in remote areas of Nepal. Cooling tower

6. Geothermal energy Steam
Turbine & generator
The inner part of the earth is very hot.
During volcanic eruption, the hot molten Fig. Hot water Injection well
magma comes out. The energy which is
obtained from the heat inside the earth is
called geothermal energy.

In the places where earthquake and Geothermal energy

volcano are active, the temperature 3.12 GREEN Science (Physics) Book-10 61
increases by 80°C while going about 1km

deep in the earth's crust. In other places, the temperature increases by 30°C. Hot rocks are
found inside the earth crust.

To produce geothermal energy, two parallel holes are drilled deep below the earth’s crust
mainly in volcanic regions. Water is sent through a hole which gets converted into steam
using enormous heat below the earth crust. The steam produced is carried out through
another hole which is used to rotate turbines to generate electricity.

Energy Crisis

Now-a-days, we are facing the shortage of energy sources due to population growth. We
are facing shortage of electricity, petrol, diesel, gas, etc. Most of the sources of energy that
we use now-a-days are petroleum products like petrol, diesel, coal, LP gas, etc. These
sources have a limited stock in nature. We are using them at so rapid rate that their stock
will be finished from nature in the very near future. It will result in great scarcity of
energy sources. The acute scarcity of energy sources is called energy crisis.

The main causes of energy crisis are given below:
1. Population growth
2. Careless use of existing energy sources
3. Lack of development of alternative sources of energy

It is our duty to contribute to push energy crisis further. Some methods of solving the
problem of energy crisis are as follows:
1. The existing sources of energy should be used wisely and economically.
2. Alternative sources of energy such as solar energy, wind energy, biomass energy, etc.

should be developed and used.
3. Population growth should be controlled.
4. Awareness should be generated for wise and economic use of existing sources of

energy.
Methods of conserving energy at home.
1. Food should be cooked in pressure cookers to save energy.
2. Computer, television, bulbs, etc. should be switched off if their operation is not

necessary.
3. Lead bulbs should be used instead of filament bulbs to save electricity.

Present Status of Energy Use

Along with the pace of development, the use of mineral oil, coal and natural gas is
increasing day by day. About 35% of the energy demand is being fulfilled by mineral oil.
Coal is widely used in industries to generate heat energy. After mineral oil, coal occupies
the second position of the source of energy. Coal fulfills about 27% of the energy demand.
The demand of energy is increasing at the rate of 2.3% per year due to population growth.
It may cause energy crisis in near future if available energy sources are not conserved and
alternative sources of energy are not developed. Therefore, it is very necessary to develop
and use alternative sources of energy throughout the world.

62 GREEN Science (Physics) Book-10

Key Concepts

1. The objects from which we get energy are called the sources of energy. For example,
the sun, running water, coal, petrol, diesel, wind, etc.

2. The sources of energy which never get exhausted and can be used again and again
are called renewable sources of energy. Solar energy, biomass energy, wind energy,
hydropower, tidal energy, biogas, geothermal energy, etc. are the examples of
renewable sources of energy.

3. The sources of energy which get exhausted after use and cannot be obtained again
and again are called non-renewable sources of energy. Coal, petrol, diesel, kerosene,
natural gas, etc. are non-renewable sources of energy.

4. Energy can be obtained from different sources like the sun, blowing wind, petrol,
diesel, coal, running water, biomass, hydroelectricity, etc. Among them, the sun is
considered as the main source of energy. All other sources of energy depend on the
solar energy directly or indirectly.

6. The energy obtained from the fossils that remain buried under the earth crust is
called fossil fuel energy.

7. Coal and mineral oil are formed from fossils of plants and animals in nature. So
they are called fossil fuels.

8. Coal is a hard black mineral found in nature. It is the fossil fuel which is found in
solid state. Coal is widely used in industries, factories and trains as the main source
of heat energy.

10. Mineral oil includes liquid fossil fuels like petrol, diesel, kerosene, etc. It is used to
run automobiles.

12. Hydropwer is the form of energy which is produced by rotating turbines with the
help of flowing water.

14. The energy sources which are used in place of conventional sources of energy
are called alternative sources of energy. Bio fuel, tidal energy, wind energy, solar
energy, etc. are the examples of alternative sources of energy.

15. The fuel which is obtained from different parts of plants and animal dung is called
bio fuel.

16. Biogas is the cooking gas which is produced by decomposition of animal dung.
Biogas consists of methane, carbon dioxide, hydrogen and hydrogen sulphide.

17. The energy which is obtained from nuclear fusion and nuclear fission reaction is
called nuclear energy.

18. The nuclear reaction in which lighter nuclei combine to form a heavy nucleus by
releasing a large amount of energy is called nuclear fusion reaction.

19. The nuclear reaction in which a heavy nucleus breaks down into two lighter nuclei
by releasing a large amount of energy is called nuclear fission reaction.

23. The energy which is obtained from tides of sea is called tidal energy. It is a renewable
source of energy.

24. Solar energy is the best alternative source of energy as it is available in all parts of
the earth.

25. The energy which is obtained from the heat inside the earth is called geothermal
energy.

GREEN Science (Physics) Book-10 63

Sequential General Exercise 1

1. Choose the best answer from the given alternatives.
a. Which of the following is an ultimate source of energy.

Coal Petrol

Sun Diesel

b. Which of the following is a mineral fuel? Hydropower
Diesel

Bio fuel Solar energy

c. Which of the following is the best alternative source of energy in context of Nepal?

Tidal energy Geothermal energy

Solar energy Wind energy

d. Which of the following source of energy is not possible in Nepal?

Tidal energy Solar energy

Biomass energy Wind energy

2. Answer the following questions.
a. What is energy?
b. What are sources of energy?
c. Name two types of sources of energy. Give any five examples.
d. Define renewable sources of energy with any two examples.
e. Define non-renewable sources of energy with any three examples.
f. Why is the sun considered as the main source of energy?
g. What is fossil fuel? Why is fossil fuel called a non-renewable source of energy?
h. What is mineral oil? Write its two uses.
i. Write any four advantages of fossil fuel.
j. What is hydropower? Write any five utilities of hydropower.
k. What are alternative sources of energy? Give any two examples.
l. What is bio fuel? Define biomass energy.
m. What is nuclear energy? How is it produced?
n. What are nuclear reactions? What are their types?
o. What is nuclear fusion reaction?
p. What is nuclear fission reaction?

64 GREEN Science (Physics) Book-10

q. What are two necessary conditions for nuclear fusion reaction?
r. What is meant by E = mc²?
s. Define tidal energy. Why is it called alternative source of energy?
t. What is geothermal energy?
u. What is energy crisis? Write any two causes of energy crisis.
v. Write any four measures to solve energy crisis in Nepal.

3. Differentiate between:
a. Renewable sources and non-renewable sources of energy
b. Biomass energy and tidal energy
c. Solar energy and fossil fuel energy
d. Nuclear fusion and nuclear fission

4. Give reason.
a. Biomass energy is called a renewable source of energy.
b. The sun is considered as the main source of energy.
c. Tidal energy is called an alternative source of energy.

5. The development and use of alternative sources of energy is very important to
solve energy crisis. Justify this statement.

6. The use of solar energy should be increased in the context of Nepal, why?
7. Hydropower is the outcome of solar energy. Justify this statement.
8. How is electricity generated with the help of tidal energy? Explain.
9. The world will face energy crisis in the near future. Justify this statement giving

three reasons.
10. Describe the present status of energy use in the world.

Grid-based Exercise

Group ‘A’ (Knowledge Type Questions) (1 Mark Each)

1. What are the various types of energy sources ? Name them.

2. What is non-renewable source of energy ?

3. What is renewable source of energy ?

4. How much energy does the sun radiate in one second ? (Ans: 27 x 1023 kW/s)

5. How much part of the solar energy reaches the earth ? (Ans: One part of 2.2 trillion)

6. What are two conditions required for nuclear fusion reaction that takes place in the sun?

7. What is hydroelectricity? What is the estimated hydroelectric potentiality of Nepal?

Write. (Ans: 83000 MW)

8. What is the main source of solar energy ? How is enormous energy produced in the
sun ? Write.

9. Write the name of any two alternative sources of energy.

GREEN Science (Physics) Book-10 65

10. What is nuclear fission reaction?
11. What is geothermal energy?
12. What are the types of energy sources based on time of replacement ? Write down their

names.
13. Give any two examples of non-renewable source of energy.
14. What is energy crisis?
15. Write any two natural phenomena from which geothermal energy can be obtained.

Group ‘B’ (Understanding Type Questions) (2 Marks Each)

16. Biomass energy is considered as a renewable source of energy. Why ?
17. The sun is considered as the main source of energy. Justify this statement.
18. Coal and petroleum are known as non-renewable sources of energy, why ?
19. Write two differences between renewable and non-renewable sources of energy.
20. The demand of fossil fuel is increasing day by day in the present day world. Why ?
21. State any two causes of energy crisis.
22. Write two differences between nuclear fusion reaction and nuclear fission reaction.
23. The use of hydroelectricity should be increased rather than that of mineral oil. Justify

this statement.
24. In the present world, the use of hydroelectricity is increasing. Write any two reasons.
25. Write two causes that Nepal Government has given higher priority for the production

of hydroelectricity.
26. Write any two points to justify that urbanization brings energy crisis.

Group ‘C’ (Application Type Questions) (3 Marks Each)

27. Write any three uses of biogas.

28. Write any three ways to solve the situation of energy crisis in the context of Nepal.

29. The development and use of alternative energy sources is very necessary in present
day world. Explain this statement.

30. Why should energy be conserved? Mention any three steps you would take at home
to conserve energy?

31. In future, the hydroelectricity will be main alternative source of energy for Nepal.
Justify it with three reasons.

Group ‘D’ (Higher Abilities Type Questions) (4 Marks Each)

32. All the sources of energy present on the earth are the outcomes of solar energy. Justify
this statement with examples.

33. Explain in brief about present status of energy use in the world and its conservation.

34. How is energy produced from nuclear fission reaction ? Describe in brief.

35. Why is the development and use of alternative energy sources necessary in today’s
world ? Explain.

36. Justify giving four reasons that the use of hydropower should be increased than that
of coal and mineral oil for energy.

66 GREEN Science (Physics) Book-10

UNIT Heat

4

Weighting Distribution Theory : 6 Practical: 0

Before You Begin

Heat is a form energy which gives us the sensation of hotness or
coldness. Heat is produced when we rub our palms against each
other. So we feel hot. When we touch a burning candle, we feel hot
but when we touch ice, we feel cold. How do we get such a sensation?
It is due to the flow of heat from a hot object to a cold one. When we
touch a burning candle, heat flows from the flame of candle to our
body and we feel hot. When we touch ice, heat flows from our body
to the ice and we feel cold. Heat is a form of energy which gives us the
sensation of warmth, i.e. hotness or coldness. The degree of hotness
or coldness of a body is called its temperature.

Learning Objectives Syllabus

After completing the study of this unit, students will be able to: • Introduction to heat and
temperature
i. differentiate between heat and temperature and
demonstrate them. • Factors affecting heat
• Thermometer and its structure
ii. describe the types of thermometer and use them. • Thermometric liquids
• Types of thermometer
iii. solve simple numerical problems related to heat • Specific heat capacity
equation. • Heat equation
• Simple numerical problems

Glossary: A dictionary of scientific/technical terms

heat : the form of energy which produces sensation of warmth
temperature : the degree of hotness or coldness of a body
calorimeter : the device which is used to measure the heat of a body
thermometer : the device which is used to measure the temperature of a body
sensation : a feeling that we get when we touch something or something affects our

expansion body
contraction : the process of becoming larger or increasing in volume
calorimetry : the process of becoming smaller or decreasing in volume
: the measurement of heat lost or gained by a substance

GREEN Science (Physics) Book-10 67

Heat

Put your hands together and rub your palms? What do you feel? When we rub our palms
against each other we feel hot due to production of heat. Heat is the form of energy which
gives the sensation of hotness or coldness.

Tea is hot and ice is cold. These sensations give us the concept of heat. Matter is made of
molecules and these molecules have potential energy. When the molecules of a body
vibrate, heat is produced due to kinetic energy of the molecules present in matter. The
kinetic energy in the molecules of hot body is more than that of the molecules of a cold
body. The objects from which we get heat energy are called sources of energy. Some
sources of heat energy are given below:

Fig.

4.1 Heater Burning coal
Sun

According to kinetic molecular theory, heat Do You Know
can be defined as the sum of kinetic energy
present in the molecules of matter. The device which is used to measure heat
is called calorimeter.
Units of heat

1. SI unit: joule (J)

2. CGS unit : calorie (Cal.)

Heat is a very useful form of energy. Plants and animals cannot survive without heat
energy. Human beings use heat to cook food, to dry clothes, to run industries and operate
automobiles. Heat is also essential to keep our body warm. Heat is the cause of change
in temperature of a body. When heat is supplied to a body, its temperature increases and
when heat is taken out of a body its temperature decreases.

Factors affecting heat energy present in a body

The total amount of heat present in a body depends on following two factors:

1. Number of molecules present in the body or mass of the body

2. Average KE (kinetic energy) of the molecules present in that body

Therefore, the total amount of heat present in a body is directly proportional to the mass
of the body and average kinetic energy of the molecules of that body.

68 GREEN Science (Physics) Book-10

In short,
Amount of heat present in a body ∝ mass of the body .....(i)
Amount of heat present in a body ∝ average kinetic energy of the molecules .....(ii)
From (i) and (ii),

Amount of heat present in a body ∝ mass of the body × average kinetic energy of the
molecules

From above relation, it becomes clear that the total amount of heat present in a body is
equal to the product of mass of the body and average kinetic energy of the molecules
present in that body.

There are different types of matter. The different molecules of matter have different

properties. Due to this when equal quantity of heat is given to equal mass of different

substances they do not have the same

temperature. Do You Know

Relation between calorie and joule In CGS system, heat is measured in calorie

1 calorie = 4.2 joules (Cal.). One calorie is the amount of heat

required by 1 gram of pure water to raise

Temperature its temperature by 1°C.

The temperature of a body is defined as the
degree of hotness or coldness of the body. It can also be defined as the average kinetic
energy of the molecules present in a body. Temperature of a body is the measure of the
average kinetic energy or thermal energy of its molecules. Temperature is the property of
a substance which determines the direction of flow of heat.

Units of temperature Do You Know

The SI unit of temperature is kelvin (K). The device which is used to measure
But temperature is commonly measured in the temperature of a body is called
degree Celsius (°C) and degree Fahrenheit thermometer. There are different types of
(°F). thermometer.

Differences between Heat and Temperature
Temperature

Heat

1. Heat is the sum of kinetic energy 1. Temperature is the average kinetic

present in the molecules of a body. energy present in the molecules of a

body.

2. Calorimeter is used to measure heat. 2. Thermometer is used to measure
3. Its SI unit is joule (J). temperature.

3. Its SI unit is kelvin (K).

GREEN Science (Physics) Book-10 69

Activity 1

Go to the science laboratory and take two beakers of equal size.
Put 100 ml of water in a beaker and 200 ml in another beaker.
Measure the initial temperature of water in both beakers by using thermometer.

Stand Thermometer Stand
Beaker
Fig. Water

Tripod stand

4.2

Heat both beakers with separate burners of equal flame.
Note down the rise in temperature in both beakers after every 3 minutes. What do

you observe? What can you conclude from this activity?

Activity 2

Take a beaker and put 50 ml of hot water into it.
Measure the temperature of the hot water and note it down.
Take another beaker and put 50ml of cold water into it.
Measure the temperature of the cold water and note it down.
Now, mix hot water into cold water and measure the final temperature of the mixture.
What can you conclude from this activity?

When hot water is mixed with cold water, the temperature of hot water decreases and that
of cold water increases. Due to this, the final temperature of the mixture is less than that
of hot water and more than that of cold water. It shows that heat always flows from the
body at high temperature to the body at low temperature.

Thermometer

We can feel hotness or coldness of a body by touching it. But we cannot measure the
temperature of that body just by touching. A device that is used to measure the temperature
of a body is called thermometer. So, thermometer is the device which is used to measure
the temperature of a body.

Principle of construction of thermometer

When a body is heated, it expands and when it is cooled, it contracts. This is the working
principle of a thermometer. Thermometer can be made by using liquid or gas as they
expand more than the solids. However, we commonly use liquid thermometers.

70 GREEN Science (Physics) Book-10

Types of Thermometer

There are different types of thermometers. In this unit, we will study three types of
thermometers. They are: Laboratory thermometer, clinical thermometer and maximum-
minimum thermometer.

1. Laboratory thermometer
The thermometer which is used to measure the temperature of various objects in the
laboratory is called the laboratory thermometer. It consists of a capillary tube made of
glass in which one end is closed and another end is connected to a cylindrical bulb.

The cylindrical bulb has a small portion of the capillary tube filled with mercury or coloured
alcohol. The air from the capillary tube is completely taken out before closing the open end
of the capillary tube. The scale in the laboratory thermometer ranges from – 10° C to 110° C.

Fig.4.3
Fig. Fig.
Laboratory thermometer

While measuring the temperature of a body, the bulb of the thermometer is kept in close
contact of the body. As the bulb comes in contact with the hot body, the thermometric
liquid inside the bulb expands and rises upwards in the capillary tube. Finally, the
liquid gives a constant level, which is the temperature of the body. When the bulb of the
thermometer is kept in a cold body, the thermometric liquid contracts upto a constant
level which shows the temperature of the cold body.

2. Clinical thermometer

The thermometer which is used to measure the temperature of the human body is called
clinical thermometer. It consists of a prismatic glass tube having a constriction in the
capillary tube near the bulb. The constriction allows the mercury to rise up in the capillary
tube while measuring the temperature. But it does not let the mercury to fall back
immediately when removed from the body. It helps to take accurate reading of the body
temperature.

4.4
Clinical thermometer

In a clinical thermometer, the temperature scale
ranges from 35°C to 42°C because the average
human body temperature is 37°C.

While measuring temperature of the human body, 4.5 Digital thermometer
the bulb of the clinical or digital thermometer

GREEN Science (Physics) Book-10 71

is kept in the mouth or armpit of the person for a while. The mercury remains at the
original level due to contraction after its removal from the body. It helps to take accurate
measurement of the body temperature.

Now-a-days clinical thermometer has been replaced by digital thermometer. It shows
body temperature in digits. It neither contains mercury nor alcohol.

3. Maximum – minimum thermometer Minimum Maximum

It is a U-shaped thermometer which is used to measure Vacuum
the maximum and minimum temperature of a certain
place for 24 hours. In this thermometer, both mercury Alcohol
and alcohol are used. This thermometer has two bulbs
one on either side. Out of two bulbs, one is small and Metal indices
another is large. Both bulbs are connected with a tube.
In maximum – minimum thermometer, the mercury
tube shows the maximum temperature and alcohol
tube shows minimum temperature.

There is an index in both tubes to measure the Fig. Mercury
temperature. A magnet is used to set the index. The
reading indicated by lower ends of the index shows 4.6
the maximum and minimum temperature of a certain
place.

Activity 3

Take a clinical thermometer and measure the body temperature of all the students
of class 10.

Take a digital thermometer and repeat the above activity.

Thermometric liquids

Two liquids, i.e. mercury and alcohol, are used in a thermometer. These liquids are
called thermometric liquids. The liquids which are used in a thermometer are called
thermometric liquids.

Advantages of mercury as a thermometric liquid
1. Mercury can be seen clearly in the capillary tube as it is a silvery white liquid.

2. It has a uniform rate of expansion and contraction.

3. It is a good conductor of heat as it is a liquid metal.

4. It can measure a wide range of temperature because the freezing point of mercury is
–39°C and boiling point is 357°C.

5. Mercury does not stick to the inner wall of capillary tube.

72 GREEN Science (Physics) Book-10

Disadvantage of mercury as a thermometric liquid

A mercury thermometer cannot measure very cold temperature as the freezing point of
mercury is –39°C. Therefore, mercury thermometer is not suitable to measure very low
temperature in cold countries.

Advantages of alcohol as a thermometric liquid

1. Alcohol is suitable to measure very low temperature in cold countries as its freezing
point is –115°C.

2. It can measure accurate temperature as its rate of expansion is six times more than
that of mercury.

3. It is not very expensive.

Disadvantages of alcohol as a thermometric liquid

1. Alcohol is not suitable to measure the temperature above 78°C because its boiling
point is 78°C.

2. It is a bad conductor of heat. So, it has no uniform rate of expansion and contraction.
3. It cannot give accurate measurement of temperature because it sticks to the inner wall

of the capillary tube.

Specific Heat Capacity

When a body is heated, its temperature increases and the increase in temperature depends
on the mass of the body, amount of heat supplied and the nature of the molecules of that
body.

The total quantity of heat required to raise the temperature of a body of 1 kg mass by 1 K
or 1° C. is called specific heat capacity of the body. Its SI unit is J/kg K or J/Kg°C.

Different objects have different specific heat capacity. For example, 1 kg of copper
requires 400 J of heat energy to raise its temperature by 1°C. So the specific heat capacity
of copper is 400J/kg° C. Similarly, 1 kg of alcohol requires 2400 J of heat energy to raise its
temperature by 1° C. So the specific heat capacity of alcohol is 2400 J/kg° C.

The values of specific heat capacity of different objects are given in the following table:

Substances Specific heat capacity (in J/kg°C)
234
Metals and alloys Silver 140
460
Mercury 400
447
Iron 900
130
Copper 380

Steel

Aluminium

Lead

Brass

GREEN Science (Physics) Book-10 73

Non-metals Water 4200
Ice 2100
Alcohol 2400
Sand 800
Cooking oil 2000
Petrol 1670
Kerosene 2200
Glass 670

Water is a very common substance having high specific heat capacity, i.e. 4200 J/kg° C. It
means that 1 kg of pure water requires 4200 J of heat energy to raise its temperature by
1°C. Water absorbs a large amount of heat energy. Therefore, it is used in hot water bags
for heating purpose and in radiators of vehicles for cooling purpose.

The molecules of different objects have different capacity to store heat energy. Similarly,
the rate of molecular vibration differs according to the nature of molecules when supplied
with equal amount of heat. Therefore, the specific heat capacity of different substance is
different.

Activity 4

Take two beakers of the same size.

Put 100 ml of water in one beaker and 100 ml of cooking oil in another. Set the
apparatus as shown in the figure.

Stand Thermometer Stand

Fig. Beaker
Oil
Water

Tripod stand

4.7

Take a laboratory thermometer and measure the initial temperature of water and
cooking oil. Note down the reading.

Now, heat both liquids, i.e. water and cooking oil, with burners of equal flame.
Note the temperature of both liquids after every three minutes.
What do you observe? Why?

74 GREEN Science (Physics) Book-10

It is observed that the temperature of oil increases faster than that of water though both
liquids are heated with equal flame. It shows that different substances have different
heat retention capacity that causes difference is rise in temperature. The kinetic energy
of the molecules of different substances in different even though same quantity of heat
is supplied to them. Due to this, different substances have different heat capacities.v

Heat Equation

According to law of conservation of energy, energy can neither be created nor be
destroyed but it can be changed from one form to another. In the same way, heat energy
also flows from the body at high temperature to body at low temperature but it does not
get destroyed.
The amount of heat lost by a body is equal to the amount of heat gained by a body
provided that no heat flows in the surroundings.

Heat lost by a hot body = Heat gained by a cold body

The quantity of heat lost by a hot body or gained by a cold body is equal to the product
of its mass, specific heat capacity and change in temperature of the body. This relation is
called heat equation. In short,

Q = msdt

where,
Q = quantity of heat lost or gained by a body
m = mass of the body
s = specific heat capacity of the body
dt = change in temperature
If t1 = initial temperature
t2 = final temperature
Then dt = t2 – t1
From the above relation, it becomes clear that the quantity of heat lost or gained by a body
is directly proportional to the mass of the body and change in temperature of the body.
Q ∝ m ………………… (1)
Q ∝ dt ……………… (2)
From (1) and (2), we get
Q ∝ m.dt
or, Q = sm dt ...................... (3)
(where ‘s’ is a constant which is known as the specific heat capacity of the body)

\ Q = msdt

GREEN Science (Physics) Book-10 75

Solved Numerical: 1
What amount of heat is required by 1500 g of water to raise its temperature by 25° C ?

Solution :

Mass (M) = 1500 g

= 1500 kg [ 1kg = 1000 g]
1000

= 1.5 kg

Change in temperature (dt) = 25°C

Amount of heat (Q) = ?

According to the formula,

Q = msdt

= 1.5 × 4200 × 25 [ Specific heat capacity of water (s) = 4200J/kg° C]

= 157500 J

\ The amount of heat required (Q) = 1.575 × 105 J
Solved Numerical: 2

A heater gives 3.6 × 106J of heat energy in one hour. What will be final temperature when
the heat is supplied to 50 kg of water at 25° C? The specific heat capacity of water is 4200
J/kg°C.

Solution :

Amount of heat (Q) = 3.6 × 106 J

Mass of water (m) = 50 kg

Specific heat capacity (s) = 4200 J/kg°C

Initial temperature (t1) = 25°C
Final temperature (t2) = ?
According to the formula,

Q = msdt

or, 3.6 × 106 = 50 × 4200 × dt

or, dt = 530.6××4120060 = 17.14° C

Now, dt = t2 – t1

or, t2 = dt + t1 = 17.14 + 25 = 42.14° C

\ Final temperature of water (t2) = 42.14°C.

76 GREEN Science (Physics) Book-10

Principle of Calorimetry

The measurement of heat lost or gained by a substance is called calorimetry. When a hot
body is mixed with a cold body, the hot body loses heat and the cold body gains heat.
So, principle of calorimetry states that, “when a hot body is mixed with a cold body, the
heat lost by the hot body is equal to the heat gained by a cold body, provided that no heat
escapes to the surroundings. In short,

Heat lost by hot body = Heat gained by cold body

or, m1s1(t1 – t) = m2s2(t – t2)
Where,

m1 = mass of hot body
m2 = mass of cold body
s1 = specific heat capacity of hot body
s2 = specific heat capacity of cold body
t1 = temperature of hot body
t2 = temperature of cold body
t = final temperature of the mixture

Solved Numerical: 3

A bucket contains 10 kg hot water at 90°C. Calculate the final temperature of the mixture
when 20 kg water at 20°C is mixed to the hot water. Specific heat capacity of water is
4200J/kg°C.

Solution :

Mass of hot water (m1) = 10 kg
Temperature of hot water (t1) = 90°C
Mass of cold water (m2) = 20 kg
Temperature of cold water (t2) = 20°C
Specific heat capacity of water (s) = 4200J/kg°C

Final temperature of the mixture (t) = ?

We know, m1s(t1 – t) = m2s(t – t2)

10 × 4200 (90 – t) = 20 × 4200 (t – 20)

or, 900 – 10t = 20t – 400

or, 900 + 400 = 20t + 10t

or, 30t = 1300

or, t= 1300
30

or, t = 43.33°C

\ The final temperature of the mixture (t) = 43.33°C.

GREEN Science (Physics) Book-10 77

Key Concepts

1. Heat is the form of energy which gives the sensation of hotness or coldness.

2. According to kinetic molecular theory heat can be defined as the sum of kinetic
energy present in the molecules of a matter.

3. The total amount of heat present in a body is directly proportional to the mass of
the body and average kinetic energy of the molecules of that body.

4. The different molecules of matter have different properties. Due to this when equal
quantity of heat is given to equal mass of different substances they do not have the
same temperature.

5. One calorie is the amount of heat required by 1 gram of pure water to raise its
temperature by 1°C.

6. The temperature of a body is defined as the degree of hotness or coldness of the
body. It can also be defined as the average kinetic energy of the molecules present
in a body.

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

8. When a body (liquid) is heated, it expands and when it is cooled, it contracts. This
is the working principle of a thermometer.

9. The thermometer which is used to measure the temperature of various objects in a
laboratory is called laboratory thermometer.

10. The thermometer which is used to measure the temperature of the human body is
called clinical thermometer or digital thermometer.

13. In maximum – minimum thermometer, the mercury tube shows the maximum
temperature and alcohol tube shows minimum temperature.

14. The liquids which are used in a thermometer are called thermometric liquids.

15. A mercury thermometer cannot measure very cold temperature as the freezing
point of mercury is –39°C. Therefore, mercury thermometer is not suitable to
measure very low temperature in cold countries.

16. Alcohol is suitable to measure very low temperature in cold countries as its freezing
point is –115°C.

17. The total amount of heat required to raise the temperature of a body of 1 kg mass by
1 K or 1° C is called specific heat capacity of the body. Its SI unit is J/kg K or J/Kg°C.

18. Water is a very common substance having high specific heat capacity,
i.e. 4200J/kg°C. It means that 1 kg of pure water requires 4200 J of heat energy to raise
its temperature by 1°C. Water absorbs a large amount of heat energy. Therefore, it is
used in hot water bags for heating purpose and in radiators of vehicles for cooling
purpose.

19. According to the law of conservation of energy, energy can neither be created nor
be destroyed but it can be changed from one form to another.

20. The amount of heat lost by a body is equal to the amount of heat gained by another
body provided that no heat flows in the surroundings.

78 GREEN Science (Physics) Book-10

Sequential General Exercise

1. Choose the best answer from the given alternatives.
a. The SI unit of heat is ..............................

J °C Cal. K

b. The device which is used for measuring temperature is called ...............

calorimeter thermometer ammeter voltmeter

c. What is the boiling point of mercury?

100°C 357°C 39°C 78°C

d. What is the freezing point of alcohol?

78° C – 115° C 39°C 0°C

e. Which of the following is the heat equation?

Q = msdt Q = mstd S = Qmdt Q = dt
ms

2. Answer the following questions.
a. What is meant by heat energy? Write its SI unit and CGS unit.
b. Define heat in terms of kinetic molecular theory.
c. What are two factors in which heat energy of a body depends?
d. What is temperature? Write its units.
e. What is thermometer? Write its working principle.
f. Describe the structure of a laboratory thermometer with a neat figure.
g. What is clinical thermometer? Draw its neat figure.
h. What is maximum–minimum thermometer? Draw its figure.
i. What is specific heat capacity of a substance? Write its unit.
j. State heat equation and prove Q = msdt.
k. The specific heat capacity of ice is 2100 J/kg°C. What does it mean?

3. Study the given table and answer the following questions.

Metals Specific heat capacity
Iron 460 J/kg°C
Copper 400 J/kg° C
Lead 130 J/kg°C

GREEN Science (Physics) Book-10 79

a. When 1000 J of heat is given to equal mass of these three metals, which one will
have the highest temperature, why?

b. If equal mass of these metal balls at 50°C are kept on wax slab, which metal ball
will penetrate to greatest depth, why?

4. Differentiate between:
a. Calorimeter and thermometer
b. Heat and temperature
c. Mercury and alcohol
d. Clinical thermometer and laboratory thermometer

5. Give reason:
a. See breeze occurs during the day and land breeze occurs during the night.
b. Water is used to cool down the engine of vehicles.
c. The water kept in earthen pot is felt to be cold in summer days.
d. Well water is felt to be warm in cold morning.
e. A wet handkerchief is kept on the forehead of a patient suffering from fever.
f. Oil cools down faster than water.

6. Describe the structure of maximum-minimum thermometer with a neat and
labelled figure.

7. Alcohol thermometer is not suitable to measure the temperature of boiling water.
Justify the statement.

8. Numerical Problems:
a. How much heat is required to raise the temperature of 10 kg of water from 25°C
to 45°C? The specific heat capacity of water is 4200J/kg°C. [Ans: 8.4 × 105 J)

b. A metal ball of 4 kg mass is cooled from 25° C to 15°C. What quantity of heat is

released? The specific heat capacity of the metal ball is 460 J/kg°C .

[Ans : 18400J)

c. Calculate the specific heat capacity of water if 1575kJ of heat is required to raise

the temperature of 15 kg water form 20°C to 45°C. [Ans : 4200 J /kg°C]

d. Calculate the specific heat capacity of a substance if 19 kJ heat is required by 5 kg
of the substance to raise its temperature from 10° C to 20°C. [Ans : 380 J/kg° C]

e. If 84000 J of heat is given to 2 kg of water at 20° C, calculate the final temperature.

[Ans : 30° C]

f. A frying pan of mass 2.5 kg has a temperature of 40°C. The specific heat capacity

of the frying pan is 1000J/kg°C. Calculate the final temperature when 4500J of

heat is supplied to the pan. [Ans: 41.8° C]

80 GREEN Science (Physics) Book-10

Grid-based Exercise

Group ’A’ (Knowledge Type Questions) (1 Mark Each)

1. Define heat energy on the basis of molecular kinetic theory.

2. State the factors on which the amount of heat present in a body depends.

3. Define one calorie of heat energy.

4. Define temperature and write its SI unit.

5. Write down formula and SI unit of specific heat capacity.

6. Write the relationship between quantity of heat gained or lost by a body with its mass.

7. How much joule is there in 1 calorie? (Ans: 4.2J)

8. Name the instrument which is made on the basis of expansion of heat.

9. Which principle is the construction of thermometer based on ?

10. What is digital thermometer?

11. Define specific heat capacity.

12. What is specific heat capacity of water?

13. Write two factors that affect heat energy.

14. What is clinical thermometer?

15. What is maximum-minimum thermometer?

Group B’ (Understanding Type Questions) (2 Marks Each)

16. Water is used to cool the engine of vehicles. Why ?

17. Write any two differences between heat and temperature.

18. What is the meaning of the statement that the specific heat capacity of water is 4200
J/kg°C ?

19. Why is water used in hot water bags?

20. Write any two differences between clinical thermometer and laboratory thermometer.

21. We wear woolen clothes during winter. Why ?

22. When hot iron nail is kept in cold water, the temperature of iron decreases, whereas
temperature of water increases, why ?

23. A wet handkerchief is kept on the forehead of the patient suffering from fever. Why ?

24. Write any two differences between mercury and alcohol.

25. Write down the cause that the night in desert is very cold and day of that place is very hot.

Group ‘C’ (Application Type Questions) (Metal) (3 Marks Each)

26. Study the given table and answer the following A (Specific heat
questions. B capacity)
C 234 J/kg°C
i. The specific heat capacity of metal ‘A’ is 234J/ 400 J/kg°C
kg°C. What does it mean? 900 J/kg°C

GREEN Science (Physics) Book-10 81

ii. If the same amount of heat energy is given to the same mass of all three metals at
the same temperature, which one will gain the lowest temperature? Why?

iii. Which metal will penetrate into the greatest depth if each with equal mass is put
on a wax slab after heating to 75°C each? Why?

27. Write any three advantages of mercury as a thermometric liquid.

28. Write any three advantages of alcohol as a thermometric liquid.

29. Write any two applications of thermometer. If a heater with power 1000W gives

3.6 × 103 KJ of heat energy in one hour. What will be the temperature raised in the 50

kilogram of water with temperature 30°C by that heat ? (Ans: 47.14°C) 1 +2

Group ‘D’ (Higher Abilities Type Questions) (4 Marks Each)

30. Write the relationship between heat gained or lost by an object with its mass. An iron

ball of mass 5000 g at 100°C temperature is dipped in water of mass 10 kg at 20°C.

Find the final temperature of water. (The specific heat capacity of water and iron are

4200 J/kg°C and 470 J/kg°C respectively.) (Ans: 24.33°C) 1+3

31. Why does frost bite occur in mountain climber? What will be the final temperature

of a mixture of 10kg of water at 70°C and 20kg of water at 10°C. Neglect the heat

absorbed by the container. (Ans: 30°C) 1+3

32. Study the given figure and answer the following questions. 10 kg of water 5 kg of water
i. What is the direction of heat and why? at 60°C at 80°C

ii. What will be the equilibrium temperature if stopper

is opened, assuming there is no heat gain or loss, in the

system? 1 + 3 (Ans: 66.66°C) A Stopper B

33. What will happen if oil is used in hot water bag instead of water ? Explain with reason.

Hot water of mass 10 kg at 90°C is cooled for taking bath by mixing 20 kg of water

at 20°C, what is the final temperature of the water? (Specific heat capacity of water is

4200J/kg°C.) (Ans: 43.33°C) 1 +3

34. Three liquids ‘P’, ‘Q’ and ‘R’ of equal mass are kept in the same type of container and
placed in the sun for 30 minutes. The increase in temperature is given in the table. 2+2

Liquid Increase in temperature
P 20°C
Q 25°C
R 18°C

i. Which liquid has the highest specific heat capacity? Why?

ii. If equal mass of all three liquids at same temperature are cooled, which one will
cool down faster? Why?

82 GREEN Science (Physics) Book-10

UNIT Light

5

Weighting Distribution Theory : 5 Practical: 2

Before You Begin

In a dark room, we cannot see the things kept there although our eyes
are open. We need light to see the things kept there. So, light is a form
of energy which makes things visible. It is produced from extremely hot
objects. The objects which emit light are called the sources of light. The
sun is the main source of light on the earth. Some other sources of light are
light bulb, burning candle, kerosene lamp, etc. Light is invisible itself but it
makes things visible. The rays of light coming from a source get reflected
when they fall on an object. These reflected rays of light enter our eye and
finally reach the brain through optic nerve. As a result, we see the object.
In some cases, we use various instruments to see objects clearly. These
instruments are called optical instruments. One or more lenses are used
in optical instruments. In this unit, we will study lenses, their types and
refraction of light through lenses, defects of vision and their remedy.

Learning Objectives Syllabus

After completing the study of this unit, students will be able to: • Introduction to light
• Lens and its types
i. introduce lenses and state the uses of lenses in our • Refraction of light through
daily life.
lenses
ii. demonstrate refraction of light through convex and • Utilities of lenses
concave lens and draw ray diagrams. • Power of a lens
• Magnification
iii. give general introduction to optical instruments and • Eye
state their uses. – Myopia
– Hypermetropia
iv. explain defects of vision and their remedy with ray
diagrams.

Glossary: A dictionary of scientific/technical terms

light : a form of energy which makes things visible
convex : curving out
concave : curving in
aperture : the total reflecting surface of a mirror
optical : related to lens
inverted : facing upside down
erect : upright in position
magnification : the ratio of image distance to the object distance
myopia : the defect of vision in which a person can see nearby objects only
hypermetropia : the defect of vision in which a person can see distant objects only

GREEN Science (Physics) Book-10 83

Lens

A piece of a transparent glass or a plastic having refracting surfaces is called lens. A
lens forms image by refraction rays of light falling on it. Lens is used in various optical
instruments like camera, microscope, telescope, etc. Human eye also contains a biconvex
lens. Eye lens refracts the rays of light and forms image in retina. A lens consists of two
refracting surfaces one of which may be plane or spherical.

Fig.5.1
Fig. Different types of lenses

Fig.Types of lens

On the basis of uses, there are two types of lens. They are:

1. Convex lens 2. Concave lens

1. Convex lens

A convex lens is the lens which is thick in the middle and thin at the edges. A convex lens
converges parallel rays of light at a point after refraction. So, it is also called a converging
lens.

5.2 Convex lens Refraction of light through a convex lens

On the basis of shape, there are three types of convex lens which are as follows:
i. Biconvex lens
ii. Planoconvex lens
iii. Concavoconvex lens

5.3 Biconvex lens Planoconvex lens Concavoconvex lens

84 GREEN Science (Physics) Book-10

2. Concave lens

A concave lens is the lens which is thin in the middle and thick at the edges. A concave
lens diverges parallel rays of light after refraction. So, it is called a diverging lens.

Fig.5.4 Concave lens Refraction of light through a concave lens
Fig.
On the basis of shape, there are three types of concave lenses. They are as follows:

i. Biconcave lens ii. Planoconcave lens iii. Convexoconcave lens

5.5 Biconcave lens Planoconcave lens Convexoconcave lensFig.

Refraction of light through lensFig.

The parallel rays of light passing through a convex lens are converged at a point after
refraction. This point is called focal point of a convex lens. It is denoted by F.

OF

5.6
Convex lens

The parallel rays of light passing through a concave lens appear to diverge from a fixed
point. This fixed point is called focal point of a concave lens. It is denoted by F.

FO

5.7
Concave lens

GREEN Science (Physics) Book-10 85

Lens as a group of prism

A lens is made up of a group of prisms. The upper half of the prism faces downward and
the lower half of the prism faces upward. The central part of the lens acts as a glass slab.
This fact becomes clear from the given figure.

Prism 1

Fig. Fig. Prism 2

5.8 Convex lens

The rays of light passing through upper edge of the prism bend more than that passing
through the middle part of the slab. However, the ray of light passing through the central
part of a lens passes straight without bending.

Like convex lens, a concave lens is also made of a group of prisms. This fact becomes clear
from the following figure.

5.9 Concave lens

The ray of light passing through upper part of the concave lens gets diverged upwards and
the ray of light passing through lower part of the concave lens gets diverged downwards.
The ray of light passing though edges of concave lens diverges more than that passing
through the middle part of the lens.

Information regarding the lens

Different terms like optical centre, centre of curvature, principal focus, focal length, radius
of curvature, etc. are used in a lens. These terms are defined as follows:

i. Optical centre

Optical centre is the geometrical centre of a lens. It is denoted by O. A ray of light passing
through optical centre does not deviate.

86 GREEN Science (Physics) Book-10

X O YX O Y

C2 F2 F1 C1 C2 F2 F1 C1
Fig.
Fig.5.10 Convex lens Concave lens

ii. Centre of curvature

It is the centre of the sphere from which the lens has been cut. It is denoted by C or 2F. A
lens has two centres of curvature.

iii. Focal length

Focal length of a lens is the distance between optical centre and principal focus of the lens.
It is denoted by ‘f’.

iv. Principal focus

The point on the principal axis where the rays of light parallel to principal axis pass after
refraction or appear to diverge after refraction is called principal focus. It is denoted by ‘F’.

v. Radius of curvature

Radius of curvature is the radius of the sphere from which the lens is cut. It is denoted by
R. It is the distance between optical centre and centre of curvature. Radius of curvature is
equal to the twice of the focal length.

\ R = 2f

vi. Principal axis

The straight line passing through optical centre and centre of curvature is called principal
axis. It can also be defined as the straight line passing through two centres of curvature
of the lens.

Activity 1

Take a hand lens and piece of thin paper.

Go in a sunny place.

Focus the fine beam of the sunlight on the piece of paper
and wait for five minutes.

Does the paper burn after some time?

Repeat the same activity with a concave lens.

Does the paper burn in this case also? 5.11
What can you conclude from this activity?

GREEN Science (Physics) Book-10 87

Fig. Fig. Fig.Activity 2

Take a convex lens and white cardboard paper.
Keep the lens in front of a distant object and put the cardboard paper behind the

lens.
Change the distance between the lens and the cardboard paper until a sharp image

is produced in the cardboard paper.
Now, measure the distance between the lens and the cardboard paper.
The distance between the lens and the cardboard paper is the rough focal length of

the lens.

Laws of refraction of light through the convex lens

1. The ray of light passing parallel to the principal axis of a convex lens passes through
principal focus after refraction.

OF

5.12
Convex lens

2. The ray of light passing through the principal focus of the convex lens becomes
parallel to the principal axis after refraction.

F

5.13
Convex lens

3. The ray of light passing through the optical centre of a convex lens passes straight
without deviation.

5.14 Convex lens

88 GREEN Science (Physics) Book-10

Image formed by a convex lens

Convex lens forms different types of images on the basis of the position of the object.
The ray diagrams produced by keeping object at different positions from a convex lens
are given below.

a. At infinity

When an object is at infinity from a convex lens, the image is formed at focus. The image is
i. real,
ii. inverted and
iii. highly diminished.

Rays of light from infinity O Image Do You Know
2F F F
Fig. The image which can be obtained
Fig. 2F on a screen is called a real image.

It is always inverted.

5.15
Convex

b. Beyond centre of curvature (C) or 2F

When an object is placed beyond centre of curvature (C) or 2F of a convex lens, the image
is formed between F and C (2F) on the other side of the lens.

The image is
i. real,

ii. inverted and

iii. diminished.

Object 2F F 2cm O F 2F
2cm 2cm Image
5.16
2cm

Convex lens

c. At centre of curvature (C) or 2F

When an object is placed at center of curvature (C) or 2F of a convex lens, image is formed
at C or 2F on the other side of the lens.

The image is

i. real,

ii. inverted and

GREEN Science (Physics) Book-10 89

iii. of the same size to the object.

Fig. Fig. Object O F 2F
Fig. 2F F 2cm 2cm Image
2cm
2cm
5.17
Convex lens

d. Between F and 2F or C

When an object is placed between F and 2F or C of a convex lens, the image is formed
beyond 2F on the other side of the lens.
The image is
i. real,
ii. inverted and
iii. enlarged.

Object F O F 2F or C
2F or C Image

5.18
Convex lens

e. At F (Principal focus)

When an object is placed at principal focus (F), the image is formed at infinity on the other
side of the lens.
The image is
i. real,
ii. inverted and
iii. highly enlarged.

Object OF 2F
2F F
Image at infinity
5.19
Convex lens
90 GREEN Science (Physics) Book-10

f. Between principal focus (F) and optical centre (O)

When an object is placed between principal focus (F) and optical centre (O), the image is
formed beyond the object on the same side of the lens.

The image is

i. virtual,

ii. erect and

iii. enlarged.

Fig. Fig. Image Object O F 2F
2F F

5.20 Convex lens

Activity 3

Take a convex lens and a burning Do You Know
candle.
The image which cannot be obtained on
Adjust the burning candle in front of the screen is called virtual image. It is
the convex lens and try to get a sharp always erect.
image of the candle on the wall.

Observe the image formed on the wall
and study its characteristics.

Rules of drawing ray diagrams from a concave lens

1. A ray of light parallel to the principle axis of a concave lens, appears to be diverging
from the principal focus.

2F F O F 2F

5.21

GREEN Science (Physics) Book-10 91

2. An incident ray coming to meet the principal focus of a concave lens is refracted
parallel to the principal axis.

2F F O F 2FFig. Fig.

5.22
3. A ray of light passing through the optical centre of a concave lens passes straight

without bending.

2F F O F 2F

5.23

Image formed by a concave lens

Concave lens always forms the same type of image. The image is always erect, diminished
and virtual. The image lies in between optical centre and principle focus on the same side
of the object.

a. Object at infinity

When an object lies at infinity from a concave lens, the image is formed on the same side
of the object.
The image is
i. virtual,
ii. erect and
iii. diminished.

Fig. Image O F
F

5.24 Concave lens

92 GREEN Science (Physics) Book-10

b. Object between infinity and optical centre

When an object is placed anywhere between infinity and optical centre, image is formed
between principal focus and optical centre.

The image is
i. virtual,
ii. erect and
iii. diminished.

Fig. Object F Image O F 2F
2F

5.25

Activity 4

Bring some chart papers, pencil and ruler.
Draw ray diagrams showing the images formed by a convex and concave lens when

the object is placed
– beyound 2 F,
– at 2F,
– between F and 2F,
– at F,
– between F and O.

Differences between convex lens and concave lens

Convex lens Concave lens

1. It is thick in the middle and thin at the 1. It is thin in the middle and thick at the

edges. edges.

2. It may form real and virtual image. 2. It always forms virtual image.

3. It converges rays of light falling on it. 3. It diverges the rays of light falling on
it.

4. Its power is positive. 4. Its power is negative.

Power of a lens

The reciprocal of the focal length (in meters) of a lens is called the power of the lens. It can
also be defined as the ability of a lens to converge or diverge the rays of light falling on it.

The power of a lens having less focal length is more and vice versa. The power of a convex
lens is positive and that of a concave lens is negative.

GREEN Science (Physics) Book-10 93

The power of a lens is calculated by the given formula.

P = f (in 1 Do You Know
metre)

Where, P = power of a lens If the focal length of a lens is 1m,
its power is 1 D (dioptre).

f = focal length of the lens

In SI system, the power of a lens is measured in dioptre. It is denoted by D. Dioptre (D) is
also measured in radian/m.

Solved Numerical: 1

The focal length of a lens is + 20 cm. Calculate the power of the lens. Also, write down the
type of the lens.

Solution: = + 20 m = + 0.2 m
Focal length (f) = + 20cm 100
Power (P) = ?

According to the formula,

Power (P) = 1
f

= + 1
0.2

= + 5D

\ Power of the lens (P) = + 5 D

Since the focal length of the lens is positive, the type of the lens is convex.

Solved Numerical: 2

The power of a concave lens is – 5D. Calculate the focal length of the lens.

Solution:

Power (P) = – 5D

Focal length (f) = ?

According to the formula,

P = 1
f

or, f = 1
P

or, f = 1
–5

94 GREEN Science (Physics) Book-10

or, f = – 0.2 m
\ The total length of the lens is –0.2m or –20cm.

Uses of convex lens

1. Convex lens is used in optical instruments like camera, telescope, binoculars,
microscope, etc.

2. It is also used as a magnifying glass.
3. It is used in spectacles to remove far-sightedness or hypermetropia.

Uses of concave lens

1. It is used to remove myopia or short - sightedness.
2. It is also used in optical instruments like terrestrial telescope.

Magnification

The image formed by a lens may be smaller or larger than the object depending on the
position of object and type of lens. The ratio of height of the image to the height of the
object is called magnification. It can be calculated by the given formula.

Magnification (m) = Height of the image (I)
Height of the object (O)

Symbolically,

m = I
O

Magnification has no unit since it is the ratio of two heights.

Magnification of a lens can also be defined as the ratio of image distance to the object
distance.

Magnification (m) = Image distance (v)
Object distance (u)

Symbolically,

m = v
u

From the above formula, it becomes clear that the magnification produced by a lens is
calculated by measuring .

i. Height of the image and height of the object

ii. Image distance and object distance.

GREEN Science (Physics) Book-10 95

B Convex lens F 2F A'
Object O Image
2F A
F

Fig. Fig. u v B'

5.26

In the given figure,
AB = Height of object
A' B' = Height of image
u = Object distance
v = Image distance

Methods of measuring magnification

The magnification (m) produced by a lens is the ration of image distance (I) to the object
distance (O). So magnification can be calculated by measuring image distance (I) and
object distance (O).

A B F 2F I
Object C Image
2F O Image distance
F (v) D

object
distance (u)

5.27

In the given ray diagram, AO is an object, ID is the image of AO and C is the optical centre
of the lens. Let, ‘u’ and ‘v’ be the object distance and ‘v’ be the image distance. Then from
the above diagram, we get,

Statement Reason
i. <ACO = ∠ICD i. Vertically opposite angles
ii. <AOC = ∠CID ii. Both are right angles
iii. <CAO = ∠CDI iii. Remaining angles of two triangles

\ DAOC is similar to DCDI

\ ID = OICC ( corresponding sides of similar triangles)
AO

96 GREEN Science (Physics) Book-10

\ Magnification (M) = ID = IC = v = Image distance
AO OC u Object distance

\ Magnification = Image distance (v) proved.
Object distance (u)

Solved Numerical: 3

A burning candle is placed at a distance of 12cm form a convex lens having focal length
6cm. Calculate the magnification.

Solution:

Object distance (u) = 12cm

Focal length (f) = 6cm

\ Image distance (v) = 12cm [ The candle is kept at 2F, i.e. 2 × 6cm = 12cm]

Magnification (m) = ?

According to the formula,

m = v
u

= 12
12

= 1

\ Magnification (m) = 1

Human Eye

Human eye is a spherical organ located in orbital cavity in the facial region. Each eye
consists of a biconvex lens. So it is called an optical instrument. The rays of light coming
from an object enter the eye through cornea and strike the eye lens. The lens refracts
the rays of light and forms a real, inverted and diminished image on the retina. The
photosensitive cells of retina convert the image into nerve impulses. These nerve impulses
are sent to the brain through optic nerve. The brain processes the impulses received. As a
result, we see the object.

1Fig.
2 3 and 4
3 1 and 2
4

5.28
Working mechanism of human eye

GREEN Science (Physics) Book-10 97

Accommodation of human eye

Accommodation of eye can be defined as the ability of an eye to focus the image of "the
objects at different distances" on the retina by changing its focal length. Ciliary muscles
contract and relax to change the focal length of the eye lens. When we see nearby object,
the ciliary muscles contract, the eye lens becomes thicker and the focal length decreases.
As a result, converging power of the eye lens increases and we can see nearby objects
clearly.

When we see distant object, the ciliary muscles relax, the eye lens becomes thinner and
the focal length increases. As result, converging power of the eye decreases and we can
see distant objects clearly.

Rays of light Rays of light from
from nearby distant objects

Fig. objects

5.29 Eye lens becomes thick when Eye lens becomes thin when
we see nearby objects we see distant objects

Range of normal eye

The nearest and the farthest distance that a normal eye can see clearly without any
difficulty is called range of normal eye. The range of normal eye is at a distance of 25cm
from the eye to the infinity.

The near point of a normal eye is at a distance of 25cm from the eye and the far point is
at infinity. Near point is the nearest distance that a normal eye can see clearly without
any difficulty whereas the far point is the farthest distance that a normal can see clearly
without any difficulty.

Defects of vision

A normal eye can see nearby as well as distant objects clearly. The shape of the eye
lens changes according to the distance between eye and objects to be seen. The eye lens
becomes thick in order to see the nearby objects clearly. Similarly, eye lens becomes thin
in order to see distant objects clearly. In some cases, the thickness of eye lens does not
change according to the distance between the eye and the objects to be seen. As a result,
the person cannot see nearby objects or distant objects clearly. If a person cannot see
nearby or distant objects clearly, it is called defect of vision. It is of two types.

1. Short-sightedness or myopia

2. Long-sightedness or hypermetropia

1. Short - sightedness Do You Know

In this defect of vision, a person can see nearby The far point of the eye suffering
objects clearly but cannot see distant objects. The from myopia is less than infinity.
main causes of short-sightedness are given below:

98 GREEN Science (Physics) Book-10

i. Thickening of eye lens or shortening the focal length of eye lens

ii. Elongation of eye balls

In myopic eye, the image of the distant objects forms in front of retina due to high
converging power of the eye lens. The main cause of this defect is thickening of eye lens.
The thick lens has high converging power and less focal length.

The ray diagram of the eye suffering from myopia is given below:

Fig. Fig. Rays of light from Image
distant objects

5.30 Retina

An eye suffering from short-sightedness

Short-sightedness can be removed by using a concave lens of suitable thickness (focal
length) in the spectacles. The concave lens diverges the rays of light coming from distant
objects and these rays are converged by the eye lens. As a result, image is formed at retina
and the person can see distant objects clearly due to combined action of those lenses.

The ray diagram showing the correction of short-sightedness is given below:

Concave lens

5.31 Image on
retina

Remedy of short-sightedness

2. Long-sightedness

In this defect of vision, a person can see distant objects clearly but cannot see nearby
objects. In this defect, the image of the nearby objects is formed beyond the retina due
to low converging power of eye lens. As a result, the person cannot see nearby objects
clearly.

The main cause of long-sightedness or far-sightedness are given below:

i. Shortening of eye ball or contraction of eye ball Do You Know

ii. Increase in the focal length of eye lens or decrease For an eye suffering from long-
in thickness of the eye lens sightedness, the near point is
more than 25cm.
The ray diagram showing long-sightedness or
hypermetropia is given below.

GREEN Science (Physics) Book-10 99

Fig. Rays of light from Image beyond
Fig. nearby objects the retina

5.32

An eye suffering from long-sightedness

Long - sightedness can be removed by using a convex lens of suitable power in the
spectacles. The rays of light coming from nearby objects are converged first by the convex
lens in spectacles and then by the eye lens. As a result, image of the nearby object is
formed at retina and the person can see those objects clearly.

The ray diagram showing the correction of long-sightedness is given below:

Convex lens

Image at retina

5.33
Correction of long-sightedness

Differences between Myopia and Hypermetropia

Myopia or short-sightedness Hypermetropia or long-sightedness

1. In myopia, a person can see nearby 1. In hypermetropia, a person can see

objects but cannot see distant objects distant objects but cannot see nearby

clearly. objects clearly.

2. It can be corrected by using concave 2. It can be corrected by using convex

lens in spectacles. lens in spectacles.

3. It is caused due to elongation of eye 3. It is caused due to shortening of eye
ball. ball.

Key Concepts

1. Light is a form of energy which makes things visible. It is produced from extremely
hot objects. The objects which emit light are called the sources of light. The sun is
the main source of light on the earth.

2. A piece of a transparent glass or a plastic having refracting surfaces is called lens.

3. Lens is used in various optical instruments like camera, microscope, telescope, etc.

4. A lens consists of two refracting surfaces one of which may be plane or spherical.

5. A convex lens is the lens which is thick in the middle and thin at the edges. A
convex lens converges parallel rays of light at a point after refraction. So, it is also
called a converging lens.

100 GREEN Science (Physics) Book-10