ICSE Selina Solutions for Class 10

Science (Physics)

Chapter 7 – Sound

1. What are mechanical waves?

Ans: The mechanical wave can be defined as a wave which is not capable of
transmitting its energy through the vacuum. To transport the energy from one
location to another the mechanical waves require a medium. The example of a
mechanical wave is a sound wave.

2. Define the following terms in relation to a wave.

(a) Amplitude

Ans: When the maximum displacement is moved by a point on a body that is

vibrating it is called the amplitude. It is almost equal to one-half to the length of the
path vibration.

(b) Frequency

Ans: The frequency is the number of waves that pass with a fixed point in the
specific time. It can also be defined as the number of vibrations during one unit of
time.

(c) Wavelength

Ans: It is the distance between the two successive troughs or crests of a wave. It can
be measured in the wave direction. The wavelength and frequency is inversely
proportional to each other.

(d) Wave velocity

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Ans: The distance travelled by a periodic motion per unit time in any direction is
called the wave velocity. The wave velocity is also known as the speed.

3. A wave passes from one medium to another medium. Mention one property
of the wave out of speed, frequency or wavelength,

i. Which changes

Ans: The wavelength or the speed of the wave changes when a wave passes from
one medium to another medium.

ii. Which does not change

Ans: There will be not any change in the frequency of the wave when a wave
travelling in one medium passes to another medium.

4. State two factors on which the speed of a wave travelling in a medium

depends.

Ans: The factors on which the speed of sound depends are the density of the medium
through which it is travelling and the elasticity. In liquid the sound travels faster as
compared to the gases and in solids it travels as compared to the liquids.

5. State two differences between the light and sound waves.

Ans: The difference between the light and sound waves are as following:

S. No. Light waves S. No. Sound waves

The light waves are The sound waves are the
1 1
electromagnetic waves mechanical waves.
They require a material medium
They can travel in the
2 2 for propagation. They cannot
vacuum.
travel in the vacuum.

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6. What do you mean by reflection of sound? State one condition for the
reflection of a sound wave. Name a device in which reflection of sound waves is
used.

Ans: When the sound strikes the surface of the other medium and bounces back in
some different direction when it travels in a given medium this is called the reflection
of sound. The condition is that the reflecting surface size should be much bigger than
that of the wavelength of the sound wave. The device in which reflection of sound
waves is used is the megaphone.

7. What is meant by an echo? State two conditions necessary for an echo to be

heard distinctly.

Ans: The echo is a sound that is caused by the reflection of the sound waves from a
surface back to the listener.

The two conditions for an echo to be heard distinctly are given as follows.

(a) There should be a minimum 17m distance between the source of sound and the
reflector in air.

(b) The size of the reflector should be large enough as compared to the wavelength
of the sound wave.

8. A man is standing at a distance of 12m from a cliff. Will he be able to hear a

clear echo of his sound? Give a reason for your answer.

Ans: The time taken to hear the echo will be calculated as follows:

t = 2d / v

(2  12)

340

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 24

340

=0.07 < 0.1 seconds

The man will not be able to hear the echo as the sensation of a sound persists in our
ears for about 0.1 second after the exciting stimulus ceases to act upon it.

9. State two applications of echo.

Ans: The applications of echo are as follows:

(a) In the medical field, doctors use this technique in the cardiograph, sonogram and
many other medical diagnoses.

(b) The dolphins use the technique of echolocation to locate their ways as they
cannot see from their eyes. Through the reflected sound they can understand if there
is any object in between. Therefore by using this technique it hunts its prey.

10. Explain how the speed of sound can be determined by the method of echo?

Ans: To hear an echo the sound has to travel from the source of the sound to the
reflector and back from the reflector to the source of the sound.

If d is the distance between the source and the reflection and t is the time taken, then
the total distance travelled is 2d.

Therefore the speed of sound can be calculated by using the relation given as
follows:

Total distance travelled [2d ]

Speed [V] =
Time taken [t ]

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11. State the use of echo by a bat, dolphin and fisherman.

Ans: The bats, dolphins and fishermen use the principle of echo for locating the
obstacles and prey. They produce the ultrasonic waves and send in all directions.
When these waves are reflected back from the obstacles they are able to hear the
echo.

12. How do bats avoid obstacles in their way, when in flight?

Ans: The bat has the ability to generate and detect sound waves of very high
frequency. If any obstacle is encountered in the path of the bat then the sound waves
generated by the flying bats get reflected from any obstacle residing in front of it.
Also the hearing ability of the bats is even stronger than any other animal in that they
come to know the positions of obstacles just by hearing the echoes. They are able to
produce and detect the sound of very high frequency up to about 100 kHz. Therefore,
without colliding with the obstacles, bats can fly safely.

13. What is meant by sound ranging? Give one use of sound ranging.

Ans: The process of detecting obstacles with the help of an echo is called the sound
ranging. Animals such as bats, dolphin use the sound ranging to detect their enemies.

14. Name the waves used for sound ranging. State one reason for their use. Why
are the waves mentioned by you not audible to us?

Ans: The waves used for sound ranging is the ultrasonic sound waves. These
ultrasonic waves have a frequency more than 20,000 Hz. The range of audibility of
the human ear is 20 Hz to 20,000 Hz therefore these are not audible.

15. What is ‘SONAR’? State the principle on which it is based.

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Ans: The SONAR is the abbreviation of Sound Navigation and Ranging, it is a
system for detecting the objects under water by emitting the sound pulses and
measuring their return after being reflected. The SONAR is based on the principle
of echo. Sonar is helpful for exploring and mapping the ocean because sound waves
travel farther in the water than that of the radar and light waves do.

16. State the use of echo in the medical field.

Ans: In the medical field, the echo method of ultrasonic waves is used for imaging
the human organs such as the liver, gallbladder, uterus, womb, etc. This is called
ultrasonography. In addition to this to obtain the image of the human heart the
echocardiography is used.

17. The minimum distance between the source and the reflector in air, so that
an echo is heard is approximately.

(a) 10 m

(b) 17 m

(c) 34 m

(d) 50 m

Ans: The minimum distance between the source and the reflector in air, so that an
echo is heard is approximately 17m. Hence the correct answer is option b.

18. To detect the obstacles in their path, bats produce.

(a) Infrasonic waves

(b) Ultrasonic waves

(c) Electromagnetic waves

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