Selina Textbook Solutions: Sound | Physics Class 10 ICSE PDF Download

 Page 1


Class X Chapter – 7 Sound  Physics 
_____________________________________________________________________________ 
 
EXERCISE. 7 (A) 
 
Question 1:  
Define following terms in relation to a wave:  
(a) amplitude  (b) frequency    (c) wavelength and  (d) wave velocity  
Solution 1: 
(a) Amplitude: The maximum displacement of the particle of medium on either side of its 
mean position is called the amplitude of wave. Its S.I. unit is metre (m). 
(b) Frequency: The number of vibrations made by a particle of the medium in one second 
is called the frequency of the waves.  
It is also defined as the number of waves passing through a point in one second. Its S.I. 
unit is hertz (Hz). 
(c) Wavelength: The distance travelled by the wave in one time period of vibration of 
particle of medium is called its wavelength. Its S.I. unit is metre (m). 
  
(d) Wave velocity: The distance travelled by a wave in one second is called its wave 
velocity. Its S.I. unit is metre per second (ms
-1
). 
 
 
Question 2: 
A wave passes from one medium to another medium. Mention the property of the wave  
(i) which changes,  (ii) which does not change. 
Solution 2: 
(i) Wavelength (or speed) of the wave changes, when it passes from one medium to another 
medium. 
(ii) Frequency of a wave does not change when it passes from one medium to another medium. 
 
 
Question 3: 
State two factors on which the speed of a wave travelling in a medium depends. 
Solution 3: 
Two factors on which the speed of a wave travelling in a medium depends are: 
(i) Density: The speed of sound is inversely proportional to the square root of density of the gas. 
(ii) Temperature: The speed of sound increases with the increase in temperature. 
 
 
Page 2


Class X Chapter – 7 Sound  Physics 
_____________________________________________________________________________ 
 
EXERCISE. 7 (A) 
 
Question 1:  
Define following terms in relation to a wave:  
(a) amplitude  (b) frequency    (c) wavelength and  (d) wave velocity  
Solution 1: 
(a) Amplitude: The maximum displacement of the particle of medium on either side of its 
mean position is called the amplitude of wave. Its S.I. unit is metre (m). 
(b) Frequency: The number of vibrations made by a particle of the medium in one second 
is called the frequency of the waves.  
It is also defined as the number of waves passing through a point in one second. Its S.I. 
unit is hertz (Hz). 
(c) Wavelength: The distance travelled by the wave in one time period of vibration of 
particle of medium is called its wavelength. Its S.I. unit is metre (m). 
  
(d) Wave velocity: The distance travelled by a wave in one second is called its wave 
velocity. Its S.I. unit is metre per second (ms
-1
). 
 
 
Question 2: 
A wave passes from one medium to another medium. Mention the property of the wave  
(i) which changes,  (ii) which does not change. 
Solution 2: 
(i) Wavelength (or speed) of the wave changes, when it passes from one medium to another 
medium. 
(ii) Frequency of a wave does not change when it passes from one medium to another medium. 
 
 
Question 3: 
State two factors on which the speed of a wave travelling in a medium depends. 
Solution 3: 
Two factors on which the speed of a wave travelling in a medium depends are: 
(i) Density: The speed of sound is inversely proportional to the square root of density of the gas. 
(ii) Temperature: The speed of sound increases with the increase in temperature. 
 
 
Class X Chapter – 7 Sound  Physics 
_____________________________________________________________________________ 
 
Question 4: 
State two differences between the light and sound waves. 
Solution 4: 
(i) The light waves can travel in vacuum while sound waves need a material medium for 
propagation. 
(ii) The light waves are electromagnetic waves while sound waves are the mechanical waves. 
 
 
Question 5: 
What is meant by an echo? What is the condition necessary for an echo to be heard distinctly? 
Solution 5: 
If a person stands at some distance from a wall or a hillside and produces a sharp sound, he hears 
two distinct sounds: one is original sound heard almost instantaneously and the other one is 
heard after reflection from the wall or hillside, which is called echo. 
 The condition for the echo: An echo is heard only if the distance between the person producing 
the sound and the rigid obstacle is long enough to allow the reflected sound to reach the person 
at least 0.1 second after the original sound is heard. 
 
 
Question 6: 
A man is standing at a distance of 12 m from a cliff. Will he be able to hear a clear echo? Give 
a reason for your answer. 
Solution 6: 
t = 2d/V = 2 × 12/340 = 24/340 < 0.1 seconds so the man will not be able to hear the echo. This 
is because the sensation of sound persists in our ears for about 0.1 second after the exciting 
stimulus ceases to act. 
 
 
Question 7: 
State two applications of echo. 
Solution 7: 
The applications of echo: 
(i) Dolphins detect their enemy and obstacles by emitting the ultrasonic waves and hearing 
their echo.             
(ii) In medical science, the echo method of ultrasonic waves is used for imaging the human 
organs such as the liver, gall bladder, uterus, womb etc. This is called ultrasonography. 
Page 3


Class X Chapter – 7 Sound  Physics 
_____________________________________________________________________________ 
 
EXERCISE. 7 (A) 
 
Question 1:  
Define following terms in relation to a wave:  
(a) amplitude  (b) frequency    (c) wavelength and  (d) wave velocity  
Solution 1: 
(a) Amplitude: The maximum displacement of the particle of medium on either side of its 
mean position is called the amplitude of wave. Its S.I. unit is metre (m). 
(b) Frequency: The number of vibrations made by a particle of the medium in one second 
is called the frequency of the waves.  
It is also defined as the number of waves passing through a point in one second. Its S.I. 
unit is hertz (Hz). 
(c) Wavelength: The distance travelled by the wave in one time period of vibration of 
particle of medium is called its wavelength. Its S.I. unit is metre (m). 
  
(d) Wave velocity: The distance travelled by a wave in one second is called its wave 
velocity. Its S.I. unit is metre per second (ms
-1
). 
 
 
Question 2: 
A wave passes from one medium to another medium. Mention the property of the wave  
(i) which changes,  (ii) which does not change. 
Solution 2: 
(i) Wavelength (or speed) of the wave changes, when it passes from one medium to another 
medium. 
(ii) Frequency of a wave does not change when it passes from one medium to another medium. 
 
 
Question 3: 
State two factors on which the speed of a wave travelling in a medium depends. 
Solution 3: 
Two factors on which the speed of a wave travelling in a medium depends are: 
(i) Density: The speed of sound is inversely proportional to the square root of density of the gas. 
(ii) Temperature: The speed of sound increases with the increase in temperature. 
 
 
Class X Chapter – 7 Sound  Physics 
_____________________________________________________________________________ 
 
Question 4: 
State two differences between the light and sound waves. 
Solution 4: 
(i) The light waves can travel in vacuum while sound waves need a material medium for 
propagation. 
(ii) The light waves are electromagnetic waves while sound waves are the mechanical waves. 
 
 
Question 5: 
What is meant by an echo? What is the condition necessary for an echo to be heard distinctly? 
Solution 5: 
If a person stands at some distance from a wall or a hillside and produces a sharp sound, he hears 
two distinct sounds: one is original sound heard almost instantaneously and the other one is 
heard after reflection from the wall or hillside, which is called echo. 
 The condition for the echo: An echo is heard only if the distance between the person producing 
the sound and the rigid obstacle is long enough to allow the reflected sound to reach the person 
at least 0.1 second after the original sound is heard. 
 
 
Question 6: 
A man is standing at a distance of 12 m from a cliff. Will he be able to hear a clear echo? Give 
a reason for your answer. 
Solution 6: 
t = 2d/V = 2 × 12/340 = 24/340 < 0.1 seconds so the man will not be able to hear the echo. This 
is because the sensation of sound persists in our ears for about 0.1 second after the exciting 
stimulus ceases to act. 
 
 
Question 7: 
State two applications of echo. 
Solution 7: 
The applications of echo: 
(i) Dolphins detect their enemy and obstacles by emitting the ultrasonic waves and hearing 
their echo.             
(ii) In medical science, the echo method of ultrasonic waves is used for imaging the human 
organs such as the liver, gall bladder, uterus, womb etc. This is called ultrasonography. 
Class X Chapter – 7 Sound  Physics 
_____________________________________________________________________________ 
 
 
Question 8: 
Explain how the speed of sound can be determined by the method of echo. 
Solution 8: 
Sound is produced from a place at a known distance say, d at least 50 m from the reflecting 
surface. The time interval t in which the echo reaches the place from where the sound was 
produced, is noted by a stop watch having the least count 0.01 s. then the speed of sound is 
calculated by using the following relation 
V = total distance travelled / time interval = 
2 ?? ?? m/s 
 
 
Question 9: 
State the use of echo by a bat, dolphin and fisherman. 
Solution 9: 
Bats, dolphin and fisherman detect their enemies or obstacles or position of fish by 
emitting/sending the ultrasonic waves and hearing/detecting the echo. 
 
 
Question 10: 
How do bats avoid obstacles in their way, when in flight? 
Solution 10: 
Bats can produce and detect the sound of very high frequency up to about 1000kHz. The sounds 
produced by flying bats get reflected back from any obstacle in front of it. By hearing the echoes, 
bats come to know even in the dark where the obstacles are. So they can fly safely without 
colliding with the obstacles. 
 
 
Question 11: 
What is meant by sound ranging? Give one use of sound ranging. 
Solution 11: 
The process of detecting obstacles with the help of echo is called sound ranging. It's used by the 
animals like bats, dolphin to detect their enemies. 
 
 
 
Page 4


Class X Chapter – 7 Sound  Physics 
_____________________________________________________________________________ 
 
EXERCISE. 7 (A) 
 
Question 1:  
Define following terms in relation to a wave:  
(a) amplitude  (b) frequency    (c) wavelength and  (d) wave velocity  
Solution 1: 
(a) Amplitude: The maximum displacement of the particle of medium on either side of its 
mean position is called the amplitude of wave. Its S.I. unit is metre (m). 
(b) Frequency: The number of vibrations made by a particle of the medium in one second 
is called the frequency of the waves.  
It is also defined as the number of waves passing through a point in one second. Its S.I. 
unit is hertz (Hz). 
(c) Wavelength: The distance travelled by the wave in one time period of vibration of 
particle of medium is called its wavelength. Its S.I. unit is metre (m). 
  
(d) Wave velocity: The distance travelled by a wave in one second is called its wave 
velocity. Its S.I. unit is metre per second (ms
-1
). 
 
 
Question 2: 
A wave passes from one medium to another medium. Mention the property of the wave  
(i) which changes,  (ii) which does not change. 
Solution 2: 
(i) Wavelength (or speed) of the wave changes, when it passes from one medium to another 
medium. 
(ii) Frequency of a wave does not change when it passes from one medium to another medium. 
 
 
Question 3: 
State two factors on which the speed of a wave travelling in a medium depends. 
Solution 3: 
Two factors on which the speed of a wave travelling in a medium depends are: 
(i) Density: The speed of sound is inversely proportional to the square root of density of the gas. 
(ii) Temperature: The speed of sound increases with the increase in temperature. 
 
 
Class X Chapter – 7 Sound  Physics 
_____________________________________________________________________________ 
 
Question 4: 
State two differences between the light and sound waves. 
Solution 4: 
(i) The light waves can travel in vacuum while sound waves need a material medium for 
propagation. 
(ii) The light waves are electromagnetic waves while sound waves are the mechanical waves. 
 
 
Question 5: 
What is meant by an echo? What is the condition necessary for an echo to be heard distinctly? 
Solution 5: 
If a person stands at some distance from a wall or a hillside and produces a sharp sound, he hears 
two distinct sounds: one is original sound heard almost instantaneously and the other one is 
heard after reflection from the wall or hillside, which is called echo. 
 The condition for the echo: An echo is heard only if the distance between the person producing 
the sound and the rigid obstacle is long enough to allow the reflected sound to reach the person 
at least 0.1 second after the original sound is heard. 
 
 
Question 6: 
A man is standing at a distance of 12 m from a cliff. Will he be able to hear a clear echo? Give 
a reason for your answer. 
Solution 6: 
t = 2d/V = 2 × 12/340 = 24/340 < 0.1 seconds so the man will not be able to hear the echo. This 
is because the sensation of sound persists in our ears for about 0.1 second after the exciting 
stimulus ceases to act. 
 
 
Question 7: 
State two applications of echo. 
Solution 7: 
The applications of echo: 
(i) Dolphins detect their enemy and obstacles by emitting the ultrasonic waves and hearing 
their echo.             
(ii) In medical science, the echo method of ultrasonic waves is used for imaging the human 
organs such as the liver, gall bladder, uterus, womb etc. This is called ultrasonography. 
Class X Chapter – 7 Sound  Physics 
_____________________________________________________________________________ 
 
 
Question 8: 
Explain how the speed of sound can be determined by the method of echo. 
Solution 8: 
Sound is produced from a place at a known distance say, d at least 50 m from the reflecting 
surface. The time interval t in which the echo reaches the place from where the sound was 
produced, is noted by a stop watch having the least count 0.01 s. then the speed of sound is 
calculated by using the following relation 
V = total distance travelled / time interval = 
2 ?? ?? m/s 
 
 
Question 9: 
State the use of echo by a bat, dolphin and fisherman. 
Solution 9: 
Bats, dolphin and fisherman detect their enemies or obstacles or position of fish by 
emitting/sending the ultrasonic waves and hearing/detecting the echo. 
 
 
Question 10: 
How do bats avoid obstacles in their way, when in flight? 
Solution 10: 
Bats can produce and detect the sound of very high frequency up to about 1000kHz. The sounds 
produced by flying bats get reflected back from any obstacle in front of it. By hearing the echoes, 
bats come to know even in the dark where the obstacles are. So they can fly safely without 
colliding with the obstacles. 
 
 
Question 11: 
What is meant by sound ranging? Give one use of sound ranging. 
Solution 11: 
The process of detecting obstacles with the help of echo is called sound ranging. It's used by the 
animals like bats, dolphin to detect their enemies. 
 
 
 
Class X Chapter – 7 Sound  Physics 
_____________________________________________________________________________ 
 
Question 12: 
Name the waves used for sound ranging. Why are the waves mentioned by you audible to us? 
Solution 12: 
The ultrasonic waves are used for the sound ranging. Ultrasonic waves have a frequency more 
than 20,000 Hz but the range of audibility of human ear is 20Hz to 20,000 Hz 
 
 
Question 13: 
What is sonar? State the principle on which it is based. 
Solution 13: 
Sonar is sound navigation and ranging. Ultrasonic waves are sent in all directions from the 
ship and they are received on their return after reflection from the obstacles. They use the 
method of echo. 
 
 
Question 14: 
Name the waves which are used in sonar to find the depth of a sea. Give one reason for their 
use. 
Solution 14: 
Ultrasonic waves are used in the sonar to find the depth of sea because of these waves 
travel undeviated through long distances. 
 
 
Question 15: 
State the use of echo in medical science. 
Solution 15: 
In medical science, echo method of ultrasonic waves is used for the imaging of human organs 
such as liver, gall bladder, uterus, womb; which is called ultrasonography. 
 
 
MUTIPLE CHOICE TYPE: 
 
Question 1: 
The minimum distance between the source and the reflector in air. So that an echo is heard is 
approximately equal to: 
Page 5


Class X Chapter – 7 Sound  Physics 
_____________________________________________________________________________ 
 
EXERCISE. 7 (A) 
 
Question 1:  
Define following terms in relation to a wave:  
(a) amplitude  (b) frequency    (c) wavelength and  (d) wave velocity  
Solution 1: 
(a) Amplitude: The maximum displacement of the particle of medium on either side of its 
mean position is called the amplitude of wave. Its S.I. unit is metre (m). 
(b) Frequency: The number of vibrations made by a particle of the medium in one second 
is called the frequency of the waves.  
It is also defined as the number of waves passing through a point in one second. Its S.I. 
unit is hertz (Hz). 
(c) Wavelength: The distance travelled by the wave in one time period of vibration of 
particle of medium is called its wavelength. Its S.I. unit is metre (m). 
  
(d) Wave velocity: The distance travelled by a wave in one second is called its wave 
velocity. Its S.I. unit is metre per second (ms
-1
). 
 
 
Question 2: 
A wave passes from one medium to another medium. Mention the property of the wave  
(i) which changes,  (ii) which does not change. 
Solution 2: 
(i) Wavelength (or speed) of the wave changes, when it passes from one medium to another 
medium. 
(ii) Frequency of a wave does not change when it passes from one medium to another medium. 
 
 
Question 3: 
State two factors on which the speed of a wave travelling in a medium depends. 
Solution 3: 
Two factors on which the speed of a wave travelling in a medium depends are: 
(i) Density: The speed of sound is inversely proportional to the square root of density of the gas. 
(ii) Temperature: The speed of sound increases with the increase in temperature. 
 
 
Class X Chapter – 7 Sound  Physics 
_____________________________________________________________________________ 
 
Question 4: 
State two differences between the light and sound waves. 
Solution 4: 
(i) The light waves can travel in vacuum while sound waves need a material medium for 
propagation. 
(ii) The light waves are electromagnetic waves while sound waves are the mechanical waves. 
 
 
Question 5: 
What is meant by an echo? What is the condition necessary for an echo to be heard distinctly? 
Solution 5: 
If a person stands at some distance from a wall or a hillside and produces a sharp sound, he hears 
two distinct sounds: one is original sound heard almost instantaneously and the other one is 
heard after reflection from the wall or hillside, which is called echo. 
 The condition for the echo: An echo is heard only if the distance between the person producing 
the sound and the rigid obstacle is long enough to allow the reflected sound to reach the person 
at least 0.1 second after the original sound is heard. 
 
 
Question 6: 
A man is standing at a distance of 12 m from a cliff. Will he be able to hear a clear echo? Give 
a reason for your answer. 
Solution 6: 
t = 2d/V = 2 × 12/340 = 24/340 < 0.1 seconds so the man will not be able to hear the echo. This 
is because the sensation of sound persists in our ears for about 0.1 second after the exciting 
stimulus ceases to act. 
 
 
Question 7: 
State two applications of echo. 
Solution 7: 
The applications of echo: 
(i) Dolphins detect their enemy and obstacles by emitting the ultrasonic waves and hearing 
their echo.             
(ii) In medical science, the echo method of ultrasonic waves is used for imaging the human 
organs such as the liver, gall bladder, uterus, womb etc. This is called ultrasonography. 
Class X Chapter – 7 Sound  Physics 
_____________________________________________________________________________ 
 
 
Question 8: 
Explain how the speed of sound can be determined by the method of echo. 
Solution 8: 
Sound is produced from a place at a known distance say, d at least 50 m from the reflecting 
surface. The time interval t in which the echo reaches the place from where the sound was 
produced, is noted by a stop watch having the least count 0.01 s. then the speed of sound is 
calculated by using the following relation 
V = total distance travelled / time interval = 
2 ?? ?? m/s 
 
 
Question 9: 
State the use of echo by a bat, dolphin and fisherman. 
Solution 9: 
Bats, dolphin and fisherman detect their enemies or obstacles or position of fish by 
emitting/sending the ultrasonic waves and hearing/detecting the echo. 
 
 
Question 10: 
How do bats avoid obstacles in their way, when in flight? 
Solution 10: 
Bats can produce and detect the sound of very high frequency up to about 1000kHz. The sounds 
produced by flying bats get reflected back from any obstacle in front of it. By hearing the echoes, 
bats come to know even in the dark where the obstacles are. So they can fly safely without 
colliding with the obstacles. 
 
 
Question 11: 
What is meant by sound ranging? Give one use of sound ranging. 
Solution 11: 
The process of detecting obstacles with the help of echo is called sound ranging. It's used by the 
animals like bats, dolphin to detect their enemies. 
 
 
 
Class X Chapter – 7 Sound  Physics 
_____________________________________________________________________________ 
 
Question 12: 
Name the waves used for sound ranging. Why are the waves mentioned by you audible to us? 
Solution 12: 
The ultrasonic waves are used for the sound ranging. Ultrasonic waves have a frequency more 
than 20,000 Hz but the range of audibility of human ear is 20Hz to 20,000 Hz 
 
 
Question 13: 
What is sonar? State the principle on which it is based. 
Solution 13: 
Sonar is sound navigation and ranging. Ultrasonic waves are sent in all directions from the 
ship and they are received on their return after reflection from the obstacles. They use the 
method of echo. 
 
 
Question 14: 
Name the waves which are used in sonar to find the depth of a sea. Give one reason for their 
use. 
Solution 14: 
Ultrasonic waves are used in the sonar to find the depth of sea because of these waves 
travel undeviated through long distances. 
 
 
Question 15: 
State the use of echo in medical science. 
Solution 15: 
In medical science, echo method of ultrasonic waves is used for the imaging of human organs 
such as liver, gall bladder, uterus, womb; which is called ultrasonography. 
 
 
MUTIPLE CHOICE TYPE: 
 
Question 1: 
The minimum distance between the source and the reflector in air. So that an echo is heard is 
approximately equal to: 
Class X Chapter – 7 Sound  Physics 
_____________________________________________________________________________ 
 
(a) 10 m          (b) 17 m          (c) 34 m           (d) 50 m  
Solution 1: 
17 m 
Explanation: An echo is heard distinctly if it reaches the ear at least 0.1 s after the original sound. 
If d is the distance between the observer and the obstacle and V is the speed of sound, then the 
total distance travelled by the sound to reach the obstacle and then to come back is 2d and the 
time taken is, 
t = Total distance travelled/Speed of sound = 2d/V 
or, d = V t/2 
Putting t = 0.1 s and V = 340 m/s in air at ordinary temperature, we get: 
d = (340 x 0.1)/2 = 17 m 
Thus, to hear an echo distinctly, the minimum distance between the source and the reflector in 
air is 17 m. 
 
 
Question 2: 
To detect the obstacles in their path, bats produce: 
(a) infrasonic waves             (b) ultrasonic waves 
(c) electromagnetic waves   (d) radio waves 
Solution 2: 
Ultrasonic waves 
 
 
NUMERICALS: 
 
Question 1: 
The wavelength of waves produces on the surface of water is 20 cm. If the wave velocity is 24 
m s
-1
, calculate: (i) the number of waves produces in one second , and (ii) the time in which one 
wave is produced. 
Solution 1: 
(i)Frequency or the number of waves produced per second 
= Velocity/Wavelength 
= 24 / 20 × 10
-2
 
=120 
(ii)Time = 1/ frequency = 1/ 120= 8.3 × 10
-3
 seconds