# Frank Solutions for Chapter 3.1 Characteristics of Wave Motion and Echoes Class 10 Physics ICSE

## Exercises

1. Distinguish between longitudinal and transverse waves.
 Longitudinal waves Transverse waves 1. As it travels through a medium, the particles of the medium vibrate to and fro about their mean positions along the direction of propagation of wave.2. As the wave propagates through the medium, it causes compressions and rarefactions.3. In case of longitudinal waves, one wavelength contains one compression and one rarefaction.4. They can travel through all media i.e. solids, liquids and gases.5. As the longitudinal wave propagates through a medium, there is change in density of the medium. 1. As it travels through a medium, the particles of the medium vibrate perpendicular to the direction of propagation of wave.2. As the wave propagates through the medium, it produces crests and troughs.3. In case of transverse waves, one wavelength contains one crest and one trough.4. They can travel only through solids and on the surface of liquids.5. As the transverse wave travels through a medium, there is no change in density of the medium.

2. What do you understand by amplitude of an oscillation?
The amplitude of an oscillation is the height of a crest or the depth of a trough measured from the mean position. The SI unit of amplitude is metre.

3. Write down the relation between wavelength, frequency and wave-velocity.
Wave-velocity = wavelength × frequency

4. State two factors on which the velocity of a wave depends.
The velocity of a wave in a medium depends on the elasticity and density of the medium.

5. How does the velocity of a wave vary with temperature?
The velocity of a wave increases with the increase in temperature.

6. The speed of a wave is 350 ms-1. Find the wavelength of the wave whose frequency is 700 Hz.
v = Î»f
∴ Î» = v/f = 350/700 = 0.5 m

8. Distinguish between crest and trough.
A transverse wave propagates by means of crests and troughs.
The high points of the waves are called crests or peaks and the low points of the waves are called troughs.

9. What do you mean by wave-velocity?
The velocity with which a wave travels in a medium is referred to as wave-velocity.

10. What is the nature of sound waves?
Sound waves are mechanical in nature.

11. Which has a greater speed: mechanical wave or electromagnetic wave?
Electromagnetic waves have greater speed.

12. Can sound waves pass through vacuum?
Sound waves being mechanical waves need a material medium for their propagation, they cannot travel in vacuum.

13. What are characteristics of a wave motion?
Characteristics of wave motion:
1. Wave motion can be produced only in a medium having elasticity and inertia.
2. When energy is given to any part of a medium, disturbance is produced in it by repeated periodic motion of the particles about their equilibrium positions.
3. During wave motion, no matter is transferred. It is only the energy that gets transferred.
4. The velocity of the wave relative to the medium depends only on the nature of the medium and not on the nature or motion of source of disturbance.
5. The velocity of the wave is different from the velocity with which the particles of the medium vibrate about their equilibrium positions.
6. Energy gets transferred from one particle of a medium to the next particle in a fixed interval of time.
7. The energy associated with the wave is the kinetic and potential energy of the matter.

14. Is energy transferred during wave motion?
Yes, energy is transferred during wave motion.

15. Distinguish between mechanical and electromagnetic waves.
 Electromagnetic waves Mechanical waves 1. They can travel in vacuum.2. These are formed by the periodic vibrations of mutually perpendicular electric and magnetic field in a plane normal to the direction of wave propagation.3. These are traverse in nature. 1. They cannot travel in vacuum. They need a material medium for their propagation.2. These are formed by the vibrations of the medium particles about their mean positions. These vibrations can be along the direction of propagation of wave or perpendicular to it.3. These waves can be transverse as well as longitudinal.

16. Can a transverse wave travel in air?
No, transverse waves cannot travel in air. They can only travel in media which posses rigidity.

17. Define wavelength. Give its SI units.
The wavelength is the distance between two successive crests or two successive troughs on a transverse wave.
It is also equal to the distance between any two points where the particles are passing through their respective mean positions in the same direction.
It is also the distance between two successive compressions or two successive rarefactions on a longitudinal wave.
The SI unit of wavelength is metre (m).

18. What is the range of hearing for a normal person?
Range of hearing of a normal person is 20 Hz to 20,000 Hz which is called the range of audibility.

19. Define an echo.
The sound heard after reflection from a rigid obstacle (such as cliff, a hillside, a wall of a building, edge of a forest etc.), is called an echo.

20. What is the full form of SONAR?
Full form of SONAR is 'sound navigation and ranging'.

21. What is the full form of RADAR?
Full form of RADAR is 'radio detection and ranging'.

22. Name the principle used to find the depth of a sea.
The depth of a sea can be found by the process of 'echo depth sounding'. This process is based on the principle of 'echo formation'.

23. What are ultrasonic?
The sound wave of frequency higher than 20,000 Hz is called ultrasonic.

24. What are infrasonics?
The sound wave of frequency below 20 Hz is called infrasonic.

25. Give two applications of supersonics.
Applications of supersonics:
1. Jet aircrafts
2. Rockets

26.What is the difference between echo and reverberation?
The sound heard after reflection from a rigid obstacle is called an echo. To hear the echo of a sound distinctly, the reflecting surface in air should be at a minimum distance of 17 m from the listener.
If the distance is less than 17 m, the reflected sound will reach the ears before the original sound dies out. In such a case, the original sound mixes up with the reflected sound. Due to repeated reflections at the reflecting surface, the sound gets prolonged. This effect is known as reverberation.

27. What are the conditions necessary for the formation of echo?
Conditions necessary for echo formation are:
1. The minimum distance between the source of sound and its reflector should be 17 m.
2. Reflected sound should reach the person atleast 0.1 second after the original sound is heard.

28. What are the lowest and highest limits of frequencies of sound which is audible to a human ear?
The lowest frequency audible to human ear is 20 Hz and the highest frequency audible to human ear is 20,000 Hz.

29. Can sound waves be reflected?
Yes, sound waves can be reflected.

30. Give a simple experiment to demonstrate the reflection of sound waves.

Experiment to demonstrate the reflection of sound:
1. Take two identical pipes, as shown in Fig. You can make the pipes using chart paper. The length of the pipes should be sufficiently long as shown.
2. Arrange them on a table near a wall.
3. Keep a clock near the open end of one of the pipes and try to hear the sound of the clock through the other pipe.
4. Adjust the position of the pipes so that you can best hear the sound of the clock.

Conclusion: Sound waves pass down the first tube and are reflected from the smooth surface of the wall. After reflection, they enter the second tube and are received by the ear at the other end. If you measure the angle of incidence and angle of reflection, you will notice that they are equal.

31. How will you find the velocity of sound waves?
A simple method for finding the velocity of sound is echo or open air method.
For example: A person stands at a known distance (d m) from a cliff and fires a pistol and simultaneously starts the stop watch. He stops the stop-watch as soon as he hears the echo. The distance traveled by sound during this time (t seconds) is twice the distance (2d m). The velocity (v) of sound is then calculated as under:
Velocity of sound = distance travelled/time taken = 2d/t
By repeating the experiment two or three times, the average velocity of sound can be calculated.

32. Explain how the principle of echo is used by
(i) The bat during its flight at
(ii) The dolphin to locate small fish as its prey.
(i) Bats can produce and detect the sound of very high frequency. The bats fly with speed much lower than the speed of sound. The sounds produced by flying bats get reflected from any obstacle in front of it. By hearing the echoes, bats come to know where the obstacles are, even in the dark. So, they can fly safely without colliding with the obstacles. This process of detecting obstacles is called sound ranging.

(ii) Dolphins detect their enemies and small fishes by emitting ultrasonic waves in all directions and then hearing their reflected sound i.e. echo. Dolphins can judge the nature of obstacles or of small fish by hearing the echo and catch their prey.

33. What do you understand by sound signaling?
The process of sending ultrasonic waves in all directions to detect obstacles and hearing the echo is called sound signaling.

34. What do you mean by sound ranging?
The process of detecting obstacles by sending ultrasonic waves and hearing the echo is called sound ranging.

35. A man fires a gun and hears its echo after 5 s. The man then moves 310 m towards the hill and fires his gun again. This time he hears the echo after 3 s. Calculate the speed of sound.

36. How do bats avoid obstacles in their way, when in flight?
Bats can produce and detect the sound of very high frequency. The bats fly with speed much lower than the speed of sound. The sounds produced by flying bats get reflected from any obstacle in front of it. By hearing the echoes, bats come to know where the obstacles are, even in the dark. So, they can fly safely without colliding with the obstacles. This process of detecting obstacles is called sound ranging.

37. State the use of echo by a bat, dolphin and fisherman.
Bats, dolphins and fishermen use the principle of echo for locating obstacles and prey.
They produce and sent ultrasonic waves in all directions. When these waves are reflected back from the obstacles or prey, they hear the echo. From the time taken to hear the echo and from the nature of the sound received, bats, dolphins and fishermen are able to gauge the distance and type of surroundings.

38. What is sonar? State the principle on which it is based.
SONAR means sound navigation and ranging.
Sonar is an instrument that makes use of ultrasonic waves for sound ranging. It is equipped to measure even short time intervals quite accurately.
Sonar works on the principle of echoes. A strong and short (ultrasonic) sound signal is sent towards the bottom of ocean. The echo of this signal is then detected by it. By noting the time after which the reflected sound (echo) reaches back, we can calculate the depth of the ocean by using the formula.
Depth of ocean = v × t/2, here v is the velocity of ultrasonic wave.

39. Name the waves which are used in sonar to find the depth of sea.
Ultrasonic waves are sent in sonar to find the depth of the sea.

40. A person standing between two vertical cliffs produces a sound. Two successive echoes are heard at 4 s and 6 s. Calculate the distance between the cliffs.
Let A1 and A2 be the distances of the two cliffs from the man then
2A1 = 320 × 4 = 1280 ...(i)
and, 2 A2 = 320 ×6 = 1920 ...(ii)
Adding (i) and (ii)
2(A1 + A2 ) = 3200
or, A1 + A2 = 1600 m
∴ distance between the two cliffs = 1600 m

41. Distinguish between compression and rarefaction.