1. What is a wave?
A disturbance that propagates from one location to another without net transport of matter is called a wave. It is an energy carrier.
2. What is wave motion?
A process by which a disturbance at one point is propagated to another point with no net transport of the medium itself is called a wave motion.
3. How many types of a wave? Define them.
There are two types of a wave.
a) Mechanical wave
b) Non-mechanical wave
Mechanical wave: The wave which requires a medium for its propagation is called a mechanical wave. e.g. waves on string and spring, sound waves, etc.
Non-mechanical wave: The wave which does not require a medium for its propagation is called a non-mechanical wave. e.g all electromagnetic waves like light, heat, etc are non-mechanical waves.
4. One can not hear sound on the moon, why?
The sound wave is a mechanical wave for the propagation of which a medium is required but there is no atmosphere (medium) on the moon and so the sound can not propagate on the moon. That is why one can not hear sound on the moon.
5. If you are walking on the moon's surface, can you hear the cracking sound behind you? Explain.
The sound wave is a mechanical wave for the propagation of which a medium is required but there is no atmosphere(medium) on the moon and so the sound can not propagate on the moon. That is why we can not hear cracking sounds on the moon.
6. Sound from the sun does not reach the earth, why?
Sound is a mechanical wave so a continuous medium is essential for its propagation, but there is no continuous medium between the sun and the earth for the propagation of sound so it can not reach the earth.
7. How many types of mechanical waves? Define them.
There are two types of mechanical waves which are longitudinal waves and transverse waves.
Longitudinal wave: A mechanical wave is said to be longitudinal if medium particles vibrate in the direction of the propagation of the wave. e.g. sound in a gas, and wave in spring are longitudinal waves.
Transverse wave: A mechanical wave is said to be transverse if medium particles vibrate perpendicular to the direction of propagation of the wave. e.g. wave on the string is transverse.
8. How can you say that sound is a wave motion?
Due to the following reasons sound is a wave motion:
(a) Like a wave, sound takes time to travel from one place to another i.e. it takes definite velocity.
(b) Like a wave, sound requires a material to travel through it.
(c) Like a wave, sound shows reflection, refraction, interference, and diffraction.
(d) Like a wave, the sound is reflected or refracted according to the same laws.
9. A common pastime at sporting events is "doing the wave" in which the spectators in one section raise and lower their arms, signaling the spectators in the next section to do the same, and so on. In the languages of this chapter, is this truly a wave? Why or why not?
Yes. The waves travel horizontally from one spectator to the next but the displacement of each spectator is vertically upward. Since the displacement is perpendicular to the direction in which a wave travels. So the wave is transverse.
10. Explain why it is necessary to include the factor 2n in equation y=Acos 2π(x/ λ-t/T). What unit does 2n have? Could you correctly replace this factor with 360 °?
The factor 2π is included in this equation because the phase of a particle is measured in the fraction of 2π and the angle of the sin represents the phase of the particle. It has a unit and it is a radian or degree. Yes, it can be replaced by 360.
11. Light can be polarized but sound can not be, why?
Only transverse waves can be polarized and since light is a transverse wave so it can be polarized but the sound is a longitudinal wave so it can not be polarized.
12. Is it possible to have a longitudinal wave in a rod? Explain.
Yes. If a rod is struck with a hammer along its length then the particles of the rod vibrate along the length of the rod in which direction the wave also travels so this is the longitudinal wave.
13. Can two astronauts talk on the surface of the moon as they do on the earth? Why.
No, the astronauts can not talk on the surface of the moon as there is no atmosphere on the moon and hence sound waves can not propagate without a medium.
14. What are the differences between sound waves and light waves? (2068)
(i) Sound waves are mechanical waves whereas light waves are non-mechanical waves.
(ii) The velocity of sound waves at NTP is 332 m/s whereas the velocity of light waves in air/vacuum is 3 x 103 m/s.
(iii) Sound waves cannot be polarized but light waves can be. polarized.
15. Explosions on other planets are not heard on the earth, why?
As we know that there is no continuous material medium over a long distance between the earth and other planets which is the required condition for the propagation of sound so explosions on other planets are not heard on the earth.
16. Solids can support both longitudinal ad transverse waves, but a gas supports only longitudinal waves. why?
A medium should have rigid modulus of elasticity for the propagation of a transverse wave through it whereas it should have a bulk modulus of elasticity for a longitudinal wave. A solid has both rigid modulus of elasticity and bulk modulus of elasticity so it supports both types of waves whereas a gas has only bulk modulus of elasticity so it supports only longitudinal 'wave.
17. Ocean waves hitting a beach are always found to be nearly normal to the shore, why?
Ocean waves are transverse waves traveling in concentric circles of ever-increasing radii. When they hit the shore their radius of curvature is so large that they can be treated as plane waves. Hence, they hit the shore nearly normally to the shore.
18. What do you mean by the persistence of hearing?
When we hear the sound, it remains for some while in the ear which is called persistence of hearing and its time is 0.1 Sec.
19. When sound travels from air into water, does the frequency of the wave change? The wave length? Explain your reasoning.
The frequency of the wave does not change but its wavelength and speed change. This is because the frequency is the characteristic of the source of the wave so it remains constant in all media but velocity changes in different ,media and according to the relation V = fλ if V changes, λ also changes keeping f constant.
20. The hero of a western movie listens for an oncoming train by putting his ears to the track .Why does this method give early warning of the approach of a train?
This is because the speed of sound is the fastest in a solid (track of a train).
21. When a rock is thrown into a pond and the resulting ripples spread in ever-widening circles, the amplitude decreases with increasing distance from the centre. Why?
This is because the energy of the wave is dissipated in the form of heat due to the viscosity of water.
22. Why do you see lighting before you hear the thunder?
As we know that speed of light( 3x108 ms-1) is much more than that of sound(332ms-1) so it will take less time for the same distance and therefore we see lighting before we hear the thunder.
23. What is principle of superposition of waves?
The principle of superposition of waves states that the sum of displacements produced by interfering waves is equal to the displacement of resultant waves. Mathematically,
Y=y1+y2+y3+......
24. State and explain stationary wave.
The resultant wave formed due to the superposition of two identical waves in opposite directions is called ·a stationary wave (standing wave). It contains nodes and antinodes at alternate positions and the energy is confined within a limited area.
25. Define node and antinode.
The region of the stationary wave where pressure is maximum is called a node. The region of the stationary wave where pressure is minimum, is called an antinode.
26. How are stationary waves formed?
When two progressive waves of same (or nearly same) amplitude and frequency are superimposed in opposite directions then the stationary wave is formed.
27. Why is a standing wave so named?
Since this wave is confined in a particular region without moving so the name stationary wave.
28. What is the distance between a compression and its nearest rarefaction in a longitudinal wave?
The distance between a compression and its nearest rarefaction in a longitudinal wave is λ/2.
29. What is the distance between any two consecutive compressions or rarefactions in a longitudinal wave?
The distance between any two consecutive compressions or rarefactions in a longitudinal wave is λ.
30. What is the distance between any two consecutive nodes or antinodes?
The distance between any two consecutive nodes or antinodes is λ/2.
31. What is the distance between a node and an adjoining antinode in a stationary wave?
The distance between a node and an antinode is λ/4.
32. What is distance between three consecutive nodes or antinodes?
The distance between three consecutive nodes or antinodes is λ.
33. What is the distance between a crest and its nearest trough in a stationary wave?
The distance between a crest and its nearest trough in a stationary wave isλ/2.
34. What is the distance between any two consecutive crests or troughs in a standing wave?
The distance between any two consecutive crests or troughs in a standing wave is λ.
35. Difference between progressive and standing wave.
Progressive Wave | Standing Wave |
It advances forward. | It doesn't move at all and hence it is confirmed in a fixed region. |
The length of the wave is not fixed. | The length of the wave is fixed. |
There is no formation of nodes and antinodes. | There is the formation of nodes and antinodes. |
There is a transmission of energy across every plane. | There is no transmission of energy across every plane. |
Each particle of the medium undergoes a similar change of pressure and density. | There is no change of pressure and density at antinodes while there is maximum change of pressure and density at nodes. |
The amplitude of each particle is same. | The amplitude varies from zero at nodes to maximum at antinodes. |
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