# Class 11 Physics Chapter 15 Important Questions Waves

Q. 1. What characteristics should a medium possess for propagation of a wave through it? Ans. A medium should possess the properties of elasticity and

Science is a complex and challenging subject, as it involves so many principles and concepts that are difficult to memorize. Those student who opt for science have to face many challenges and work hard to get good marks in the exam. In this lesson, students will learn about chapter 15 Waves. The best solution of the problem is to practice as many Important Questions of 'Physics Class 11 Chapter 15' as possible to clear the doubts.

Q. 1. What characteristics should a medium possess for propagation of a wave through it?
Ans. A medium should possess the properties of elasticity and inertia so that a wave may be propagated through it.

Q. 2. What is the direction of vibrations of a medium particle through which (a) transverse, and (b) longitudinal waves are propagating?
Ans. (a) For a transverse wave, a medium particle must vibrate in a direction perpendicular to the direction of wave propagation.

(b) For a longitudinal wave, the medium particle must vibrate along same line as the direction of propagation of wave.

Q. 3. What is the nature of sound waves?
Ans. Sound waves are longitudinal waves propagating through air whose frequencies lie within the audible range (i.e., from 20 Hz to 20 kHz).

Q. 4. What type of mechanical waves can be produced (a) in gases, (b) in vacuum, (c) in water, and (d) on the surface of water?
Ans. (a) In gases, only longitudinal waves can be set up.

(b) In vacuum, mechanical waves cannot be set up at all.

(c) Inside the water, only longitudinal waves can be set up.

(d) On the surface of water, water ripples are set up which are almost transverse waves, set up under the action of surface tension of water.

Q. 5. Is an oscillation a wave? Give reason too.
Ans. No, an oscillation is not a wave. A wave is formed when along with oscillations of individual particles of a medium, there is a continuous propagation of disturbance from one particle of medium to another and so on.

Q. 6. Are Newton's laws of motion applicable to material waves? Are these also applicable to electromagnetic waves?
Ans. Yes, Newton's laws of motion are applicable to material waves. However, these laws are not applicable to electromagnetic waves.

Q. 7. How can you show experimentally that energy is associated with a wave?
Ans. A floating cork piece on water ripples formed on the surface of a water in a pond clearly suggests that energy is associated with a wave.

Q. 8. Why is flash of lightning seen before one hears the sound of a thunder?
Ans. It is because speed of light is extraordinarily large (3 x 108 m/s) in free space and air but speed of sound in air is only about 343 m/s at a room temperature of 20°C.

Read also: Waves Class 11 Physics Notes Chapter 15

Q. 9. What are the basic differences between sound waves and light waves?
Ans. Basically, sound waves are mechanical waves which require a material medium for their propagation. Light waves are electromagnetic waves which do not need any material medium and can travel even through free space. Again, sound waves are longitudinal waves but light waves are transverse in nature. Moreover, frequency range of audible sound waves is from 20 Hz to 20 kHz, whereas frequency of visible light varies from 4.0 x 1014 Hz to 7.5 x 1014 Hz.

Q. 10. Distinguish between sound waves and radio waves of exactly the same frequency (say 10 kHz).
Ans. (a) Sound waves travel as longitudinal waves through a material medium only but radio waves travel as transverse electromagnetic waves without the need of any material medium.

(b) Speed of sound waves depends on nature of medium and is generally small (only 331 m s through air under STP conditions) but speed of radio waves is exceedingly large (3 x 108 m s in vacuum or air).

Q.11. In which gas sound travels the fastest, and why?
Ans. The speed of sound is maximum in hydrogen gas among all gaseous media because density of hydrogen gas is, least. and speed of sound is inversely proportional to the square root of the density of gas.

Q. 12. What is the nature of the thermal changes in air (or a gaseous medium) when a sound wave propagates through it? Why?
Ans. During the propagation of sound waves through air or any other gaseous media, thermal changes are under adiabatic conditions. It is due to the fact that:

(a) air and other gases are very poor thermal conductors, and

(b) compressions and rarefactions occur in quick succession. Therefore, heat exchanges are not possible and the process is adiabatic.

Q. 13. How does speed of sound in air depend on (a) frequency of sound wave, (b) wavelength, (c) amplitude, and (d) intensity of sound?
Ans. (a) Speed of sound in air does not depend on the frequency of sound waves.

(b) Speed of sound in air does not depend on the wavelength of sound waves.

(c) Ordinarily, speed of sound does not depend on amplitude of wave provided that it is not exceptionally high (as in a high power explosion).

(d) Speed of sound is independent of intensity of sound provided that intensity is not exceptionally high.

Q. 14. How is propagation constant k related to wavelength of a travelling wave?
Ans. Propagation constant k=\frac{2π}{λ}

where λ = wavelength of the given wave.

Q. 15. What is a harmonic wave function?
Ans. A periodic wave function which is expressed in terms of a single sine or cosine function of position (x) and time (t) is called a harmonic wave function, e.g. y(x, t) = A sin (ωt - kr).

Q. 16. A harmonic wave is expressed as y(x, t) = A sin (ωt - kx). How do you obtain its wave velocity?
Ans. The wave velocity of the given wave v = \frac{dx}{dt} = rate at which the wave propagates along positive direction of x. It can be easily shown that magnitude of wave velocity v = ω/k.

Q. 17. How does particle velocity differ from wave velocity?
Ans. The particle velocity dy/dt varies both with position and time whereas wave velocity v=\frac{dx}{dt}=\frac{ω}{k} remains the same for a given wave propagating through a given medium.

Q. 18. What is the phenomenon of interference of waves?
Ans. Interference is phenomenon of redistribution of energy and formation of alternate regions of maximum and minimum amplitudes (and hence energy) when two waves of equal frequencies, equal or nearly equal amplitude and travelling along a given direction superpose on each other.

Q. 19. Why is a standing wave called so?
Ans. In a standing wave, the medium particles vibrate to and fro about their respective mean positions but the disturbance does not travel forward from one part of medium to another. That is why, the wave is called a standing wave.

Q. 20. Is superposition principle valid only for transverse waves formed on a string or true for other waves too?
Ans. The principle of superposition of waves is true for all sort of waves whether mechanical waves or electromagnetic waves.

Q. 21. What type of waves are formed on a stretched string fixed at both the ends?
Ans. A transverse wave is set on a stretched string. When string is fixed at both the ends, reflection of wave takes place from either of the two fixed ends. Thus, we have waves of same frequency travelling in mutually opposite directions. Due to their superposition, transverse standing waves are formed on a stretched string fixed at both the ends.

Q. 22. What do you mean by intensity of a wave? How is it related to amplitude of a wave?
Ans. Intensity of a wave is defined as energy flowing in the wave per unit area per unit time. Intensity (I) of a wave is proportional to the square of amplitude of a wave, i.e., I ∝ A2.

Q. 23. What is the difference between an open organ pipe and a closed organ pipe and the notes produced by them?
Ans. An open organ pipe is open at both its ends but a closed organ pipe is open at one end but closed at the other end. When an organ pipe vibrates in its normal resonant mode, antinodes are formed at both the ends in an open organ pipe. For a closed organ pipe, a node is formed at the closed end and an antinode is formed at the open end of pipe.

Q. 24. What do you mean by the term 'overtones'?
Ans. Whenever, a string clamped at its ends or an organ pipe vibrates, it can vibrate in number of normal modes of vibration. In such a situation, the note of the smallest frequency is called the fundamental note. Notes of higher frequencies which are integer multiple of fundamental frequency are called the overtones.

Q. 25. Two sound waves superimpose in such a way that the resultant sound intensity falls from maximum sound to a faint sound in 1/10 s and again rises to maximum (a loud) sound in 1/5 s. What is the frequency of beats?
Ans. In this question, time of formation of one beat, i.e., time interval between two successive maximum sound instants is 1/5 s. Hence, frequency of beats, i.e., the number of beats formed per second is 5 Hz.

Q. 26. What is the cause of Doppler effect when an observer is in motion towards a stationary sound source?
Ans. When an observer moves towards a stationary sound source, the relative speed of sound with respect to observer becomes (v + v0). As a result, the observer receives more waves per unit time and the observed frequency appears to be higher.

Q. 27. Does a vibrating source always produce sound? If not, under what condition is a sound produced?
Ans. When a source vibrates, a sound will be produced only if its vibrational frequency lies between the limits 20 Hz and 20 kHz (i.e., within the audible frequency range).

Q. 28. Explain why waves on strings are always transverse. ог Explain why longitudinal waves cannot be formed on strings.
Ans. Longitudinal waves are pressure waves and are transmitted as compression and rarefaction in a medium. As a string is nonstretchable, it can neither be compressed nor be elongated. Thus, compression and rarefaction cannot be produced in a string. Thus, longitudinal waves cannot be formed on strings. A string can sustain a transverse wave only when the string is under tension. If tension in the string is zero, it cannot sustain even transverse waves.

Q. 29. Why can astronauts on the Moon's surface not talk to each other as on the Earth?
Ans. Sound waves are mechanical waves and require a material medium for their propagation. Since there is no atmosphere on the Moon's surface, hence astronauts on the Moon's surface cannot talk to each other as on the Earth.

Q. 30. What characteristics of a medium determine the speed of longitudinal waves in a medium?
Ans. Speed of longitudinal waves propagating through a medium depends on its elasticity and density.

Q. 31. An explosion occurs inside a lake. What type of waves are produced in the lake water, and why?
Ans. Longitudinal waves are produced in the lake water. It is because water possesses bulk modulus and can sustain longitudinal waves. However, shear modulus of water is practically zero, hence transverse waves cannot be sustained in lake water.

Q. 32. A person can very well detect a distant coming train by placing his ear near the rails, although train cannot yet be seen by the eye. Explain how.
Ans. If the train is situated at a long distance or railway track is curved one, then a person cannot see the incoming train. However, if the person places his ear near the rails, he can listen the sound of incoming train. It is because speed of sound in steel rails is very high (more than 5 km/s) and due to high elasticity of steel, the waves do not die out easily. Hence, one can detect the incoming train by listening its sound.

Q. 33. When one end of a long metal pipe is struck a blow, what does a listener hear at the other end of the pipe, and why?
Ans. The listener will hear two distinct sounds at the other end of metal pipe. The first sound is due to propagation of wave through the metal pipe and the other sound is due to propagation of wave through air medium. As the speed of sound in metal of pipe is more, hence the first sound heard is heard earlier and is louder too.

Q. 34. Speed of sound in air at STP is 331 m/s. What will be the speed of sound if air pressure is doubled at constant temperature? Give reason.
Ans. The speed of sound will remain unchanged. As per Laplace formula, the speed of sound in air is v=\sqrt{\frac{\gamma P}{\rho}}. At constant temperature, if pressure of air is doubled, then in accordance with Boyle's law, the density of air is also doubled. As a result, the ratio P/ρ and hence, speed of sound in air remains unchanged.

Q. 35. Sound travels faster on a hot humid day than on a cold dry day. Explain why.
Ans. We know that speed of sound waves in air increases with increase in temperature (in fact, v ∝ √TK). Moreover, if humidity is higher, it means more water vapour is present in air. As density of water vapour is less than that of dry air, the speed of sound increases with increase in humidity. Thus, it is clear that speed of sound more on a hot humid day as compared to that on a cold dry day.

Q. 36. On what factors does the wave speed in a medium depend? Do longitudinal and transverse waves travel with same speed in a medium?
Ans. We know that for propagation of a wave through a given medium, the medium must possess inertia (or inertial mass) and elasticity. Therefore, elastic properties of the medium and linear mass density in case of a linear system and density for a bulk system determine the speed of wave propagation in that medium.

Generally, longitudinal and transverse waves travel with different speeds through same medium.

Q. 37. What causes the rolling sound of a thunder?
Ans. Rolling sound of a thunder is due to multiple reflections of sound produced at the time of lightning.

Q. 38. What happens when a wave pulse is reflected from (a) a rigid boundary, and (b) a free boundary?
Ans. (a) When a wave pulse is reflected from a rigid boundary, it undergoes a phase change of n rad at the instant of reflection. Thus, a crest is reflected as a trough and vice versa.

(b) When a wave pulse is reflected from a free boundary, it is reflected as such without any phase change. Thus, a crest is reflected back as a crest.

Q. 39. For a standing sound wave in a pipe, a displacement node behaves as a pressure antinode and vice versa. Explain why.
Ans. For a standing sound wave formed in a pipe at displacement node point, the two superposing waves travelling in mutually opposite directions are exactly out of phase. Consequently, pressure changes are the largest there. On the other hand, at an antinode, there is maximum displacement but minimum pressure change. Therefore, a displacement node behaves as a pressure antinode, and vice versa.

Q. 40. Why are several holes provided in a flute?
Ans. A flute is an open organ pipe. The length of vibrating air column can be changed by putting fingers on the holes or removing fingers from the holes. By changing length of air column, one can produce notes of different frequencies.

Q. 41. Bats can ascertain distances, directions, nature and size of the obstacles without any "eyes". Explain how.
Ans. Bats emit high frequency longitudinal waves (ultrasonic waves) which travel in all directions: If an obstacle is present somewhere, the wave is reflected from it and the reflected wave is received by the bat. From the reflected wave received, bat can ascertain distance of obstacle by estimating the time of echo.

He estimates the nature and size of the obstacle by knowing the intensity of reflected wave received. The direction of obstacle is estimated by bat by judging the time gap between reflected waves received by his two ears. Bats have their senses perfected to get all these results without the need of actual calculations.

Q. 42. How does the frequency of a tuning fork change, when the temperature is increased?
Ans. The frequency of a tuning fork decreases with increase in temperature.

Q. 43. Will there be any Doppler effect if both the source of sound and the observer are moving with same speed in the same direction?
Ans. As both the source of sound and the observer are moving with same speed in same direction, there is no relative motion between them. So, there will be no variation in frequency due to Doppler effect.

Q. 44. Is Doppler effect applicable for light waves too? Give an example.
Ans. Yes, Doppler effect is applicable for light waves too. Wavelengths of the spectral lines of light emitted by stars are found to shift slightly towards the red end of the spectrum. It means there is an increase in wavelength of light or decrease in observed frequency of light emitted by stars. As per Doppler effect, it is possible only if stars are moving away from us. The phenomenon is called red shift and it supports the expanding universe theory of cosmos.

Q. 45. What would a person hear when he moves away from a stationary sound source with the speed of sound?
Ans. The person will not be able to listen any sound because he is moving away from the source of sound with speed of sound. Thus, relative velocity of sound with respect to the moving observer will be zero and consequently, sound wave will not be able to reach the observer.

Q. 46. Sound is produced due to vibratory motion of particles. However, a vibrating pendulum does not produce sound. Why?
Ans. A vibrating pendulum does not produce sound because in a vibrating pendulum, motion remains confined to the pendulum bob itself and energy of vibration is not passed to the neighbouring medium particles. Moreover, frequency of pendulum vibrations is small enough and below the audible range.

Q. 47. When a source of sound moves towards a stationary observer, the apparent frequency of sound appears to be higher than its natural frequency. What is the cause of this observed change in frequency? Is it due to change in speed of sound or change in wavelength?
Ans. When a source of sound moves towards a stationary observer, the speed of sound remains unchanged but wavelength of sound waves decreases. Consequently, frequency of sound appears to be increasing.

Q. 48. Is there a Doppler effect for sound when the observer and the sound source are moving at right angles to each other?
Ans. No, there will be no Doppler effect when sound source and the observer are moving at right angles to each other because there is no component of motion of one in the direction of motion of the other and vice versa.

Q. 49. Explain why transverse mechanical waves cannot be propagated through fluids (i.e., liquids and gases).
Ans. In transverse waves, the particle motion is normal to the direction of propagation of the wave. Therefore, as the wave propagates, each element of the medium undergoes a shearing strain. Hence, transverse waves can be propagated only in those media which can sustain shearing stress. As fluids (liquids and gases) cannot sustain shearing stress, transverse waves 50 cannot be propagated through fluids.

Q. 50. Energy in the Sun is generated due to thermonuclear fusion reaction going on at the centre of the Sun. As a result of this reaction, light, heat as well as sound is produced. Light and thermal effects reach the Earth but sound does not reach the Earth. Why?
Ans. Light and thermal radiations are both electromagnetic waves and they can travel through free space so as to reach the Earth. However, sound waves are mechanical waves and they cannot be propagated through free space. Consequently, sound produced at the Sun cannot be detected from the Earth.

Q. 51. When a pebble is thrown on the surface of water in a pond, waves travel out on the water surface. From where does the energy come?
Ans. The energy of the wave spreading on the surface of water comes from the kinetic energy of the pebble striking the water surface. This energy is shared by the water molecules.

Q. 52. "The most fundamental property of a wave motion is its frequency". Comment on the statement.
Ans. The statement is perfectly alright. When a wave passes from one medium to another medium, the wave speed as well as its wavelength change. However, the frequency of the wave remains unchanged. It is due to this reason that we say the frequency to be the most fundamental property of a wave.