Wave: Question Bank for Class 11 Physics

Get important questions of Wave for Board exams. Download or View 11th Physics important questions for exam point of view. These important questions will play significant role in clearing concepts of Physics. This question bank is designed by NCERT keeping in mind and the questions are updated with respect to upcoming Board exams. You will get here all the important questions for class 11 Physics chapter wise CBSE.

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Q.1. What is the angle between particle velocity and wave velocity in ( 1) Transverse wave ( 2) Longitudinal wave?

Ans. Angle between particle velocity and wave velocity

for transverse wave is $\pi / 2$ and that in case of

longitudinal wave is either zero or $\pi$.

Q.2 Explain why it is possible to detect the approaching of a distant train by placing the ear very close to railway line?

Ans. This is so because sound travels 16 times faster in iron than in air.

Q.3 What will be the nature of graph between the pressure of a gas and the speed of sound waves passing through the gas?

Ans. The graph will be a straight line parallel to pressure axis, as speed of sound is independent of the pressure of gas.

Q.4 When sound waves travel from air to water, does the frequency of wave changes? The wavelength ? The speed?

Ans. The frequency does not change. The speed and wavelength change.

Q.5 What are those two important characteristics which a material medium must have for the propagation of mechanical waves?

Ans. Elasticity and Inertia.

Q.6 Why explosions on the other planets can not be heard on the earth?

Ans. Because there is no material medium in the space between the planets, through which sound may

Q.7 the violin is taken to a hot room, how will the pitch of the note produced be affected?

Ans. The frequency of the note emitted gets increased.

Q.8 What do you mean by overtones ?

Ans. The notes produced by an instrument of sound other than the fundamental note are called overtones.

Q.9 What physical quantity changes when a source of sound moves and the listener is stationary ?

Ans. Wavelength of sound waves changes.

Q.10 Will be Doppler effect, when the directions of motion of the source or observer is perpendicular to the direction of propagation of sound ?

Ans. No, there is no Doppler’s effect, when the source or observer moves perpendicular to the direction of propagation of sound.

Q.11 Given below are some examples of wave motion. State in each case, if the wave motion is transverse, Longitudinal or a combination of both

(i) Motion of a kink in a long coil spring produced by displacing one end of spring sideways.

(ii) Waves produced in a cylinder containing a liquid by moving its piston back and forth.

(iii) Waves produced by motor boat sailing in water.

(iv) Light waves travelling from sun to the earth.

(v) Ultrasonic waves in air produced by a vibrating quartz crystal.

Ans. (i) Wave motion is transverse, because the vibration of particles (Kinks) of the spring are at right angles to the direction of wave propagation.

(ii) The wave motion is longitudinal, because the molecules of the liquid vibrate to and fro about their mean position along the direction of propagation of the wave.

(iii) The given wave motion is a combination of longitudinal and transverse waves.

(iv) The wave motion is transverse, because the light wave is an electromagnetic waves in which electric and magnetic fields vibrate in the direction perpendicular to each other and also to the direction of propagation of the wave.

(v) Ultrasonic waves produced by a quartz crystal in air are longitudinal, because the molecules of air vibrate to & fro about their mean positions along the direction of propagation of wave due to vibration of quartz crystal.

Q.12 Why the shape of a pulse gets deformed during propagation in a dispersive medium ?

Ans. When a pulse passes through a dispersive medium, the wavelength of wave changes. Hence, the shape of pulse changes i.e. it gets deformed.

Q.13 When a stone is thrown on the surface of water. Waves travel out, from where does the energy come ? [

Ans. The energy carried by the water waves comes from Kinetic energy of stone hitting the water surface.

Q.14 Why is sound heard in $\mathrm{CO}_{2}$ more intense in comparison to sound heard in air ?

Ans. This is because intensity of sound increases with increase in density of the medium.

Q.15 Which type of hearing aids are required by persons on the surface of moon and why?

Ans. We require those hearing aids, which may transmit and detect electromagnetic waves, because there is no atmosphere on the moon.

Q.16 Explain why frequency is the fundamental property of a wave ?

Ans When a wave travels from one medium to other, its length as well as velocity may change but frequency does not change. This is the reason, we say that frequency is the fundamental property of a wave.

Q.17 Explain, why does sound travels faster on a cloudy day as compared to that on a dry day ? [

Ans. The amount of water vapour present in the atmosphere is much greater on a cloudy day than on a dry day. As the water vapours are lighter than dry air, hence density of wet air, becomes, less than that of dry air. Now, because the speed of sound is inversely proportional to square root of the density, hence sound travels faster on a cloudy day than on a dry day.

Q.18 If a balloon is filled with $\mathbf{c} o_{2}$ gas then how will it behave for sound as a lens ? On filling of hydrogen gas ?

Ans. In $\mathrm{CO}_{2}$, the velocity of sound is less than air, when, the balloon filled with will behave as convex lens. On the other hand the balloon filled with gas will behave as concave lens because velocity of sound is greater in hydrogen than in air.

Q.19 Why is the sound produced in air not heard by a person deep inside the water ?

Ans. There is a great difference in speed of sound in air and in water. Hence, there is little refraction of sound from air into water, most of the sound is reflected from surface of water. It is due to this reason that where there is a glass wall between two rooms, the conversation in one room can not be heard in other room.

Q.20 Explain why sound of very high velocity can not be transmitted through a material of high density but of low elasticity ?

Ans. The speed of sound wave is given by,

$v=\sqrt{\frac{E}{\rho}}$

Q.21 Explain why interference is not possible in the sounds produced by two violins ?

Ans. For interference between two waves the phase difference between the waves must remainconstant. Each violin produces wave-train of sound discontinuously. Hence, the phase of a wave-train arriving at a point varies with time. Clearly, the phase difference between waves coming at a point from two separate violins will vary with time. Hence, an average intensity of sound will be heard everywhere.

Q.22 As in sound can beats be observed by two light sources ? Explain.

Ans. No, to observe beats by two light sources the phase difference between the sources should change regularly. In light sources however this change occur at random, because the light source consists of a large number of atoms and each atom emits wave independently

Q.23 Explain why there are so many holes in a flute ?

Ans. The flute is basically an open organ pipe. The location of the open end can be changed by keeping the one hole open and closing the other holes. Thus, the frequency of the note produced by the flute can be changed.

Q.24 A bucket is placed below a water top. We can estimate the height of water level reached in the bucket from a distance simply by hearing the sound. Explain, why it is so ?

Ans. The frequency of note emitted by an air column is inversely proportional to its length $\left(n \propto \frac{1}{l}\right)$. Therefore, as the length of air column decreases, frequency increases. i.e. the note becomes shrill. As the water level in bucket rises, the length of air column of the bucket goes on decreasing and the sound produced goes on becoming more and more shrill. On this basis we can estimate from a distance the height up to which the bucket has been filled with water.

Q.25 Why does the pitch of a note produced by a wooden open end pipe becomes sharper when the temperature rises ? [2]

Ans. With rise in temperature, the velocity of sound increases. The fundamental frequency of an open

prgan pipe is given by $n=\frac{v}{2 l}$, hence with increase

in value of v, n increases and the pitch of note becomes sharper.

Q.26 What is the difference between an echo and a reverbration ?

Ans. An echo is produced when sound reflected from a distant obstacle comes back after an interval of

$\frac{1}{10}$ second or more. In an echo, the original and reflected sounds are heard separately.

Reverbration, on the other hand, consists of successive reflections which follow each other so quickly that they can not produce separate echos.

Q.27. A violin note and a sitar note may have same frequency and yet we can distinguish between two notes. Explain, why it is ?

Ans. This is due to the fact that overtones (integral multiples of fundamental frequency) produced by two sources may be different, in other words the quality of sound produced by two instruments of same fundamental frequency is different.

Q.28 Of what material is the running fork made and why ?

Ans. A tunning fork is made of an alloy of steel, Nickel and chromium, called elinvar i.e. the material for which the elasticity does not change.

Q.29 Why a given sound is louder in a hall than in the open ?

Ans. In a hall, repeated reflections of sound takes place from the walls and the ceiling. The reflected sounds mix up with the direct sound, resulting in increase in the intensity of sound in the hall. But in open, no such reflections of sound are possible and as such faint sound is heard in open.

Q.30 Why all the stringed instruments are provided with hollow boxes ?

Ans. The stringed instruments are provided with hollow boxes in order to increase the surface area of vibration, which increases the loudness or intensity of sound produced, $(\text { As } I \propto A)$ . Moreover the air inside the hollow box is set into forced vibrations which also increases the loudness of sound produced.

Q.31 Explain how the bats ascertain distances, directions, nature and size of the obstacles without any eyes ?

Ans. The bats can emit ultrasonic waves (sound waves of frequency above . The waves emitted by bats are reflected by the obstacles and is received by it. Bats can also detect the ultrasonic waves. The time interval between the emission of ultrasonic wave and its reception back after reflection give the information of distance, nature of obstacle and its direction. Then bats can ascertion distances, direction, nature and size of the object inspite of their poor eyesight.

Q.32 In summer, sound of a siren is heard louder in the night than in the day to a person on earth. Explain why it is so ?

Ans. During the day time in summer the surface of the earth becomes hot due to sun rays. Hence the air near the earth is hotter than the air above. i.e. as we move above the earth, the air becomes denser. Hence, the wave starting from the sound source situated on the earth bends continuously towards the normal and goes away from the earth.

Therefore, the sound of siren is heard feeble on earth.

On the other hand, in night time the air near the earth becomes rapidly cold and so now in going from the earth the air becomes rare or less dense. Hence, the sound waves ultimately, after total internal reflection, return to earth. Hence, the sound of siren is heard loud on the earth.

Q.33 Explain the factors on which the pitch of a tunning fork depends.

Ans. The factors on which the pitch of a tunning fork depends are –

Length of the prongs (l) : The pitch is inversely proportional to square of the length$(l)$

of the prongs i.e. $v \propto \frac{1}{l^{2}}$

(ii) Thickness of the prongs (b): The pitch is

directly proportional to the thickness (b) of the

prongs i.e.

(iii) Elasticity of material (v): The pitch is directly

proportional to the square root of the elasticity

(Y) of the material i.e..

(iv) Density of material : The pitch is inversely

proportional to the square root of the density of

the material ie..

Q.34 State Newton’s formula for velocity of sound in air. Point out the error and discuss Laplaces correction.

Ans. According to Newton the velocity of longitudinal waves through any medium (solid, liquid or gas) depends upon the elasticity and density of medium as

$v=\sqrt{\frac{E}{\rho}}, \mathrm{E}=$ coeff. of elasticity of medium

$\rho=$ density of medium.

For air or gas (since gas has only one type of

elasticity) has bulk modulus only.

$v=\sqrt{\frac{K}{\rho}}$ ……………….(i)

Newton assumed that sound travels through a gas in the form of compression and rarefactions and this phenomenon takes place so slowly that temperature of gas (or medium) remains constant. i.e. the process is isothermal.

Under isothermal conditions, constant.

Differentiating on both sides, we get,

$P d V+V d P=0$

$\Rightarrow P=-\frac{V d P}{d V}=-\frac{d P}{\frac{d V}{V}}=K$

Hence, from equation ( 1), velocity of sound,

$v=\sqrt{\frac{P}{\rho}}$

Where $P=$ atmospheric pressure at N.T.P.

$=76 \mathrm{cm}$. of $\mathrm{Hg} \quad=0.76 \times 13.6 \times 10^{3} \times 9.8$

$=1.013 \times 10^{5} \mathrm{N} / \mathrm{m}^{2}$

and density of air of N.T.P.

But experimental value of velocity of sound in air at N.T.P. is i.e. there is a large difference between theoretical and experimental values. This shows that there is an error in Newton’s formula. Later on, this error was solved by french mathematician Laplace and is called Laplace’s correction

Laplace’s Correction

According to Laplace the change in pressure and volume of a gas, when sound wave propagates through it are not isothermal but adiabatic.

For adiabatic process, gas equation is, constant.

Where

Differentiating above equation, we get,

(by definition)

From equation (i), we have,

Velocity of sound in air,

(Since air is a mixture of diatomic gas;

The above value agrees fairly well with the experimental values.

Q.35 Establish relation between particle velocity and wave velocity.

Ans. Wave Velocity : It is the velocity with which a progressive wave travels through the medium. It is a constant quantity and is given by, .

Where, n = frequency of wave & is wavelength.

Particle Velocity

A wave propagates through a medium due to repeated periodic motion of the particle of the medium i.e. velocity of a particle continuously changes with time.

The instantaneous particle velocity of a particle is defined as the time rate of change of its displacement. It is denoted by ‘u’.

The displacement of a particle in a progressive wave at any time ‘t’ is given by,

Hence, the instantaneous particle velocity is given by,

Now,

i.e.

maximum particle velocity

The slope of the displacement curve at position x is given by

Now,

i.e.

Hence, particle velocity at a given position at a given time is equal to product of wave velocity

and negative slope of the wave curve at the given position and time.

Q.36 What are standing (or stationary) waves ? Give analytical treatment of formation of standing waves.

Ans. When two sets of progressive waves of same type (i.e. both longitudinal or both transverse) having the same amplitude and same time period (or frequency) travelling with same speed along the same straight line in opposite directions superimpose, a new set of waves are formed. These are called standing or stationary waves.

In stationary waves, there are certain points of the medium, which are permanently at rest i.e. their displacement is zero all throughout. These points are called nodes. Similarly, there are other points which vibrate with largest amplitude. These points are called antinodes.

Analytical method

Let a plane progressive wave of amplitude ‘a’ is travelling with speed along positive direction of axis. The equation of the wave is,

Let this wave is reflected from a free boundary and the reflected wave advances in negative direction of x–axis. The equation. of reflected wave.

If, however, the waves were reflected from a rigid boundary, then the equation. of reflected wave will be,

sign shows that a phase change of takes place on reflection).

According to principle of superposition, the resultant displacement,

i.e.

Variation of y w.r.t. x

Substituting in above equation., the value of becomes alternately +1 and –1. Thus, at these points the displacement y is always max. These points are called ‘antinodes’ and are separated from one another by a distance of . Similarly, substituting in above equations, the value of becomes zero. Thus at these points, the displacement y is always zero & are called nodes and these are also separated from one another by a distance of .

Variation of y w.r.t. time (t)

Substituting is above equation. the value of becomes zero i.e. at these points, all particles of the medium pass through their mean position simultaneously twice in each vibration.

(2) Substituting the value of becomes alternately +1 and –1. At these points the displacements y is max. Hence, all the particles of the medium (except nodes) are in position to maximum displacement twice in each vibration.

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