CBSE Class 11 Physics Chapter-14 Important Questions - Free PDF Download
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Study Important Questions for Class 11 Physics Chapter 14 – Waves
Very Short Answer Type Questions (1 Mark)
1. Explosions on other planets are not heard on earth. Why?
Ans: Explosions on other planets are not heard on earth because there is no material medium between the earth and the planets over a long distance, and without a material medium for propagation, sound waves cannot travel.
2. Why longitudinal waves are called pressure waves?
Ans: Longitudinal waves are called pressure waves because the propagation of longitudinal waves through a medium consists of the variations in the volume and the pressure of the air, these variations in volume and air pressure result in the formation of compressions and rarefactions.
3. Why do tuning forks have two prongs?
Ans: The tuning fork has two prongs because the two prongs of a tuning fork produce resonant vibrations that help to keep the vibrations going for longer.
4. Velocity of sound increases on a cloudy day. Why?
Ans: Velocity of sound increases on a cloudy day, because the air is wet on a cloudy day, it contains a lot of moisture, the density of air is lower, and because velocity is inversely proportional to density, velocity increases.
5. Sound of maximum intensity is heard successively at an interval of 0.2 second on sounding two tuning fork to gather. What is the difference of frequencies of two tuning forks?
Ans: The beat period is 0.2 second so that the beat frequency is
6. If two sound waves have a phase difference of
Ans, Phase difference,
Now, in general for any phase difference,
Given
7. If the displacement of two waves at a point is given by: -
Calculate the resultant amplitude?
Ans: Given data :
If
Then,
In our case,
8.A hospital uses an ultrasonic scanner to locate tumors in a tissue. What is the wavelength of sound in the tissue in which the speed of sound is
Ans: Given data :
Speed of sound in the tissue,
Operating frequency of the scanner,
Now,
The wavelength of sound in the tissue is given as:
9. Given below are some functions of
(a)
(b)
(c)
(d)
Ans: (a) It is a stationary wave because the harmonic terms are present individually in the equation, the preceding equation indicates a stationary wave
(b) There is no harmonic term in the provided equation. As a result, it doesn't represent either a moving or stationary wave.
(c) The harmonic terms in the preceding equation describe a moving wave and are in the combination of
(d) The given equation represents a stationary wave because the harmonic terms
10. A narrow sound pulse (for example, a short pap by a whistle) is sent across a medium. (a) Does the pulse have a definite (i) frequency, (ii) wavelength, (iii) speed of propagation? (b) if the pulse rate is 1 after every
Ans: (a) No, the pulse doesn’t have a definite .
(ii) No, It doesn’t have frequency.
(iii) Yes, It have a wavelength.
(b) No, the frequency of the note produced by the whistle is not equal .
Explanation:
(a) There is no defined wavelength or frequency for the narrow sound pulse. However, the sound pulse's speed remains constant, i.e., it is equal to the speed of sound in that medium.
(b) The short pip produced after every 20 s does not mean that the frequency of the whistle is
2 Marks Questions
1. A pipe
Ans: Given data :
Length of pipe
Frequency of
Velocity of sound
Now,
Hence, it will be the first normal mode of vibration, ln a pipe, open at both ends We have ,
As
2. Can beats be produced in two light sources of nearly equal frequencies?
Ans: No, because light emission is a random and fast occurrence, and we obtain uniform intensity instead of beats.
3. A person deep inside water cannot hear sound waves produces in air. Why?
Ans: Person deep inside water cannot hear sound produces in air because the speed of sound in water is roughly four times that of sound in air, refractive index is four times that of sound in air.
index
For, refraction
Since
Thus, Sounds are only reflected in the air, and people deep in the water are unable to hear them.
4. If the splash is heard 4.23 seconds after a stone is dropped into a well. 78.4 meters deep, find the velocity of sound in air?
Ans: Given data,
depth of
Total time after which splash is heard
Assume that,
If
then
Now, for downward journey of stone;
Now,
If
5. How roar of a lion can be differentiated from bucking of a mosquito?
Ans: The roaring of a lion produces a low-pitched, high-intensity sound, but the buzzing of mosquitoes produces a high-pitched, low-intensity sound, and therefore the two noises may be distinguished.
6. The length of a sonometer wire between two fixed ends is
Ans: Assume that,
Since,
T and m are fixed quantities, and 2 are constant
Equating equation 1), 2) & 3)
Also,
Now, Total length
1.e
i. e.
Now_
7. If string wires of same material of length
Ans: Given data ,
Since frequency
Tension
Let for first case,
For second case,
so,
and
Divide equation 1) by equation 2)
or
Hence, the ratio of tensions is 1: 9 .
8. Two similar sonometer wires of the same material produces 2 beats per second. The length of one is
Ans: Given data,
The frequency (f) of a Sonometer wire of length (L), mass (m) and
Tension (T) is given by
Let,
so,
Now,
In first case;
Subtract equation
Now, given
so,
9. Why are all stringed instruments provided with hollow boxes?
Ans: The sound box is a hollow box that comes with stringed instruments. Forced vibrations are produced in the sound box when the strings are set into vibration. The enormous surface area of the sound box causes a big amount of air to vibrate. The result is a loud sound with the same frequency as the string.
10. Two waves have equations:
If in the resultant wave, the amplitude remains equal to the amplitude of the super posing waves. Calculate the phase difference between
Ans: Given data,
The first wave
The second wave
Where ,
a = amplitude
Assume that,
The resultant amplitude
Now, in oar case,
so,
So, the phase difference between
11 A Tuning fork of frequency
Ans: According to the question,
Frequency of air column at
Let 1 = length of air column and speed of sound
For a pipe, closed of one end, the frequency of
1 = length of air column
So, Let at
Now,
or
Using
The frequency of tuning fork remains
12. A vehicle with horn of frequency '
Ans: The apparent change in wave frequency caused by relative motion between the source of waves and the observer is known as the Doppler effect.
If,
But in our case, the source and observer more at right angles to each other. The Doppler Effect is not observed when the source of the sound and the observer are moving at right argyles to each other.
S0, if
13. We cannot hear echo in a room. Explain?
Ans: We all know that in order for an echo to be heard, the obstruction must be hard and vast in size. In addition, the obstruction must be at least a distance from the source. The parameters for the generation of Echo are not met since the length of the room is usually less than. As a result, there is no echo in the room.
14. Why do the stages of large auditoriums give curved backs?
Ans: The backs of big auditorium stages are curved because a speaker's voice is rendered parallel following reflection from a concave or parabolic seer face when he stands at or near the focal of a curved surface. As a result, the voice can be heard from afar.
15. Show that Doppler effect in sound is asymmetric?
Ans: It is seen that apparent frequency of sound when source is approaching the stationary listener (with velocity
Apparent frequency
This is when source approaches station any listener
Apparent frequency
since
16. An organ pipe
Ans: Given data,
Length of pipe closed at one end for first overtone,
Length of pipe closed at both ends for third overtone;
We know that,
17. A simple Romanic wave has the equation
Another wave has the equation.
Deduce the phase difference and ratio of intensities of the above two waves?
Ans: Given data,
If y is in meters, then equation becomes:
The standard equation of plane progressive wave is
Now,
Comparing equation 1) & 2)
frequency
wave velocity,
wavelength
On inspection of the equations of the given two waves,
Phase difference,
Ratio of amplitudes of two waves
Ratio of intensities
18. The component waves producing a stationary wave have amplitude, Frequency and velocity of
Ans: Since the ware equation of a travelling wave
Let
By principle of superposition, wave equation for the resultant wave
Using
Here
19. A wine of density a
Ans: The lowest frequency of transverse vibrations is given by: -
Area
Density
Here
because Density
F=35.3 v/sec
20. Given two cases in which there is no Doppler effect in sound?
Ans: The two circumstances in which there is no Doppler effect in sound (i.e. no change in frequency) are as follows: -
1) When both the sound source and the listener are moving in the same direction and at the same speed.
2) When one of the source listeners is in the circle's center and the other is travelling around it at a constant speed.
21. A string of mass
Ans: Given data,
Mass of the string,
Tension in the string,
Length of the string,
Mass per unit length,
The velocity (v) of the transverse wave in the string is given by the relation:
Time taken by the disturbance to reach the other end,
22.
Ans: Given data ,
Length of the steel wire,
Mass of the steel wire,
Velocity of the transverse wave,
Mass per unit length,
For tension
23. A bat emits ultrasonic sound of frequency
Ans: (a) Frequency of the ultrasonic sound,
Speed of sound in air,
The wavelength
(b) Frequency of the ultrasonic sound
Speed of sound in water,
The wavelength of the transmitted sound is given as:
24. (1) For the wave on a string described in Exercise 15.11, do all the points on the string oscillate with the same (a) frequency, (b) phase, (c) amplitude? Explain your answers. (i) What is the amplitude of a point 0.375 m away from one end?
Ans: (1)
(a) Yes, except at the nodes
(b) Yes, except at the nodes
(c) No
(ii) 0.042 m
Explanation:
(i)
(a) All the points on the string oscillate with the same frequency, except at the nodes which have zero frequency.
(b) All the points in any vibrating loop have the same phase, except at the nodes.
(c) All the points in any vibrating loop have different amplitudes at vibration.
(ii) The given equation is:
For
Amplitude = Displacement
25. A wire stretched between two rigid supports vibrates in its fundamental mode with a frequency of
Ans: Given data,
(a) Mass of the wire,
Linear mass density,
: Length of the wire,
The wavelength of the stationary wave
Where,
For fundamental node,
The speed of the transverse wave in the string is given as:
(b) The tension produced In the string is given by the relation:
26. Two sitar strings
Ans: Given data ,
Frequency of string A,
Frequency of string
Beat's frequency,
Beat's frequency is given as:
Frequency decreases with a decrease in the tension in a string. This is because frequency is directly proportional to the square root of tension. It is given as:
Hence, the beat frequency cannot be
3 Marks Questions
1. Explain briefly the analytical method of formation of heats?
Ans: Assume that,
Let us consider two wares’ trains at equal amplitude 'a' and with different frequencies
Let displacements are
According to superposition principle, the resultant displacement
Using
we get,
Where,
Now, amplitude A is maximum when,
i.e. resultant intensity of sound will be maximum at times,
Time interval between 2 successive Maximax’s
similarly, a will be minimum,
i.e. resultant intensity of sound will be minimum at times
Hence time interval between 2 successive minim as are
Combining 1) &: 2) frequency of beats
2. Show that the frequency of nth harmonic mode in a vibrating string which is closed at both the end is 'n" times the frequency of the first harmonic mode?
Ans: When a sting under tension is placed into vibration, transverse harmonic waves propagate along its length, and reflected waves exist when the length of the sting is fixed. In sting, the incident and reflected waves will superimpose on one other, resulting in transverse stationary waves.
Let a harmonic wave be set up in a sting of length
Let the incident wave travels from left to right direction, the wave equation is:
The reflected wave equation will have the same amplitude, wavelength, velocity, and time as the incident wave equation, but the only difference between the incident and reflected waves will be in their propagation direction.
Reflection, the wave will suffer a phase change of
According to the principle of superposition, the wave equation of resultant stationary wave will be-
Using equation 1) & 2)
Using
Now, at
1) At
2)
Now,
Sin
i.e
1) Let
Let
If
i.e. frequency of second harmonic Mode is twice the frequency of first harmonic Mode Similarly,
3. Differentiate between the types of vibration in closed and open organ pipes?
Ans: 1) In closed pipe, the wavelength of nth mode
where
where as in open pipe,
2) The fundamental frequency of open pipe is twice that of closed pipe of same length.
3) A closed pipe of length
4) For an open pipe, harmonics are present for all integers and for a closed pipe, harmonics are present for only odd integers hence, open pipe gives richer note.
Put the value of
Now, Force = Tension and
Now, young's Modulus
Or Stress
Stress
Put the value of Stress in equation
2)
Put
Since fundamental frequency of a stretched spring
4. A bat is flitting about in a eave, navigating via ultrasonic beeps. Assume that the sound emission frequency of the bat is
Ans: Ultrasonic beep frequency emitted by the bat,
Velocity of the bat,
Where,
The apparent frequency of the sound striking the wall is given as:
This frequency is reflected by the stationary wall
The frequency (
5. A stone dropped from the top of a tower of height
Ans: Given data ,
Height of the tower,
Initial velocity of the stone,
Acceleration,
Speed of sound in air
The time
second equation of motion, as:
Time taken by the sound to reach the top of the tower,
Therefore, the time after which the splash is heard,
6. You have learnt that a traveling wave in one dimension is represented by a function
(a)
Ans: No;
(a) Does not represent a wave
(b) Represents a wave
(c) Does not represent a wave
The converse of the given statement is not true. The essential requirement for a function to represent a travelling wave
Explanation:
(a) Far
Hence, for
(b) For
Since the function does not converge to a finite value for
(c) For
Since the function does not converge to a finite value for
7: For the travelling harmonic wave
Where
(a)
Ans: Equation for a travelling harmonic wave is given as:
Where,
Propagation constant,
Amplitude,
Angular frequency,
Phase difference is given by the relation:
(a) For
(b) For
(d) For
8. A meter-long tube open at one end, with a movable piston at the other end, shows resonance with a fixed frequency source (a tuning fork of frequency
Ans: Given data,
Frequency of the turning fork,
Because one end of the supplied pipe is connected to a piston, it will behave as a pipe with one end closed and the other open, as illustrated in the diagram.
Such a system produces odd harmonics. The fundamental note in a closed pipe is given by the relation:
Where,
Length at the pipe,
The speed of sound is given by the relation:
9. A steel rod
Ans: Given data,
Length of the steel rod,
Fundamental frequency of vibration,
When the rod is plucked at its middle, an antinode (A) is formed at its center, and nodes (N) are formed at its two ends, as shown in the given figure.
The distance between two successive nodes is
The speed of sound in steel is given by the relation:
(e) A pulse is made up of many waves with varying wavelengths. Depending on the nature of the medium, these waves travel at varying speeds in a dispersive medium. The shape of a wave pulse is distorted as a result of this.
10. A train, standing at the outer signal of a railway station blows a whistle of frequency
Ans: (i) Given that,
(a) Frequency of the whistle,
Speed of the train,
Speed of sound,
The apparent frequency
the relation:
(b) The apparent frequency
(ii) The apparent shift in sound frequency is produced by the source and observer's relative motions. The speed of sound is unaffected by these relative motions. As a result, the speed of sound in air is constant in both circumstances. i.e,
11. A SONAR system fixed in a submarine operates at a frequency
Ans: Given that,
Operating frequency of the
Speed of the enemy submarine,
Speed of sound in water,
The source is at rest and the observer (enemy submarine) is moving toward it. Hence, the apparent frequency
The frequency (v") received by the enemy submarine is given by the relation:
where,
4 Marks Questions
1. A pipe
Ans: Given data ,
First (Fundamental); No
Length of the pipe,
Source frequency
Speed of sound,
In a closed pipe, the
Hence, the first mode of vibration frequency is resonantly excited by the given source.
In a pipe open at both ends, the nth mode of vibration frequency is given by the relation:
Since the number of the mode of vibration
2. Explain why (or how):
(a) In a sound wave, a displacement node is a pressure antinode and vice versa,
(b) Bats can ascertain distances, directions, nature, and sizes of the obstacles without any "eyes",
(e) A violin note and sitar note may have the same frequency. yet we can distinguish between the two notes.
(d) Solids can support both longitudinal and transverse waves, but only longitudinal waves can propagate in gases, and
(e) The shape of a pulse gets distorted during propagation in a dispersive medium.
Ans: (a) A node is a position where the vibration amplitude is the smallest and the pressure is the highest. An antinode, on the other hand, is a place where the vibration amplitude is greatest and the pressure is lowest.
As a result, a pressure antinode is nothing more than a displacement node, and vice versa.
(b) Bats produce ultrasonic sound waves at a very high frequency. Obstacles re-direct these waves back toward them. With the help of its cerebral sensors, a bat receives a reflected wave (frequency) and calculates the distance, direction, nature, and size of an obstacle.
(c) A sitar's and a violin's overtones, as well as the strength of these overtones, are different. Hence, one can distinguish between the notes produced by a sitar and a Violin even if they have the same frequency of vibration.
(d) Shear modulus is a property of solids. They are able to withstand shearing force. Fluids yield to shearing stress because they have no defined shape. A transverse wave propagates in such a way that it causes shearing stress in a medium. A wave of this nature can only propagate in solids, not in gases.
The bulk moduli of solids and fluids are different. They have the ability to withstand compressive stress.
3. One end of a long string of linear mass density
Ans: The equation of a travelling wave propagating along the positive
Linear mass density,
Frequency of the tuning fork,
Amplitude of the wave,
Mass of the pan,
Tension in the string.
The velocity of the transverse wave
Angular frequency
Wavelength,
displacement equation:
4. Earthquakes generate sound waves inside the earth. Unlike a gas, the earth can experience both transverse
Ans: Assume ,
Let
Let
We have:
L.
Where,
It is given that:
From equations (i) and (ii), we have:
It is also given that
And
From equation (ii), we get
Hence, the earthquake occurs at a distance of
5. A train, standing in a station-yard, blows a whistle ot frequency
Ans: Given that,
For the stationary observer:
For the running observer Not exactly identical
For the stationary observer:
Frequency of the sound produced by the whistle,
Speed of sound
Velocity of the wind,
As there is no relative motion between the source and the observer, the frequency of the sound heard by the observer wit be the same as that produced by the source, Le.
The wind is blowing toward the observer. Hence, the effective speed of the sound increases by 10 units, i.e.,
Effective speed of the sound,
The wavelength (
For the running observer:
Velocity of the observer,
The observer is moving toward the source. As a result of the relative motions of the source and the observer, there is a change in frequency
This is given by the relation:
since the air is still, the effective speed of sound
The source is at rest. Hence, the wavelength of the sound will not change, Le.,
Hence, the given two situations are not exactly identical
5 Marks Questions
1. Use the formula
(a) is independent of pressure,
(b) increases with temperature,
(e) Increases with humidity-
Ans: Given that,
(a) Take the relation:
Where,
Density,
Hence, equation(i) reduces to
Now from the ideal gas equation for
For constant
Since both
Hence, at a constant temperature, the speed of sound in a gaseous medium is independent at the change in the pressure of the gas.
(b) Now,
Tale the relation:
For one mole at an ideal gas, the gas equation can be written as:
Substituting equation (ii) in equation (i), we get:
Where,
We conclude from equation (iv) that
As a result, the speed of sound in a gas is proportional to the square root of the temperature of the gaseous medium, Le. In other words, the speed of sound increases as the temperature of the gaseous medium rises and vice versa.
(e) Now assume that,
And
Take the relation:
Hence, the speed of sound in moist air is:
And the speed of sound in dry air is:
On dividing equations
However, the presence of water vapor reduces the density of air, 1e.,
As a result, sound travels faster in moist air than in dry air. As a result, the speed of sound increases with humidity in a gaseous medium.
2. A transverse harmonic wave on a string is described by
Where
(a) Is this a travelling wave or a stationary wave?
If it is travelling, what are the speed and direction of its propagation?
(b) What are its amplitude and frequency?
(c) What is the initial phase at the origin?
(d) What is the least distance between two successive crests in the wave?
Ans: (a) Yes; Speed =
(b)
(c)
(d)
Explanation:
(a) The equation of a progressive wave travelling from right to Left is given by the displacement function:
The given equation is:
On comparing both the equations, we find that equation
Now, using equations
We know that:
Also,
Hence, the speed of the given travelling wave is
(b) Amplitude of the given wave,
Frequency of the given wave:
(c) On comparing equations
(d) The distance between two successive crests ơ troughs is equal to the wavelength of the wave.
Wavelength is given by the relation:
3. For the wave described in Exercise 15.8, plot the displacement
Ans: All the waves have different phases.
The given transverse harmonic wave is
For
Also,
Now, plotting
For
4. The transverse displacement of a string (clamped at its both ends) is given by
Where
Answer the following:
(a) Does the function represent a travelling wave or a stationary wave?
(b) Interpret the wave as a superposition of two waves travelling in opposite directions. What is the wavelength, frequency, and speed of each wave?
(c) Determine the tension in the string.
Ans: (a)The general equation representing a stationary wave is given by the displacement function:
This equation is similar to the given equation:
Hence, the given function represents a stationary wave.
(b) A wave travelling along the positive
The wave travelling along the negative
The superposition of these two waves yields:
The transverse displacement of the string is given as:
Comparing equations
Wavelength,
It is given that:
Frequency,
Wave speed,
(e) The velocity of a transverse wave travelling in a string is given by the relation:
Where,
Velocity of the transverse wave,
Mass of the string,
Length of the string,
Mass per unit length of the string
Tension in the string
From equation (i), tension can be obtained as:
5. A travelling harmonic wave on a string is described by
(a) What are the displacement and velocity of oscillation of a point at
(b) Locate the points of the string which have the same transverse displacements and velocity as the
Ans: (a) The given harmonic wave is:
Far
Where,
The velocity of the oscillation at a given point and time is given as
A
Now, the equation of a propagating wave ss given by:
Where,
And
speed,
Where.
Hence, the velocity of the wave oscillation at
(b) Propagation constant is related to wavelength as:
Therefore, all the points at distances
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Important Study Materials for Class 11 Physics
FAQs on Important Questions for CBSE Class 11 Physics Chapter 14 - Waves
1. Is Class 11 Physics Chapter 14 difficult?
All the chapters of Class 11 Physics require serious effort and striving hard on the part of the students. The students may find some topics difficult. But it is also an important chapter. Thus, you need to devote time, effort, and energy to understanding, reading and practising this chapter with all that you got. In fact, without sheer hard work, all the chapters of Class 11 Physics will seem challenging.
2. Why are waves important?
The area of "Waves" forms an important aspect of Physics. If you understand the workings of a wave, you will be able to explain various physical phenomena like the wave properties of matter. We encounter several waves in our daily lives including light waves, radio waves, microwaves, X-rays, etc. If you aspire to have a career in Engineering or a field of expertise related to Physics, then you need to have a good understanding of the working of waves.
3. What are some important questions from Chapter 14 “waves”?
As we have mentioned previously, Chapter 14 "Waves'' is an important chapter that carries significant marks in exams. Hence students should practice the important questions from this chapter, such as;
Why are all stringed instruments provided with hollow boxes?
Show that the Doppler effect in sound is asymmetric?
The chapter has many more important questions, which can be accessed from the page Important questions for Class 11 Physics and can also be downloaded at absolutely no cost.
4. Is “Waves” important for NEET?
"Waves" and "Oscillations" form an important part of the NEET syllabus. Wave mechanics usually carry around 4% of the weightage in NEET exams. This chapter is important also for CBSE exams and also for understanding basic concepts of Physics for Class 12th. So even NEET aspirants need to practice this chapter well. You can always refer to Vedantu's app to access the resources for help in exam preparation; be it for CBSE or NEET exams. You can find all the important questions from this chapter by visiting the page Important questions for Class 11 Physics.
5. What are beats?
When two sound waves of similar but different frequencies and amplitudes travel in the same direction, they form beats. Two slightly different frequencies having comparable amplitudes, superpose forming beats. The concept might appear complicated. But interestingly we experience beats around us in our lives very frequently. However, the fact that these beats are not physically visible makes them hard to comprehend. We experience beats while striking a fork or playing the piano. Beats are necessary for musicians.

















