# 11th Physics Chapter 8 Waves Short Question Answers ## 11th Physics Chapter 8 Waves Short Question Answers Below

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1. what features do longitudinal waves have in common with transverse waves?
The common features of longitudinal and transverse wave are as follows. (i) in both types of waves, the particles oscillate (vibrate) about their mean position. (ii) Both types of waves transport energy from one place to another but no matter. (iii) In both types of waves, the relation between frequency, wavelength and speed of the waves is given by V=f λ (iv) Both longitudinal and transverse are mechanical waves. (v) Stationary waves can be studied with both type of waves.
2.Is it possible for for two identical waves travelling in the same direction along a stering to give rise to a stationary wave?
No, It is not possible because the two identical waves produce stationary waves only when they travel in opposite directions along the same string.
3. A wave in produce along a stretched string but some of its particles permanently show zero displacement. What type of wave is it?
This is stationary wave some of the particales of the medium remain permanently zero to show displacement. These points are called nodes.
4. Explain the terms, crest trough, node and antinode?
CREST:- In a transverse wave, it is the portion of wave it mean level (equilibrium position) TROUGH:- In a transverse wave, it is the portion of wave below its mean level (equilibrium position) NODE:- a node is a point in a vibrating body where the strain is maximum and amplitude is zero ANTINODE:- An antinode is a point in a vibrating body where the strain is minimum and the amplitude is maximum.
5. Why does sound travel faster in solides than is gases? 6.How are beats useful in tuning musical instrument?
We know that the number of beats produced per second is equal to the difference between the frequencies of two sounding bodies. By tuning a musical instrument we mean to set it to produce a note (sound) of desired frequency. For this purpose, we take a standard instrument of known frequency. The unturned musical instrument and the standard instrument are sounded together. At first the number of beats will be produced due to slightly frequency difference between them. The number of beats becomes equal to zero. When this happens, the musical instrument will produce the note of desired frequency and is said to be tuned. In this way, beats become useful in tuning a musical instrument.
7. As a result of a distant explosion, an observer sense a ground tremor and then hear the explosion?
The time difference is due to the difference between the speed of sound travelling through Earth and air. As the explosion occurs, the disturbance in the medium (air) is produced. Some of them travel through air while the other pass through the ground. But we know that sound travel faster in solids than in gases (air). Therefore an observer senses a ground termer first and then hears the explosion i.e. later.
8. Explain why sound travels faster in warm air than in cold air? 9. How should a sound source move with respect to the observer so that the frequency of its sound does not change?
According to Doppler’s effect, the apparent change in frequency of sound is produced due to relative motion of the source of sound and observer. If sound source and observer move in such a way that their relative velocity is zero, then the apparent frequency of the sound does not change. In other words, we can say that if sound source and observer move with same velocity along the same direction then there will be no change in frequency of sound due to zero relative velocity.
10. How wave length and frequency are related with the speed of propagation of a wave?
The relation of the speed of wave with its frequency and wavelength is given as velocity=frequency * wavelength. Let v be the velocity, λ be the wavelength and f be the frequency of the wave, then mathematically it can be written as v= fλ
11. what is effect of pressure on the speed of sound? 12. Two waves of slightly different frequencies moving in the same direction along the same line produce beats. How beats are produced?
The phenomenon of beats is a special example of interference of sound waves with respect to time at a fixed point. When there is a slight difference in the frequencies of the waves, then the phase difference at any point varies regularly with respect to time i.e. at that point the waves go on meeting alternately in intensity of sound at that point rises and falls alternately. These rises and falls are called beats.
13. A transverse wave traveling in a denser medium is incident on a rarer medium. How the phase of the wave is affected?
If a transverse wave traveling in a denser medium is incident on a rarer medium, it is reflected without any change in phase. Thus, in this case incident crest is reflected as a crest.
14. How the velocity of a wave will change if “Tension” is made 16 times? 15.Why the pitch of sound increases when a observer moves towards a stationary source?
If an observer moves with velocity U0 towards a soure at rest, then the relative velocity of the sound waves with respect to the observer will be v ―(- U0) = v+ U0, because the waves and the observer are moving in opposite direction, Where ‘v’ is the velocity of the sound. As the resultant velocity has increased i.e v+uo so the pitch od sound heard by the observer will increase anh hence higher sound will be heard by him.
16.Why Radar, cannot detect under water objects?
This is because electromagnetic waves are observed by water so they cannot penetrate into it.
17. Why stars moving towards Earth show a blue shift and those moving away show a red shift?
(i) It has been generally found that stars moving towards the Earth show a blue shift. This is become the emitted waves by the star have shorter wavelength than if the star had been at rest. So, the spectrum is shifted towards shorter wavelength i.e. to the blue end of the spectrum. (ii) It has been found that when stars moving away from the Earth, they show a red shift. This is because the emitted waves have a longer wavelength than if the stars had been at rest. So, the spectrum is shifted towards longer wavelength i.e. towards the red end of the spectrum.
18. Cleary explain the difference between transverse and longitudinal waves?
TRANSVERSE WAVES 1.It is that type of waves in which particles of a medium moved at right angle to the direction of propagation of waves . 2. The portion of medium above the mean level is called crest. (ii) The portion of medium below the mean level is called trough. 3. These waves consist of crests and through. LONGITUDINAL WAVES 1. The type of wave in which particles of the medium move along the direction of propagation of the waves. 2. The region where crowding of the particles of medium is maximum is called compression. (ii) The region where crowding is minimum is called rarefaction. (3) These waves consist of compression and rarefaction.
19. Explain how the speed of transverse wave in a string will change if its tension is made four times? 20. What are the condition of constructive and destructive interference?
Constructive interference:- The two waves interfere constructively if the crests and trough of one wave fall on the crests and trough of the other wave respectively. Similarly, if compressions and rarefactions and of one wave fall on the compression and rarefaction of the other wave respectively. This condition is satisfied only if the path difference between them is integral multiple of wave length. This is Path difference = nλ Where, n = 0,1,2,3,4,………… (ii) Destructive Interference:= The two waves are said to interfere destructively if crest one wave falls upon the trough of the other wave. This condition is satisfied only if the path difference between them is odd integral multiple of λ/2 That is ,. Path difference = (2n+1) λ/2 Where n = 0,1,2, 3,……………..
21. What is mean by the phase coherence? How does it affect the interference?
Phase Coherence:- When sources send waves at the same frequency and they produce crests at the same instant, they are said to have phase coherence. For example, two generators with the same period have phase coherence when they always dip into water together, producing crests at the same instant. If two wave generators do not have phase coherence, no definite interference pattern can be seen on the screen of the ripple tank.
22. Define longitudinal wave?
The wave of which the particles of the medium are displayed in a direction parallel to the direction of propagation of wave are called longitudinal wave are There waves are also called compressional wave. Example:- (i) Sound wave (ii) Compressional waves in the spring (iii) Railway engine connected to boggies with buffer springs in between them.
23. Define transverse waves?
Transverse waves are those in which particles of the medium are displayed in a direction perpendicular to the direction of propagation of waves. Example. (1) Radio waves (2) Light waves (3) Water waves (4) Microwaves (5) Waves produced in string
24. What are stationary waves?
When two waves of the same period, amplitude and speed travel along the same straight line in opposite direction, these waves superimpose to given rise to stationary waves.
25. What are beats?
When two tuning forks of slightly different frequencies are sounded together at the same time, the periodic alternation of sound between maximum and minimum loudness are called beats. The number of beats produced per second is equal to the difference between the frequencies of the tuning forks. Beats are produced as a result of interference. Uses:- (1)The beats can be used to tune a string instrument. (2) They can be used to find the unknown frequencies.
26.What is the principle of superposition?
When a particle of the medium is simultaneously acted upon by two or more waves, then the resultant displacement of the particle is the algebraic sum of their individual displacements. This is called principle of superposition.
Y= y1, y2,y3,………………………………………….. yn be the displacement due to individual waves, then the resultant ‘y’ is given by
Y = y1+y2+y3+………………………………………. yn
27. What is effect of pressure and density on the velocity of sound? 28. Why sound travels in hydrogen than that in oxygen? 29. What is Doppler’s effect?
The apparent change in the pitch of the sound caused by the relative motion of either the source of sound or the listener is called Doppler’s effect. This effect was observed by j. Doppler when he was observing the frequency of light emitted from distant stars. He found that the change in frequency of light depends upon the motion of star relative to Earth.
30. Write three uses of Dopplerls effect?
(i) In sonar, the Doppler effect is used in the detection and location of submarines, control of anti – weapons and depth measurement of sea. (ii) Astronomers use Doppler’s effect to calculate the speed of a star ang galaxy. (iii) In a radar system, the Doppler’s effect is applied in the working of a radar system. Radar uses radio waves to find the elevation and speed of an aeroplane. If an aeroplan approaches towards radar, then the wavelength of the wave reflected from aeroplane should be shorter and if it moves away, then wavelength would be longer.
31. What is the effect of temperature on the speed of sound? 32. Write the formula for speed of speed at o0C ?
Let v0 and vt be speed of sound at 00C respectively, then the relation between them can be written as
Vt =(V0+ o.61t)
Or          Vo = vt – 0.61t
This relation shows that one degree Celsius rise in temperature produces approximately 0.61 ms -1 increase in the speed of sound.
33. A transverse wave has a speed 200/sec. Find the wave –length λ of wave if frequency is kHz?
Speed = v= 200ms
Frequency =f= 1kHz= 1000Hz
V= fλ
Or     λ= v/f
putting the values, we get
λ= 200/1000= 0.2 m Ans
34. What are progressive waves?
The wave which transfers energy in moving away from the source of disturbance is called as progressive (or traveling) wave. Example. Drop a pebble into water. Ripples will be produced and spread out across the water. The ripples are the examples of progressive waves because they carry energy across the water surface.
35. What do you mean by interference?
When two waves of the same frequency traveling in the same direction meet each other in a medium, then at some points they reinforce the effect of each other and at some other points they cancel the effect of each other’s effect. This phenomenon is called interference. There are two types of interference (1) constructive interference. (2) Destructive interference
36. What is reflection of waves? Describe the general rules for reflection of transverse wave from the boundary of two different media?
The bouncing (turning) back of a wave from the boundary of two different media is called reflection of waves. The reflection of the waves from the boundary of two different media obeys the following rules. (1) If a transverse wave traveling in a rarer medium is incident, it is reflected such that it undergoes a phase change of 1800 or a path difference of λ/2 (2) If a transverse wave traveling in a denser medium is incident on a rarer medium, it is reflected without any change in phase.
37. What is Netwon’s formula of spend of sound and describe the Laplace correction in Newton’s formula for air? 38. What is the frequency of the wave in the first mode of vibration of stretched string? 39. Describe the law of transverse vibrations of stretched strings? 40. Prove that the open pipe is richer in harmonics that of closed pipe?

First case:-
When both ends of an organ pipe are open , it is called an open pipe.
In the case of open pipe, the frequency of any harmonic is given by
fn= nv/2/= nf1…………………. (1)
where n = 1,2,3,4,……………..
v = speed of sound in air and ‘/’ is the length of pipe.
f1 = Fundamental frequency.
The above equation (1) shows that all harmonic in an open pipe are of frequencies f1,2f1, 3f3,4f1 and so on. It means all harmonic are present in an open pipe.
Second case.
In a closed pipe, only odd harmonics are produced by using the equation
fn = n v/4l = nfl……………………….. (2)
where n= 1,3,5, ………………………………….
The above equation (2) shows that only first, third, fifth harmonics and so on are present in an closed pipe.
Conclusion.
Comparing both the case (1) and (2) we see that the pipe which is open at both ends is richer in harmonic (or overtones).

41. What do you understand by harmonic series?
The stationary waves can be set up on the string only with a discrete set of frequencies f1, 2f1, 3f1, 4f1…………………. Nf1
Which is called harmonic series. The lowest of these (i.e.f1) is called fundamental frequency and the others which are integral multiples of the fundamental are called harmonic or overtones.
42. Write characteristics of stationary waves?
(i) No energy is transferred from particle to particle in a stationary wave. (ii) All particles, except nodes perform SHM with the same period as the component waves (iii) At nodes, strain is maximum and the amplitude is zero. But at antinode, strain is minimum and the amplitude is maximum (iv) Distance between two consecutive nodes or antinodes is equal to half of the wave length (λ/2).(V) Distance between node and neighboring antinode is λ/4.
43.Discribe three important cases of superposition?
Following are the cases of superposition. (1) Interference. Two waves having same frequency and traveling in the same direction produce the phenomenon of interference (2) Two waves of slightly difference frequencies are traveling in the same direction produce beats. (3) Two waves of equal frequencies traveling in opposite directions produce stationary waves.
44. What are the factors upon which speed of sound in air depends? 45. Why are both odd harmonics produced in an open pipe?
When the pipe is open, antinodes are produced at both the ends of pipe. This is due to this reason that all the harmonics are produced (i.e. present) in an open pipe.
46. Why are both only odd harmonic produced in a closed pipe?
In the case of closed of pipe, one end is close while the other end is close open. As we know that at closed end, a node is produced and at open end , and an antinode is produced. This is way only odd harmonic are produced in closed open.
47. What are the type of organ pipes?
Following are two type of organ pipes. (i) open pipe (ii) closed pipe (i) Open pipe When both the ends of an organ pipe are open, it is called an open pipe. The open always acts as an anti- node (ii) closed pipe When one end of the pipe is closed, it is called closed pipe. Closed end of an organ pipe always acts as a node
48. Expain, how stationary waves are produced in air column organ pipes?
An organ pipe is an wind instrument. It consist of a hollow long tube with both ends open or with one end open and the other closed. When air is forced from one end, the enclosed air column is set into is vibrations. When the incoming wave arrives at the other end, it is reflected back and stars moving in the opposite direction. The superposition of incoming and reflected waves moving along the pipe give rise to the formation of stationary waves within the pipe. Due to the resonance between the forced air and stationary waves, sound is produced.
49. What are three kinds of waves?
Following are the three kinds of waves. (1) Mechanical waves (1) Electromagnetic waves (3) Matter waves
50. What are mechanical waves?
Mechanical waves. The waves which require a material medium for their propagation are known as mechanical waves. These waves propagate by the oscillation of material particles. Example:- (i) rope (ii) string (iii) coil of springs (iv) water (v) air are all mechanical waves.

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