# Important Short Questions and Answers of Physics for class 12 NEB

#### 1. How are stationary waves formed? [NEB 2055]

Stationary waves are those which cannot carry energy from one end to another. Their formation is possible when two waves of the same frequency traveling with the same velocity in opposite directions interact with each other. This makes the resultant velocity of the waves zero. However since energy is flowing in both directions, it gets locked in between the nodes. This makes their amplitude increase to very high values.

#### 2. Do sound waves need a medium to travel from one point to another point in space? What properties of the medium are relevant? [HSEB 2062]

Of course, a medium is necessary for the propagation of a sound wave. The medium must meet two requirements. First, the medium should have elasticity so that particles can return to equilibrium or mean position after having been disturbed. The medium must have inertia so that particles are able to maintain their previous state of rest or motion. (This is useful when particles move towards the mean position. Their inertia helps them in going beyond the mean position.)

#### 3. When water is poured into an extremely hot vessel, it gives a sizzling sound, why?

Such a phenomenon is more related to the sudden high pressure created in the water – vessel boundary. It is to be noted that sound is produced due to compression and rarefactions produced in the producing substance and then in the medium, which requires that there be some pressure difference. When water comes in contact with a hot vessel, water suddenly reaches boiling point there and turns to vapor. It tries to expand suddenly causing a large pressure. This pressure pushes neighboring atoms and suddenly starts a series of compressions and rarefactions which then give the impression of sound.

#### 4. What types of waves propagate in a liquid? Explain. [HSEB 2063]

Liquid molecules are mobile and also can easily slide (shear) over one another. Besides they are highly incompressible. So, inside the liquid, the waves are longitudinal, the velocity is higher than in air. At the surface, however, the waves are transverse.

#### If you are walking on the moon surface, can you hear the cracking sound behind you? [HSEB 2058]

The direct answer is ‘NO’. It is impossible to talk to each other. Sound propagates only through the material medium. There is no atmosphere on the moon. Therefore sound waves cannot travel there.

#### 6. Why do the stages of large auditoriums have curved backs (behind the speaker)?

Auditoriums are designed in such a manner that the sound waves produced by the speakers are heard over large distances. If their shape is concave, and if the speakers are at the focal point of such surface, then the waves are sent parallel after reflection. Such a voice can be heard at large distances.

#### 7. Differentiate between forced vibration and free vibration. [HSEB 2052]

When a body or a particle vibrates naturally without any external influence (except in the beginning), the vibration is called Free Vibration. E.g. The vibration of a rod. It depends on the inertial and elastic property of the vibrating body. Such vibration is often the result of the formation of standing waves in them. When a body is made to vibrate by means of an external factor or external vibrating agency, such vibration is called Forced Vibration. The innate properties of the suffering body do not have a role in the vibrations.

#### 8. What do you mean by Resonance?

When the externally imposed frequency and the natural frequency of a body are equal by some means, the amplitude of vibration increases very much. Such a condition is called Resonance. During the phenomenon, the amplitude can sometimes become so high that bodies can actually break or shatter due to the vibrations.

#### 9. What are the shock waves?

When an object traveling faster than the sound in the air then it leaves_ disturbances behind it which are conical in shape. Such disturbances are called Shock Waves. They are destructive because of a very high concentration of sound energy. When they are incident on something, they cause severe damage.

#### 10. Point out the difference between damped and un-damped oscillations.

Damped oscillations are the type of oscillations where the amplitude will continuously decrease over time. This decrease in amplitude is due to resistance forces such as air resistance.
If the oscillations occur with constant amplitude, they are called Undamped oscillations. They are possible if there were no resistance forces at all. If oscillations are undamped, their oscillations will continue up to infinite time.

#### 11. Why is some degree of reverberation considered useful?

Since times immemorial, humans have always encountered some degree of reverberations and echo in nature. When they exist in certain settings, the sound seems natural and pleasant. But when all of them are absent, the sound seems very artificial. Such sound is ‘, often called as being ‘acoustically dead’.

#### 12. When sound waves travel through a medium, does the temperature at various points remain constant? Explain. [HSEB 2072]

When sound propagates through a medium, at the compression region heat is produced, and the temperature increases but at the rarefaction region, temperature decreases. So if we consider the temperature at various points, it will be different. But the process of the formation of compression and rarefaction in the same region takes place so fastly that it will be difficult to notice the variation in temperature.

#### 13. Why is the loud sound heard at resonance?

This phenomenon is based on the amplitude of the vibrating substances; resonance, the forced and free vibration up-ness of sound depends around the same frequency y amplitude of the resultant vibrations will be the highest at t at moment. This makes the loudness also highest.

#### 13. Explain why soldiers are ordered to break steps while crossing a bridge.

A bridge is made of elastic substances and its different portions appear as stings fixed at both ends. So they have their natural frequency of vibration (roughly about 1Hz). (When anything (including the soldiers) travel through them in unison which is also roughly 1Hz), the imposed and natural frequency matches, making the amplitude very high which can collapse the bridge. If however, they walk randomly, the frequency varies greatly. This with the natural frequency and c frequency match can not cause e vibrations. high amplitudes. This makes the bride much safer to handle.

#### Sometimes, the glasses of the windows of a house are found to be shattered when large sounding bodies like airplane pass near the house. Explain how?

Every substance has its natural frequency of oscillation when it is set into vibrations, including glass. When the sound produced by anything matches this frequency, the amplitude of the vibration of glass will be very high, which brings shear to them. When the amount of shearing force exceeds the braking force, they crack. Sometimes the explosions can be such that the frequency of their sound matches this natural frequency, causing large scale vibration, at least for some time. This cracks them.

#### 15. One of the nine jewels of Emperor Akbar, widely known as Tanserk (the king of Music), was able to break the glass by singing appropriate notes. What physical phenomenon could account for this? [HSEB 2059]

Every substance has its natural frequency of oscillation when it is set into vibrations, including glass. When the sound produced by a person can match the frequency of the glass, the amplitude of the vibration of the glass walls will be very high, which brings shear to them. When the amount of shear exceeds the breaking point they crack. Revered Maestro Tansen had a voice of such matching frequency and he could utter such frequency for a long time while singing. This could ensure that the glasses crack. In the case of other people, either the frequencies do not match or even if they match, they can not emit that frequency for a long time to significantly amplify the vibrations.

#### 16. What is the relation between displacement nodes and antinodes to the pressure nodes and antinodes?

In a standing sound wave, a displacement node N will be a pressure antinode (a point where the pressure varies the most) whereas the displacement antinode A is a pressure node (a point where the pressure does not fluctuate at all). Therefore a pressure node is always a displacement antinode and a pressure antinode is always a displacement node.

#### 17. Why are bells made of metal and not of wood?

Bells are meant to produce specific sounds for long durations of time. This requires that they have low damping. Since wood produces very high clamping, it rapidly dampens the vibrations. On the other hand, metals have low damping. So, bells are made of metal and not of wood.

#### 18. Why do we hear more clearly in a curtained room than in a non-curtained room?

This phenomenon is also based on the phenomenon of absorption of sound waves by certain substances. Curtains are soft materials and are able to absorb sound waves. When sound waves are produced in such rooms, there is less chance of reflection, which prevents unnecessary interference and gives clearer sound. If there are no curtains, there will be random reflections and they interfere with the original sound. So, hearing is clearer in a curtained room than in a non-curtained one.

#### 19. A beating drum with a large surface area produces a loud sound as compared to the beating of drums with a small surface area.

This phenomenon is based on the surface area of the vibrating substances. One of the factors on which the loudness of a sound depends is the surface area of the area available for vibration. When drums with a large area are used, the vibration of its surface causes the vibration of a large area of air. This makes its sound appear loud. If drums with small areas are used, a small area of air vibrates and so the sound will be feeble.

#### 20. Why does the sound produce by a tuning fork become louder when the stem of the fork is pressed on the top of a table?

This phenomenon is based on the surface area of the vibrating substances. One of the factors on which the loudness of a sound depends is the surface area of the area available for vibration. When only the tuning fork is used, it causes vibration of air of a very small area, which makes it appear feeble. But when its stem is pressed against a tabletop, the whole of the top vibrates, which causes the vibration of a large area of air. This makes it appear loud.

#### 21. Why are temple bells made larger?

This phenomenon is based on the surface area of the vibrating substances. One of the factors on which the loudness of a sound depends is the surface area of the area available for vibration. When large bells are used, the vibration of its surface causes the vibration of a large area of air. This makes its sound appear loud. If small bells are used, a small area of air vibrates and so the sound will be feeble.

#### 22. A loudspeaker produces a louder sound than an earpiece does. Explain.

This phenomenon is based on the surface area of the vibrating substances. One of the factors on which the loudness of a sound depends is the surface area of the area available for vibration. When loudspeakers are used, the vibration of its surface causes the vibration of a large area of air. This makes its sound appear loud. If small earpieces are used, a small area of air vibrates and so the sound will be feeble.

#### 23. Why is the Sonometer box hollow from the inside? [HSEB 2055, 2066]

The main function of the sonometer is performing measurements related to sound, which requires that the sound is of high amplitude during the time of measurement. When the tuning fork is struck, it gives a feeble sound due to its small area of vibration. When its stem is placed on the hollow sonometer box, the walls as well as the air inside the box comes into vibration. Since the wall and the air can vibrate up to large ranges, the sound produced will be loud (because loudness depends on amplitude).

#### 24. Explain why the guitar is provided with hollow boxes. OR Explain why string type musical instruments are provided with hollow boxes.

The loudness of a musical sound is related to the surface area of the vibrating body. If the strings are vibrated then they will cause a vibration of less amount of air outside. This makes the sound feeble. If a hollow box is provided then the strings, as well as the air inside the box, is set into vibrations. As the box is of large surface, it transmits more energy into the air and loud sound is heard.

#### 25. Striking harder on a bell or a tuning fork produces a louder sound. Why?

This phenomenon is based on the amplitude of the vibrating substances; one of the factors on which the loudness of a sound depends on. When struck hard, the amplitude of vibration of either the bell or the tuning fork will be the highest. This causes very large amplitude vibrations of the air around and causes louder sound.

#### 26. A violin note and sitar note may have the same frequency, yet we can distinguish between the two notes. How?

This phenomenon is based on ‘Quality’ or ‘Timbre’ of sound which depends on the presence or absence of overtones in addition to the fundamental tone present in the note being produced. Any device can not produce totally pure note, it always has overtones. The relative number and intensities of these overtones make sound from the devices different.

#### 27. Two of your friends speak with the same frequency. How are you able to distinguish the voice of one from the other?

This phenomenon is based on ‘Quality’ or ‘Timbre’ of sound which depends on the presence or absence of overtones in addition to the fundamental tone present in the sound being produced. Any person can not produce totally pure frequency, it always has overtones. The relative number and intensities of these overtones make sound from the devices different. Two people who can produce the sound of the same fundamental, as well as the same number and intensities of overtones, are almost impossible on earth. So, the voice of one person can be distinguished from others.

#### 28. How is it that one can recognize a friend from his voice without seeing him/her? [HSEB 2056]

This phenomenon is based on the similarity or difference in frequency. The sounds from each people have different frequencies. Even if the frequency is the same (very rarely), the other harmonics present in their sound varies, which ultimately will vary the quality or timbre of the sound, making people distinguishable without looking.

#### 29. When a certain person talks over the phone with the microphone strapped with a handkerchief, his/her voice become indistinguishable, Le why?

The voice of a human voice is characterized by the number and amplitudes of the harmonics present in the fundamental frequency. o When a handkerchief is used, it also comes into vibration and its o fundamental frequency also adds up to the sound. So, the overall quality of sound is slightly altered, possibly to make it indistinguishable.

#### 30. Which is the most fundamental property of a wave? Why?

The most fundamental property of a wave is the frequency. This is because whatever be the change in other quantities as wavelength and velocity, frequency does not change and it continues its existence though it has gone through many processes and phenomena.

#### 31. Can sound waves be polarized? [HSEB 2060] OR Is polarization possible for longitudinal waves? Why [HSEB 2066, 067]

No, polarization is not possible for sound waves (because of their longitudinal nature). This is because of the very innate nature of sound waves. In a transverse wave, the particles vibrate in a plane perpendicular to the direction of propagation of energy. So, if appropriate gaps (slits) are used, the vibrations in some planes can be limited or totally eliminated, which makes polarization possible.

However, in longitudinal waves, the vibrations occur in (or against) the direction of the propagation of the waves. So the gaps used there will not limit or eliminate vibrations of any direction. So, polarization is not possible for them.

#### 32. Why does an empty vessel produce more sound than a filled one? [HSEB 2058, 069] OR An empty vessel sounds more than a filled one when it is struck. Why? [HSEB 2070 Set D]

This phenomenon is related to the ability of air or liquids to vibrate around their mean positions. An empty vessel contains an air column in it. When this vessel is struck then the air column comes into vibrations. Since air molecules are loosely held, they can vibrate around with large amplitude, which makes the sound large (Loudness depends on the amplitude of vibrations). However, if a filled vessel is stuck, the liquid molecules are set in vibrations (instead of air molecules), which can have smaller amplitudes only compared to that of air molecules, making the sound feeble. So, empty vessels produce more noise than the filled ones.

#### Explain with the figure the meaning of Beats and Beats Frequency. [HSEB 2065]

When two waves of nearly equal frequencies and similar amplitudes are sounded together the loudness of the sound increases and decreases periodically. This is known as the phenomenon of beats.

The time period of beats is given by T(beat)= 1/(f1-f2) which gives the beat frequency as f(Beats) = f1 – f2. If a person is to hear two events, the frequencies should be such that the time gap between them should be at least 0.1s. However, sometimes the frequencies are so nearby each other that the time period might become hours or days. It becomes impractical to record such beats. So, the value of the two frequencies should be such that the time period or the frequency falls in the audible range.

#### 34. Can two sound waves of equal frequency produce beats? Explain.

The time period of beats is given by T(beats) = 1/f1-f2 which gives the beat frequency as f(beats)= f1-f2. If the two sound waves have the same frequencies, f1 = f2, so the time period of beats will be infinity, or the frequency becomes zero. This means no beats occur altogether.

#### 35. In order to hear distinct beats, why should the difference in the frequencies of the two sources be small (less than 10Hz)?

If a person is to hear two events, the frequencies should be such that the time gap between them should be at least 0.1s. Since f = 1/T, the frequency should be at most 10Hz. This means it should be less than 10Hz.

#### 36. An airplane mechanic notices that the sound from a twin-engine aircraft varies in loudness when both engines are running. What could be causing this variation from loud to soft and vice versa?

This phenomenon is due to the formation of Beats. The engines of the plane will have slightly different frequencies whatever perfectly they have been designed. When both the engines operate, the waves from them undergo interaction and are able to produce beats, i.e. the overall amplitude sometimes becomes maximum and sometimes minimum. When only one engine operates, no such phenomenon exists because there is no interaction at all.

#### 37. When the same strings of two guitars are pluck, they give a sound which sometimes gets loud and sometimes feeble, why?

Whatever be the design, the strings of guitar vary somewhat in either thickness or purity of material or the tension. So, when they are pluck, they are bound to give slightly different frequencies according to the relation f1 = 1/2l√(T/m) where ‘l’ is the vibrating length, ‘T’ is the tension, and ‘m’ is the mass per unit length of the string. So, when their waves interact, they give rise to beats, which causes the amplitude (and thus loudness) to vary with time; sometimes becoming loud and sometimes feeble. [This phenomenon is utilized to compare the frequency of two wires and also checking the frequency of newly bought ware by sounding together with the old one]

#### 38. Why is the roaring of a lion different from the sound of a mosquito?

The main points of difference are in the pitch and loudness. The sound of a mosquito has a higher pitch (determined by frequency) than the roaring of a lion. In contrast, the lion roars loudly (with high amplitude) than a mosquito.

#### 39. In a machine run saw, a high pitch is heard initially and a low pitch sound is heard after the saw touches the wood. Explain why.

This phenomenon depends on the frequency of vibration of the wood particles when they are touched by the circular saw. Initially, before touching the wood, the speed and rotational frequency of the saw is high, as it is free to rotate. This rate goes on decreasing as the wood is gradually pressed into the saw since wood poses resistance to the movement of the saw. So, the rate at which wood particles vibrate also decreases, resulting in a decrease in frequency and pitch.

#### 40. Two loudspeakers have been stalled in an open space to listen to a speech. When both the speakers are in operation, the listener sitting at a particular place receives very feeble sound. What can be the reason for it?

This phenomenon is due to the interference of sound waves. When the difference in the distances of loudspeaker i.e. path difference of sound wave from the listener at a location is equal to the odd multiple of the wavelength, destructive interference occurs there. This will make the amplitude very less there. So, the listener receives a very feeble sound at that location.

#### 41. Why is a given sound louder in a hall (large closed room) than in the open?

When a sound is produced in a hall, the waves are reflected repeatedly from the walls and ceiling. The reflected sounds produce constructive interference with the direct sound at different locations; as a result, the intensity of sound increases. However, in open space, there are no such reflections and the intensity of the sound will be low there.

#### 42. What do you mean by ultrasonics and infrasonics? [HSEB 2054, 2055]

The frequency range of 20Hz to 20,000Hz is called Audible Range of Hearing. We cannot hear sounds having frequencies lying outside this range. The inaudible sound waves having frequencies greater 20,000Hz are called Ultrasonics and those having frequencies less than 20Hz are called Infrasonics.

#### 43. Explosions on other planets are not heard on earth. Why?

There are mainly two factors involved. One is the distance. From inverse square law, the intensity of sound is inversely proportional to the square of the distance, i.e. I ∝ 1/r2. Since the distances of the planets are very large from the earth, the intensity of sound on earth will be very less, so less that it would be very difficult to hear them. The other factor is the absence of any medium between them and earth. Sound needs a material medium to travel, the elastic, and inertia property of the substances playing their roles. Since there aren’t any such medium, the sound will thus not travel up to earth.

#### 44. If something goes wrong in a geostationary satellite and it explodes, can we hear the enormous sound of the explosion?

The answer is ‘NO’. One of the factors is the ‘distance’. From inverse square law, the intensity of sound is inversely proportional to the square 1 of distance, i.e. I ∝ 1/r2. Since the distances of the planets are very large from the earth, the intensity of sound on earth will be very less, so less that it would be very difficult to hear them. The other factor is the absence of any medium between them and earth. Sound needs a material medium to travel, the elastic, and inertia property of the substances playing their roles. Since there aren’t any such medium, the sound will thus not travel up to earth.

#### 45. As we move away from a sounding body, the sound we receive goes on decreasing. Why?

One of the factors on which the intensity of sound depends is the ‘distance’. From inverse square law, the intensity of sound is inversely 1 proportional to the square of the distance, i.e. I ∝ 1/r2. So when people move farther away from the source, the distance goes on increasing. This decreases the intensity of the waves and ultimately becomes inaudible.

#### 46. Why do you make your hand like a cup call your distant friend?

When sound travels freely in all directions, it follows inverse square law 1 and varies according to the relation i.e. I ∝ 1/r2So, the intensity decreases very rapidly. When the fingers are cupped, the sound energy is directed in a specific direction. Hence the intensity decreases with the distance to a lesser degree compared to inverse square law. So, sound can be heard over large distances.

#### 47. Two ears help in determining the direction from which sound is coming. How?

The main cause for this is the phase difference or path difference between the two waves reaching the two ears. When the same sound is perceived by the two ears, sound waves will travel larger distances to reach the farther ear compared to that needed for the nearer ear. This introduces a certain path or phase difference between the two waves. Such a difference helps in determining the direction of the source. Such a phenomenon is called the ‘Binaural Effect’.

#### 48. What would have happened if we have had only one ear?

When there are two ears, there exists a phase difference or path difference between the two waves reaching the two ears. When the same sound is perceived by the two ears, sound waves will travel larger distances to reach the farther ear compared to that needed for the nearer ear. This introduces a certain path or phase difference between the two waves. Such a difference helps in determining the direction of the source. Such a phenomenon is called the ‘Binaural Effect’. If there is only one ear, that is not possible.

#### 49. When a ball is hit hard, the sound produced is louder, why?

This phenomenon is based on the dependence of loudness on amplitude. When a bell is struck hard. then the amplitude of the vibrations is large right from the beginning. The loudness of sound is directly proportional to the amplitude. So the sound that is produced is louder.

#### 50. How can we distinguish between the music of a sitar from that of a harmonium without looking at the instruments when the sound is of the same pitch and loudness?

The quality of a note emitted by certain instruments depends on the presence or absence of overtones. Thus various instruments might have the same pitch or loudness, but the number and amplitude of overtones present in the note might be different. So the music produced can be easily distinguished.

#### 51. What is noise pollution?

Noise is any unwanted sound that produces a bad effect on our ears. Any unwanted sound at the wrong place and at the wrong time is called noise. When sound strikes the air drum in our ears, it exerts mechanical pressure on them due to which eardrum vibrates and sensation of sound is produced. If the amplitude of vibration of the air drum exceeds its normal value, the sound is felt to be noisy. Such unwanted and excess loudness of sound which produces unpleasant effects in ears is called noise pollution.