Bihar Board - Class 12 Physics - Chapter 10: Wave Optics Short Answer Question
Short Answer Type Question
1. What are coherent sources of light?
Answer:
Coherent sources are those sources which emit continuously the waves of the same wavelength and amplitude either in the same phase or with a constant phase difference. This is possible if two sources are obtained out of a single source.
2. The polaroids are placed at 90° to each other and the transmitted intensity is zero. What happens when one of them is rotated?
Answer:
The intensity of light transmitted from the second polaroid increases from zero and will become maximum when the two polaroids are parallel to each other.
3. Why two independent sources cannot produce sustained interference?
Answer:
The two independent sources cannot act as coherent sources because the phase difference changes 108 times in one second and due to this fast variation of phase, the position of maxima and minima also varies, so the interference pattern is produced but undetected i.e. the two independent sources cannot produce sustained interference.
4. Light from two coherent sources is reaching the screen. If the path difference at a point on the screen for the yellow light be 32, then what will be the color of the fringe at that point?
Answer:
If the light is white, the fringes will be coloured, however, the yellow and neighboring colors shall be absent. If the source is emitting yellow light (i.e. monochromatic) there will be a dark fringe at that point.
5. Why can light travel in vacuum whereas sound cannot do so?
Answer:
Light waves are electromagnetic waves and hence do not require any material medium for propagation hence light can travel in vacuum. The sound waves are mechanical waves and material medium is a must for them to travel, hence they cannot travel in vacuum.
6. What should be the amplitude of the interfering beams to produce a completely dark fringe? Why?
Answer:
To produce a completely dark fringe the amplitude of the interfering beams should be equal because at destructive interference (i.e. for dark fringe) the amplitude of two beams is equal to the difference of the amplitudes of two superimposing waves. Thus for complete darkness, the amplitude must be zero. This is possible only when the amplitude of two beams are equal in magnitude.
7. Radio waves diffract pronouncedly around buildings, while light waves, which are electromagnetic waves, do not. Why?
Answer:
The phenomenon of diffraction is observed only when size of the obstacle is comparable to the wavelength of the wave. The wavelength of radio waves (particularly short waves) have wavelength comparable to general objects like buildings and hence easily get diffracted. But the wavelength of light is very small (3900 Å to 7800 Å). So they are not diffracted.
8. When a tiny circular obstacle is placed in the path of light from a distant source, a bright spot is seen at the center of shadow of the obstacle. Explain.
Answer:
The light waves diffracted from the edges of the circular obstacle interfere destructively at the center of the shadow producing bright spots at the center of the shadow.
9. What is the condition regarding the size of the obstacle for observing the diffraction?
Answer:
The size of the obstacle should be of the same order as the wavelength of light.
10. Coloured spectrum is seen when we look through a muslin cloth. Why?
Answer:
Muslin cloth is made of very thin threads which act as slits. White light passing through these slits is diffracted giving rise to coloured spectrum.
11. What is the difference between diffraction and interference?
Answer:
Differences between interference and diffraction:
Interference is the result of superposition of light from two different wavefronts produced by two coherent sources whereas diffraction is the superposition of secondary waves emitted from various points of the same wavefront.
Interference fringes are of the same intensity whereas intensity of diffraction fringes fall rapidly.
Interference fringes are equally spaced whereas diffraction fringes are unequally spaced.
Minimas of interference fringes are of zero intensity but in case of diffraction fringes, minimas are never perfectly dark.
12. Why does the electric vector in the electromagnetic waves determine the polarization rather than the magnetic vector?
Answer:
Since our eye is more sensitive to electric vectors than the magnetic vector, so the electric vector describes polarization as well as other phenomenon than the magnetic vector.
13. Does the value of polarizing angle for a transparent medium depend upon the wavelength of the light?
Answer:
Yes. Because polarizing angle ip and refractive index µ of the material of transparent medium are related by the relation µ = tan ip
14. A polariser and analyser are so oriented that the intensity of light transmitted is maximum. What fraction of maximum light is transmitted when the analyzer is rotated through an angle of 60°?
Answer:
From Malus law, we have
I = I0 cos2 θ
Here θ = 60°
∴ II0 = cos2 60° = 122 = 14
15. The phase difference between the light waves emerging from the slits of the Young’s experiment is π radian. Will the central fringe be dark or bright?
Answer:
The path difference between the waves reaching at the central fringe is zero, and if π radian is the initial phase difference between two waves, then the central fringe will be dark.
16. Can two electric bulbs of the same power and having filaments of the same material placed close to each other produce interference on the screen?
Answer:
No, the interference pattern will not be obtained on the screen because the two electric bulbs are not coherent sources and the path difference from two incoherent sources will change randomly. Thus sustained interference patterns will not be observed but there will be general illumination on the screen.
17. The polarizing angle of a medium is 60°. What is the refractive index of the medium?
Answer:
Since µ = tan ip = tan 60°.
So µ = √3 = 1.732.
18. Why is the diffraction of sound more evident in daily life than light waves?
Answer:
It is because the wavelength of sound is much greater than that of light waves and is comparable with size of the obstacle which is the essential condition for diffraction of waves.
19. Compare single slit diffraction patterns due to monochromatic light and white light.
Answer:
When the source of light is monochromatic, the diffraction pattern consists of alternate bright and dark bands of unequal width. The central bright fringe has maximum intensity. The intensity of secondary maxima falls off rapidly.
When the source is emitting white light, the diffraction pattern is coloured. The central maxima is white, but other bands are coloured. As band width ∝ λ, therefore, red band with higher wavelength is wider than the violet band with smaller wavelength.
20. Can white light produce interference? What is its nature?
Answer:
Yes, white light can produce interference. In this case, the central fringe will be white and first few coloured fringes will be formed and then the fringes of different colors overlap to produce general illumination, white in color.
21. Doppler’s effect in sound is different depending on whether the source or observer is in motion. However, in case of light, the Doppler’s shift is the same irrespective of whether the source or observer is in motion, explain how?
Answer:
The velocity of sound is comparable to velocity of source or observer, hence the velocity of sound depends on the motion of source or observer. But the velocity of light is very large as compared to the velocity of source or observer and in free space (or air) is independent of velocity of source or observer. Hence Doppler’s shift is the same irrespective of whether the source or observer is in motion.
22. What is plane polarized light?
Answer:
Plane polarized light may be defined as the light, in which the vibrations of the light (electric vector) are restricted to a particular plane.
23. Why is the central maxima is white with white light?
Answer:
For central maxima y = 0, n = 0 for all values of λ. Hence all coloured waves reach O in the same phase and make central maxima white.
24. Why are two bulbs lighting the same wall considered as incoherent sources? How do their intensities add up?
Answer:
The bulbs are incoherent sources because at any frequency, the radiation from one has no stable phase relation with the other. This is because independent atoms and electrons are responsible for the radiation for each source.
For two different frequencies, the interference term becomes zero
∴ the intensities of two bulbs get added up.
i.e., I= I1 + I2.
25. What information do we get about the nature of light by polarization?
Answer:
Polarization is possible only in transverse wave motion. So polarization of light means light is transverse in nature.
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