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Wave Optics Class 12 MCQs Questions with Answers
Question 1.
Interference was first observed in the interference chamber, when air was present, now the chamber is evacuated, and if the same light is used then a careful observer will see:
(a) no interference
(b) interference in which breadth of the fringe will be slightly increased.
(c) interference with bright bands.
(d) interference with dark bands.
Answer
Answer: (b) interference in which breadth of the fringe will be slightly increased.
Question 2.
Huygen’s wave theory of light can not explain
(a) Photoelectric effect
(b) interference
(c) diffraction
(d) polarisation
Answer
Answer: (a) Photoelectric effect
Question 3.
Wave optics is valid when characteristic dimensions are
(a) of the order of 1 mm.
(b) of the same order as the wavelength of light.
(c) much smaller than the wavelength of light
(d) much larger than the wave length of light.
Answer
Answer: (b) of the same order as the wavelength of light.
Question 4.
Ray optics is valid when the characteristic dimensions are
(a) of the order of 1 mm.
(b) of the same order as the wavelength of light.
(c) much smaller than the wavelength of light.
(d) much larger than the wavelength of light.
Answer
Answer: (d) much larger than the wavelength of light.
Question 5.
Interference pattern is obtained on a screen due to two identical coherent sources of monochromatic light. The intensity of the central bright fringe is 1. When one of the sources is blocked, its intensity becomes I0. The intensity in two situations is related as:
(a) I = 2I0
(b) I = I0
(c) I = 4I0
(d) I = 3I0
Answer
Answer: (c) I = 4I0
Question 6.
If a source of light is moved away from a stationary observer, then the frequency of light wave appears to change because of:
(a) interference
(b) diffraction.
(c) Doppler’s effect
(d) All of these
Answer
Answer: (c) Doppler’s effect
Question 7.
When seen in green light, the saffron and green portions of our. National Flag will appear.
(a) green
(b) black
(c) black and green respectively.
(d) green and yellow respectively.
Answer
Answer: (c) black and green respectively.
Question 8.
Light travels through a glass plate of thickness t and having a refractive index µ. If c be the velocity of light in vaccum, the time taken by light to travel this thickness of glass is
(a) \(\frac {t}{c}\) µ
(b) \(\frac {t}{µc}\)
(c) \(\frac {t}{µ}\) c
(d) tµc
Answer
Answer: (a) \(\frac {t}{c}\) µ
Question 9.
In Young’s Double Slit experiment using sodium light (λ = 5898 Å), 92 fringes are seen. If green colour (5461 Å) is used, how many fringes will be seen?
(a) 62
(b) 99
(c) 67
(d) 85
Answer
Answer: (b) 99
Question 10.
Young’s experiment established that:
(a) light consists of waves.
(b) light consists of particles.
(c) light consists of both waves and particles.
(d) none of these
Answer
Answer: (a) light consists of waves.
Question 11.
The condition for observing Fraunhoffer diffraction from a single slit is that the light wavefront incident on the slit should be:
(a) spherical
(b) cylinderical
(c) plane
(d) none of these
Answer
Answer: (c) plane
Question 12.
Which of the following establishes the transverse nature of light.
(a) polarisation
(b) interference
(c) Diffraction
(d) refraction
Answer
Answer: (a) polarisation
Question 13.
Light appears to travel in straight line since
(a) Its wavelength is very large.
(b) Its wavelength is very small.
(c) It is absorbed by atmosphere.
(d) none of these
Answer
Answer: (b) Its wavelength is very small.
Question 14.
If n represents the order of a half period zone, the area of this zone is nearly proportional to nm. where m is equal to
(a) 1
(b) \(\frac {1}{2}\)
(c) 2
(d) zero
Answer
Answer: (d) zero
Question 15.
Two coherent monochromatic light beams of intensities I and 4I are superimposed. The maximum and minimum possible intensities in the resulting beam are
(a) 9I and I
(b) 5I and I
(c) 9I and 3I
(d) 5I and 3I
Answer
Answer: (a) 9I and I
Question 16.
Two waves, orginating from sources S1 and S2 having zero phase difference and commonwave length λ, will show completely destructive interferene at a point Pif (S2P – S1P) is
(a) \(\frac {11}{2}\)λ
(b) 2λ
(c) 5λ
(d) \(\frac {3}{4}\)λ
Answer
Answer: (a) \(\frac {11}{2}\)λ
Question 17.
The principle of super position in wave motion tells that in a motion in which two or more waves are simultaneously producing their displacement in a particle, then the resultant:
(a) amplitude is the sum of the individual amplitudes.
(b) amplitude is the difference of the individual amplitudes.
(c) displacement is the vector sum of the individual displacements.
(d) none of these
Answer
Answer: (c) displacement is the vector sum of the individual displacements.
Question 18.
A diffraction pattern is obtained by using a beam of red light. What will happen if the red light is replaced by the blue light.
(a) No change will take place.
(b) Bands become broader and farther apart.
(c) Bands disappear
(d) Diffraction bands become narrower and crowded.
Answer
Answer: (d) Diffraction bands become narrower and crowded.
Question 19.
Interference takes place due to
(a) Amplitude change
(b) Velocity change
(c) Intensity
(d) Phase difference.
Answer
Answer: (d) Phase difference.
Question 20.
The frequency of e.m. wave which is best suited to observe a particle of radius 3 × 10-4 cm is of the order of
(a) 1014
(b) 1013
(c) 1012
(d) 1015
Answer
Answer: (d) 1015
Question 21.
One important similarity between sound and light waves is that both:
(a) can pass through even in the absence of any medium.
(b) can show interference effects.
(c) are transverse waves.
(d) travel at the same speed in air.
Answer
Answer: (b) can show interference effects.
Question 22.
Two waves of the same frequency and same intensity are super impossed in the same phase. The intensity of the resultant wave at the Central point will be
(a) equal to that of the individual wave.
(b) 3 times that of individual wave.
(c) 2 times that of individual wave.
(d) 4 times that of individual wave.
Answer
Answer: (d) 4 times that of individual wave.
Fill in the Blanks
Question 1.
Width of the central diffraction maximum on immersing the apparatus in a denser medium of refrictive index µ becomes ……………….. its width in air and wavelength becomes ………………..
Answer
Answer: \(\frac {1}{µ}\) times, \(\frac {1}{µ}\) times the wavelength
Question 2.
The half angular width θ, of the central maximum is given by ………………..
Answer
Answer: sin θ1 = \(\frac {λ}{d}\)
Question 3.
For studying the spectrum of ultra-violet region, the lenses and prisms are made of ………………..
Answer
Answer: quartz.
Question 4.
Path difference = ……………….. x phase difference.
Answer
Answer: \(\frac {2π}{λ}\)
Question 5.
The path difference between two waves at a pointon the screen for a wavelength λ, is \(\frac {5π}{2}\). The point of the screen is ……………….. and the order of ……………….. band is ………………..
Answer
Answer: Minima, dark, third
Question 6.
The dark lines in solar spectrum are called ……………….. lines.
Answer
Answer: Fraunhoffer.
Question 7.
If the critical angle of a crystal is 45°, the polarising angle is ………………..
Answer
Answer: tan-1 √2
Question 8.
The refractive index is equal to the tangent of the angle of polarisation. It is called ……………….. law.
Answer
Answer: Brewster’s.
Question 9.
The wave theory of light in its original form was first postulated by ………………..
Answer
Answer: Christian Huygens.
Question 10.
A ray of light incident at an angle of incidence i on one surface of a prism of small angle A and emerges normally from the opposite surface. The refractive index of the prism material is µ. The angle of incidence i and θ angle of prism are related as ………………..
Answer
Answer: i = µA
Question 11.
In Young’s double slit experiment, the pringe width is minimum for ……………….. colour.
Answer
Answer: Violet.
Question 12.
In propagation of light waves, the angle between the plane of vibration and the plane of polarization is ………………..
Answer
Answer: 90°
Question 13.
Light waves spreading from two sources produce interference only if they have ………………..
Answer
Answer: Same frequency and constant phase difference.
Question 14.
The refracting angle of a biprism is nearly equal to ………………..
Answer
Answer: 180°
Question 15.
A light of frequency 5 × 1014 Hz, enters a medium of refractive index 1.5. In this medium the velocity of light wave is ……………….. cm/s and it wavelength is ……………….. Å.
Answer
Answer: 2 × 1010, 4000
Question 16.
Non, reflecting glass owes its property to ……………….. interference.
Answer
Answer: Destructive.
Question 17.
Fringe width of the fringes is ……………….. when the apparatus is immersed in water.
Answer
Answer: decreased.
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