# MCQ Questions for Class 12 Physics Chapter 10 Wave Optics with Answers

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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: (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: (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: (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: (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: (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: (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: (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: (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

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: (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

Question 12.
Which of the following establishes the transverse nature of light.
(a) polarisation
(b) interference
(c) Diffraction
(d) refraction

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: (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

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: (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: (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: (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: (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: (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

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: (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: (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: $$\frac {1}{µ}$$ times, $$\frac {1}{µ}$$ times the wavelength

Question 2.
The half angular width θ, of the central maximum is given by ………………..

Answer: sin θ1 = $$\frac {λ}{d}$$

Question 3.
For studying the spectrum of ultra-violet region, the lenses and prisms are made of ………………..

Question 4.
Path difference = ……………….. x phase difference.

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: Minima, dark, third

Question 6.
The dark lines in solar spectrum are called ……………….. lines.

Question 7.
If the critical angle of a crystal is 45°, the polarising angle is ………………..

Question 8.
The refractive index is equal to the tangent of the angle of polarisation. It is called ……………….. law.

Question 9.
The wave theory of light in its original form was first postulated by ………………..

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: i = µA

Question 11.
In Young’s double slit experiment, the pringe width is minimum for ……………….. colour.

Question 12.
In propagation of light waves, the angle between the plane of vibration and the plane of polarization is ………………..

Question 13.
Light waves spreading from two sources produce interference only if they have ………………..

Answer: Same frequency and constant phase difference.

Question 14.
The refracting angle of a biprism is nearly equal to ………………..

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: 2 × 1010, 4000

Question 16.
Non, reflecting glass owes its property to ……………….. interference.