1. According to stoke’s law, the expression for maxima is: 2μtcosr = ____________
a) nλ
b) 2nλ
c) (2n + 1) λ/2
d) (n + 1) λ/2
Explanation: Stoke’s relation states that when a light ray is incident on the interface of the optically denser medium, then the reflected wave suffers a phase change of π. Thus, for maxima 2μtcosr = (2n + 1) λ/2 and for minima 2μtcosr = nλ.
2. The interference pattern of soap bubble changes continuously.
a) True
b) False
Explanation: The top of the soap bubble is excessively thin, while the middle portion is thin and the bottom of the bubble is very thick. Thus, as the thickness varies, the interference pattern varies
3. When a thin plate of refractive index 1.5 is placed in the path of one of interfering beams of Michaelson Interferometer, a shift of 30 fringes is observed. If the thickness of plate is 0.018 mm, the wavelength of the used light is ___________
a) 4000 Å
b) 5000 Å
c) 6000 Å
d) 7000 Å
Explanation: Here, μ = 1.5, n = 30, t = 1.8 X 10-5 m
Path difference due to the glass plate = 2(μ – 1) t
2(μ – 1) t = nλ
λ = 2(μ – 1) t/n
= 2 X 0.5 X 1.8 X 10-5/30
= 6000 Å.
4. A thin layer of colorless oil is spread over water in a container (μ = 1.4). If the light of wavelength 640 nm is absent in the reflected light, what is the minimum thickness of oil layer?
a) 179.6 nm
b) 198.3 nm
c) 207.6 nm
d) 214.3 nm
Explanation: Let t be the thickness of the oil layer spread on water.
For minima, 2μtcosr = nλ
For minimum thickness, we take n = 1.
Therefore, tmin = λ/2μ
= 600 nm/2.8
= 214.3 nm.
5. In Newton’s ring experiment, the diameter of the 10th ring changes from 1.40 to 1.23 cm when a liquid is introduced between the lens and glass plate. What is the refractive index of the liquid?
a) 1.05
b) 1.15
c) 1.25
d) 1.35
Explanation: We know, Dn2 = 4nλR/μ
Without liquid in air, μ = 1, D102 = 40λR
With liquid, D102’ = 40λR/μ
Dividing both we get, μ = D102 D102’
= {1.40/1.27}2
= 1.25.
6. In Young’s Double Slit Experiment, if instead of monochromatic light white light is used, what would be the observation?
a) The pattern will not be visible
b) The shape of the pattern will change from hyperbolic to circular
c) Colored fringes will be observed with a white bright fringe at the center
d) The bright and dark fringes will change position
Explanation: When white light is used instead of monochromatic light in Young’s Double Slit experiment, all the seven constituent colors produce their interference pattern. At the center of the screen, all the wavelengths meet in phase and, therefore, a white bright fringe is formed. Then the next fringe will be formed due to violet color as the wavelength is shortest for violet color. This will be followed by indigo, blue till red color
7. Young’s Double Slit Experiment was conducted in ______________
a) 1802
b) 1889
c) 1925
d) 1930
Explanation: Interference of light was first discovered by Thomas Young, in 1802. He established the wave nature of light.
8. What kind of sources are required for Young’s Double Slit experiment?
a) Coherent
b) Incoherent
c) Intense
d) Bright
Explanation: For the interference pattern to be visible, coherent sources are required. If the sources are coherent, only then would there be constructive interference.
9. If the distance between the two slits is doubled, the fringe width _________
a) Doubles
b) Halves
c) Four-times
d) Remains same
Explanation: The fringe width is inversely proportional to the distance between the two slits. As the distance between the slits doubles, the fringe width becomes half of its original value
10. What change is observed when the whole pattern is immersed in water?
a) Fringe width decreases
b) Fringe width increases
c) Colored fringes
d) The bright and dark fringes will change position
Explanation: When the whole apparatus of Young’s Double slit experiment is immersed in water, the only change is the pattern shrinks a bit. The width of the fringes decreases, as an external path difference is present because of the water.