# Physics Questions and Answers – Wave Optics – Huygen’s Principle

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This set of Physics Multiple Choice Questions & Answers (MCQs) focuses on “Wave Optics – Huygen’s Principle”.

1. Which among the following isn’t a suitable phenomenon to establish that light is wave motion?
a) Interference
b) Diffraction
c) Reflection
d) Polarization

Explanation: Light undergoes interference, diffraction, and polarization. These phenomena establish that light is a wave motion. Therefore, out of the options, reflection isn’t a suitable phenomenon to establish that light is wave motion.

2. The optical path of monochromatic light is the same if it travels 2 cm thickness of glass or 2.25 cm thickness of water. If the refractive index of water is 1.33, what is the refractive index of glass?
a) 2.5
b) 1.5
c) 3.5
d) 4.5

Explanation: Optical path = μ × Path in medium.
The optical path for glass = Optical path for water.
μg = $$\frac {(1.33 \times 2.25)}{(2.0)}$$
μg = 1.50.

3. Identify the condition which is not necessary for two light waves to be coherent.
a) The two waves must be continuous
b) The two waves should be of the same frequency or wavelength
c) They should have a constant or zero phases difference
d) They two light sources should be narrow

Explanation: The essential conditions for two light waves to be coherent includes that the two waves must be continuous, the two waves should be of the same frequency or wavelength, and they should have a constant or zero phases difference. So, the unnecessary condition for two light waves to be coherent is the statement that the sources should be narrow.
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4. The absolute refractive indices of glass and water are $$\frac {3}{2}$$ and $$\frac {4}{3}$$. Determine the ratio of the speeds of light in glass and water.
a) 5:7
b) 9:8
c) 7:5
d) 8:9

Explanation: $$\frac {v_g}{v_w} = \frac {\mu_w}{\mu_g}$$
$$\frac {v_g}{v_w} = \frac {(\frac {4}{3} )}{(\frac {3}{2}) }$$
$$\frac {v_g}{v_w}$$ = 8:9

5. The refractive index of glass is 1.5 and that of water is 1.3, the speed of light in water is 2.25 × 108 m/s. What is the speed of light in glass?
a) 7.95 × 108 m/s
b) 9.95 × 108 m/s
c) 1.95 × 108 m/s
d) 3.95 × 108 m/s

Explanation: $$\frac {v_g}{v_w} = \frac {\mu_w}{\mu_g}$$
Vg = $$(\frac {\mu_w}{\mu_g})$$ × vw
$$\frac {v_g}{v_w} = ( \frac {1.3}{1.5} )$$ × 2.25 × 108
$$\frac {v_g}{v_w}$$ = 1.95 × 108 m/s.
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6. In Huygens’ theory, light waves are longitudinal and do not require a material medium for their propagation.
a) True
b) False

Explanation: In Huygens’ theory, light waves are longitudinal and require a material medium for their propagation. That is why, Huygens’ assumed the existence of an all-pervading hypothetical medium, called ether.

7. The speed of light in air is 3 × 108 m/s. If the refractive index of glass is 1.5, find the time taken by light to travel a distance of 10 cm in the glass.
a) 0.5 × 10-10 s
b) 5 × 10-10 s
c) 50 × 10-10 s
d) 500 × 10-10 s

Explanation: vg = $$\frac {c}{\mu } = \frac {(3 \times 10^8)}{(1.5)}$$ = 2 × 108 m/s.
Time, t = $$\frac {Displacement}{v_g}$$
t = $$\frac {(10 \times 10^{-2})}{(2 \times 10^8)}$$
t = 5 × 10-10 s.

8. The speed of yellow light in a certain liquid is 2.4 × 108 m/s. Find the refractive index of the liquid.
a) 1.25
b) 5.55
c) 6.25
d) 12.25

Explanation: μ1 = $$\frac {c}{v_1}$$
μ1 = $$\frac {(3 \times 10^8)}{(2.4 \times 10^8) }$$
μ1 = 1.25.

Sanfoundry Global Education & Learning Series – Physics – Class 12.