# Engineering Physics Questions and Answers – Polarisation of Light

This set of Engineering Physics Multiple Choice Questions & Answers (MCQs) focuses on “Polarisation of Light”.

1. A linearly polarized wabe is always __________
a) In x-y plane
b) A Transverse wave
c) A Longitudinal wave
d) In y-z plane

Explanation: A linearly polarized is always a transverse wave, i.e., the displacement of each point is always at right angles to the direction of propagation of the wave.

2. The direction in which the electric vectors oscillate in a polarized wave is called as _______
a) Polarizing axis
b) Plane of polarization
c) Pass axis
d) Propagating axis

Explanation: When a light wave is polarized, the electric vectors along the direction of propagation are absorbed by the polaroid. Thus, the electric vectors oscillate perpendicular to the propagation of wave. This direction is known as pass axis.

3. Sound waves can be polarized.
a) True
b) False

Explanation: Longitudinal waves cannot be polarized as their direction is the same as its vibration. Thus, it’s intensity will not change as it passes through a rotating analyzer.

4. A window which can transmit all the incident light without any reflection is called as ___________
a) Polarized Window
b) Malus Window
c) Brewster Window
d) Non-reflecting window

Explanation: Brewster window is the practical use of Brewster law. When light is incident on a clean glass plate, most of the incident light (>> 92%) is transmitted while a small fraction is reflected.

5. If the phase difference between two rays is π/2 and the angle of incidence is equal to π/4, the emergent light is __________
a) Linearly Polarized
b) Elliptically Polarized
c) Circularly Polarized
d) Non-Polarized

Explanation: In this case, the equation of the emergent light becomes the equation of a circle, which is x2 + y2 = a2. Thus, the emergent light is elliptically polarized and the plane of the ellipse is normal to the direction of propagation.
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6. The velocity of light in water is 1.5 X 108 m/s. What is the polarizing angle of incidence?
a) 47.23°
b) 51.02°
c) 53.74°
d) 63.43°

Explanation: Refractive index of water = Speed of light in space/Speed of light in water
= 3/1.5
= 2
Using Brewster’s Law, Angle of incidence = tan-1µ
= tan-12
= 63.43.

7. An electromagnetic beam has an intensity 16 W/m2 and is linearly polarized vertically. What is the intensity of the transmitted beam, if the angle of incidence on the polaroid is 45° with the vertical?
a) 8 Wm-1
b) 12 Wm-1
c) 16 Wm-1
d) 20 Wm-1

Explanation: Here, Io = 16 Wm-2, θ = 45°
Using Malus law, I = Iocos2θ
I = 16 X cos245
I = 8 Wm-1.

8. What should be the thickness of quarter-wave plate for a light of wavelength 5000 Å if µe = 1.553 and µo = 1.544?
a) 1.38 X 10-3 cm
b) 1.43 X 10-3cm
c) 1.53 X 10-3cm
d) 1.63 X 10-3 cm

Explanation: $$t =\frac{λ}{4(μ_E-μ_0)}$$
Λ = 5000 Å = 5000 X 10-8 cm, µe = 1.553 and µo = 1.544
Therefore, t = 5000 Å X 10-8/4 X 0.009
= 1.38 X 10-3 cm.

9. A tube of length 20cm containing a 10% sugar solution rotates the plane of polarization by 13.2°. What is the specific rotation of sugar solution?
a) 66°
b) 55°
c) 44°
d) 33°

Explanation: The specific rotation is given by S = 10θ/lC
Here, θ = 13.2°, C = 0.1 g/cc and l = 20 cm
Therefore, S = 10 X 13.2/20 X 0.1
= 66°.

10. Unpolarized light is incident on a plane glass surface. What should be the angle of incidence such that the reflect and refracted rays are perpendicular to each other?
a) 90°
b) 45°
c) 57°
d) 60°

Explanation: Using Brewster’s Law, µ = tanip
In this case, i + r is equal to π/2.
For glass, µ = 1.5
Therefore, ip = tan-1µ = 57°.

11. A plate which induces the desired amount of phase difference between two rays is known as ____________
a) Polaroid
b) Phasor plates
c) Retardation Plates
d) Quartz plates

Explanation: Sometimes, it is required to induce a certain phase difference between E-ray and O-ray. For that, a plate with a specific thickness is chosen to induce that phase difference. This plate is known as the retardation plate as it retards the motion of one of the rays.

12. What does the following graph show?

a) Linearly Polarized Light
b) Unpolarized Light
c) Partially Polarized Light
d) Circularly Polarized Light

Explanation: The following graph shows a circularly polarized light. The vibrations are restricted in the x-y plane. Such a polarized light is obtained by using a polarizer/Polaroid and the phenomenon is called polarization of light.

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