A plane polarized light is incident on a polariser with its pass axis making angle θ with x-axis, as shown in the figure. At four different values of θ, θ = 8°, 38°, 188° and 218°, the observed intensities are same. What is the angle between the direction of polarization and x-axis? (a) 98° (b) 128° (c) 203° (d) 45°

Answer: (c). If the polarisation makes angle φ with the x-axis, I = I₁cos ²(θ - φ), which is symmetric about θ = φ and repeats every 180°. 8° and 38° give the same I, so they are symmetric about φ = (8° + 38°)/(2) = 23° (or 23° + 180°). Likewise 188° and 218° are symmetric about 203°, the same direction as 23°. So the answer is (c) 203°.

A plane polarized light is incident on a polariser with its pass axis… | JEE Main 2018 (15 Apr, Shift 2)

Q53JEE Main 2018 · 15 APR · SHIFT 2Medium

Polarisation

A plane polarized light is incident on a polariser with its pass axis making angle $\theta$ with x-axis, as shown in the figure. At four different values of $\theta, \theta$ = 8°, 38°, 188° and 218°, the observed intensities are same.

What is the angle between the direction of polarization and x-axis?

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Answer: (c)

If the polarisation makes angle $\phi$ with the x-axis, $I = I_1\cos^2(\theta - \phi)$ , which is symmetric about $\theta = \phi$ and repeats every $180^\circ$ .
$8^\circ$ and $38^\circ$ give the same $I$ , so they are symmetric about $\phi = \frac{8^\circ + 38^\circ}{2} = 23^\circ$ (or $23^\circ + 180^\circ$ ).
Likewise $188^\circ$ and $218^\circ$ are symmetric about $203^\circ$ , the same direction as $23^\circ$ .
So the answer is (c) $203^\circ$ .

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Polarisation question from JEE Main 2018 (15 Apr, Shift 2)

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