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JEE Main 2021 (26 Aug, Shift 2) · Polarisation

Polarisation question from JEE Main 2021 (26 Aug, Shift 2)

Q45JEE Main 2021 · 26 AUG · SHIFT 2Medium

Polarisation

A source of light is placed in front of a screen. Intensity of light on the screen is I. Two Polaroids P1 and P2 are so placed in between the source of light and screen that the intensity of light on screen is I/2. P2 should be rotated by an angle of (degrees) so that the intensity of light on the screen becomes 3I8\frac{3I}{8}.

Numerical answer type. Work it out, then open the answer.

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Answer: 30

  1. After P1P_1: I/2I/2. Screen intensity is still I/2I/2, so P2P_2 is parallel to P1P_1.
  2. Malus's law after rotating P2P_2 by θ\theta: I2cos⁡2θ=3I8\frac{I}{2}\cos^2\theta = \frac{3I}{8}
  3. cos⁡2θ=34⇒cos⁡θ=32\cos^2\theta = \frac{3}{4} \Rightarrow \cos\theta = \frac{\sqrt{3}}{2}
  4. So θ=30∘\theta = 30^\circ.
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