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PhysicsNEET 2025Diffraction and polarisation

Wave Optics

The intensity of transmitted light when a polaroid sheet is placed between two crossed polaroids at 22.5∘22.5^\circ from the polarisation axis of one of the polaroids is (I0I_0 is the intensity of polarised light after passing through the first polaroid):

  1. A.I02\dfrac{I_0}{2}
  2. B.I04\dfrac{I_0}{4}
  3. C.I08\dfrac{I_0}{8}✓
  4. D.I016\dfrac{I_0}{16}

Solution

First polaroid → I0I_0. The middle sheet at 22.5∘22.5^\circ: I0cos⁡222.5∘I_0\cos^2 22.5^\circ. The last, crossed with the first, is at 67.5∘67.5^\circ to the middle sheet: I0cos⁡222.5∘cos⁡267.5∘=I0(sin⁡45∘2)2=I08I_0\cos^2 22.5^\circ\cos^2 67.5^\circ = I_0\left(\frac{\sin45^\circ}{2}\right)^2 = \frac{I_0}{8}.

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