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A beam of unpolarised light of intensity I₀ is passed through a polaroid A and then through another polaroid B which is oriented so that its principal plane makes an angle of 45° relative to that of A. The intensity of the emergent light is

1. I₀
2. I₀/2
3. I₀/4
4. I₀/8

Correct answer: I₀/4

Solution

When unpolarised light passes through the first polaroid (A), its intensity is reduced to half, resulting in I₀/2. As this light then passes through the second polaroid (B) at a 45° angle, the intensity is further reduced by a factor of cos²(45°), which is 1/2. Therefore, the final intensity is (I₀/2) * (1/2) = I₀/4.

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