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A polarizer - analyser set is adjusted such that the intensity of light coming out of the analyser is just 10% of the original intensity. Assuming that the polarizer - analyser set does not absorb any light, the angle by which the analyser need to be rotated further to reduce the output intensity to zero, is:

1. 71.6°
2. 18.4°
3. 90°
4. 45°

Correct answer: 18.4°

Solution

With I = I0*cos^2(theta), output is 10% of input when cos^2(theta)=0.10 -> theta=71.6deg. To make output zero the analyser axis must be at 90deg, so it must be rotated a further 90-71.6 = 18.4deg.

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