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Wave Optics

Question

Two beams, A and B, of plane polarised light with mutually perpendicular planes of polarisation are seen through a Polaroid. From the position when the beam A has maximum intensity (and beam B has zero intensity), a rotation of Polaroid through 300 makes the two beams appear equally bright. If the initial intensities of the two beams are IA and IB respectively, then I_A/I_B equals:

3

3/2

1

1/3

Solution

1/3

I_A cos² 30 = I_B cos² 60

straight I subscript straight A over straight I subscript straight B space equals space 1 third