As polaroid examines two adjacent plane polarised beam A and B whose planes of polarisation are mutually perpendicular. In the first position of the analyser, beam B shows zero intensity. From this position a rotation `30^(@)` shows that thebeams have same intensity. The ratio of intensity of the two beam `I_(Z) & I_(B)`

Options

A. `1//2`
B. `1//3`
C. `3//2`
D. `0.83`

Answer

B

Explanation

From Malus law,
`I = (I_(0))/(2) cos^(2) theta`
In one position the beam B shows zero intensity
`:. 90` and for ba, A, `theta = 0`
When analyser is rotated through `30^(@)` then `theta = 30^(@)` for beam A and `theta = 60^(@)` for beam B.
Now, `I = (I_(A))/(2) cos^(2) 30 = (I_(B))/(2) cos^(2)60`
`(I_(A))/(I_(B)) = (cos^(2) 60)/(cos^(2) 30) = (1//4)/(3//4) = (1)/(3)`