Question

19. Two beams, A and B, of plane polarized light with
mutually perpendicular planes of polarization 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 ia and ib are Iand Irespectively, then..
iA/iB equals
a. 1
b. 1/3
c. 3
d. 3/2
(JEE Main 2014)​

Answer 1

Solution :

Initial position of the polarizer is 0° with A and 90° with B. Now when polarizer is rotated by an angle 30°, angle with A is 30° and with B is 90 - 30 = 60°. So the intensity after transmission is:

$\sf{I_{A1} =I_{A} cos^{2} 30}$

$\sf{I_{A1} = \frac{3I_{A}}{4}}$ - - - - (For A)

And,

$\sf{I_{B1} =I_{B} cos^{2} 60}$

$\sf{I_{B1} = \frac{I_{B}}{4}}$ - - - - (For B)

We know that:

$\implies$ $\boxed{\sf{I_{A} cos^{2}30 = I_{B} cos^{2}60}}$

So we have,

$\implies$ $\sf{\frac{3I_{A}}{4} = \frac{IB}{4}}$

$\implies$ $\sf{\frac{I_{A}}{I_{B}} =\frac{1}{3}}$

Therefore,

Correct option: (b) 1/3

___________________

Answered by: Niki Swar, Goa❤️

Answer 2

Correct answer: b) 1/3

Two beams, A and B, of plane polarized light with

mutually perpendicular planes of polarization 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 ia and ib are Iand Irespectively, then..

iA/iB equals.