JEE Main 2014 · 6 Apr · Shift 1 · Physics Q25

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PhysicsSingle correct+4 / -1Question 25 of 90

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 30° makes the two beams appear equally bright. If the initial intensities of the two beams are $I_A$ and $I_B$ respectively, then $\dfrac{I_A}{I_B}$ equals :

\dfrac32

\dfrac13

Answer and worked solution

Official answer: D

After a $30^\circ$ rotation, Malus's law gives A: $I_A\cos^230^\circ$ and B: $I_B\sin^230^\circ$ . Equal brightness:
$\frac34I_A=\frac14I_B\implies\frac{I_A}{I_B}=\frac13.$

Topics:Polarisation and Brewster's law

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JEE Main 2014 · 6 Apr · Shift 1, Physics question 25