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

• A. 3
• B. 44622
• C. 1
• D. 44564

Answer 1

Answer: (D)

44564

Answer 2

By law of Malus, $I=I_{0} \cos ^{2} \theta$ Now, $I_{A^{\prime}}=I_{A} \cos ^{2} \,30$ $I_{B'}=I_{B} \cos ^{2} \,60$ As $I_{A'}=I_{B'}$ $\Rightarrow I_{A} \times \frac{3}{4}=I_{B} \times \frac{1}{4}$ $\frac{I_{A}}{I_{B}}=\frac{1}{3}$