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