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 ${30^ \circ }$ makes the two beams appear equally bright. If the initial intensities of the two beams are ${{\rm I}_A}$ and ${{\rm I}_B}$ respectively, then ${{{{\rm I}_A}} \over {{{\rm I}_B}}}$ equals: