a
Polariser \(A\) and \(B\) have same alignment of transmission axis. Lets assume polariser \(C\) is introduced at \(\theta \) angle \(\frac{1}{2}{\cos ^2}\,\theta \, \times {\cos ^2}\,\theta \, = \,\frac{1}{3}\) or, \({\cos ^4}\,\theta \,\, = \,\frac{2}{3}\, \Rightarrow \,\cos \,\theta \, = {\left( {\frac{2}{3}} \right)^{1/4}}\)