A) \[\frac{1}{64}\]
B) \[\frac{1}{128}\]
C) \[\frac{1}{32}\]
D) \[\frac{1}{256}\]
Correct Answer: B
Solution :
After passing through first polarising plates unpolarised light becomes polarised and intensity becomes \[\frac{{{I}_{0}}}{2}\]. When polarised light passing through second polarizing plates intensity becomes \[\left( \frac{{{I}_{0}}}{2} \right)\,{{\cos }^{2}}\,\frac{\pi }{3}=\frac{{{I}_{0}}}{8}\] \[\left[ \because \,\,{{I}_{after}}\,={{I}_{before}}\,{{\cos }^{2}}\theta \right]\] After third polarising plates intensity becomes \[\frac{{{I}_{0}}}{8}{{\cos }^{2}}\,\frac{\pi }{3}\,=\frac{{{I}_{0}}}{32}\] After fourth polarising plates intensity becomes \[\frac{{{I}_{0}}}{32}\,{{\cos }^{2}}\,\frac{\pi }{3}\,=\frac{{{I}_{0}}}{128}\]
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