Question

When the angle of incidence is 60? on the surface of a glass slab, it is found that the reflected ray is completely polarised. The velocity of light in glass is

• A. $ \sqrt{2}\times {{10}^{8}}m{{s}^{-1}} $
• B. $ \sqrt{3}\times {{10}^{8}}m{{s}^{-1}} $
• C. $ 2\times {{10}^{8}}m{{s}^{-1}} $
• D. $ 3\times {{10}^{8}}m{{s}^{-1}} $

Answer 1

Answer: (B)

Refractive index of glass, $ {{\mu }_{g}}=\tan {{\theta }_{p}} $ where $ {{\theta }_{p}}= $ polarising angle $ \Rightarrow $ $ {{\mu }_{g}}=\tan {{60}^{o}}=\sqrt{3} $ Now, $ {{\mu }_{g}}=\frac{c}{{{v}_{g}}} $ $ \therefore $ $ \frac{c}{{{v}_{g}}}=\sqrt{3} $ $ \Rightarrow $ $ {{v}_{g}}=\frac{3\times {{10}^{8}}}{\sqrt{3}} $ $ =\sqrt{3}\times {{10}^{8}}m/s $