Question

When the angle of incidence is $60^\circ$ 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. $3\times10^{8}{ms^{-1}}$
• B. $2\times10^{8}{ms^{-1}}$
• C. $\sqrt {3} \times10^8 {ms^{-1}}$
• D. $\sqrt {2} \times 10^8 {ms^{-1}}$

Answer 1

Answer: (C)

$\sqrt {3} \times10^8 {ms^{-1}}$

Answer 2

Refractive index of glass, $\mu_g = \tan \; \theta_P$ where $\theta_P$ = polarising angle $\Rightarrow \mu_{g} =\tan60^{\circ} =\sqrt{3}$ Now , $\mu_{g} = \frac{c}{v_{g}}$ $\therefore \frac{c}{v_{g} } = \sqrt{3}$ $\Rightarrow v_{g} = \frac{3 \times10^{8}}{\sqrt{3}}$ $=\sqrt{3 } \times 10^{8} \,m/s^{-1}$