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

Answer 1

Answer: (C)

Refractive index of glass,
${\mu }_g=\tan {\theta }_P$
where ${\theta }_P$ = polarising angle
$\Rightarrow {\mu }_g=\tan 60^{\circ }=\sqrt{3}$
Now , ${\mu }_g=\frac{c}{v_g}$
$\therefore \frac{c}{v_g}=\sqrt{3}$
$\Rightarrow v_g=\frac{3\times 10^8}{\sqrt{3}}$
$=\sqrt{3}\times 10^{8}m/s^{-1}$