Question

A plane light wave travelling with velocity '$v$' in a medium A reaches a point on the interface of medium A and medium B. If velocity of sound in medium B is $2v$, the angle of incidence for total internal reflection of the wave will be greater than _____. $(\sin {30}^o=0.5$ and $\sin {90}^o=1)$

• A. ${15}^o$
• B. ${30}^o$
• C. ${45}^o$
• D. ${90}^o$

Answer 1

The correct option is B ${30}^o$

By Snell's law, we have:

$\mu =\frac{\sin i}{\sin r}=\frac{v}{2v}=0.5$

For, total internal reflection, the angle of incidence has to be greater than that value for which the angle of refraction becomes 90 degrees.

So, $\mu =\sin i=0.5⟹i=30°$. (Since, $\sin \frac{\pi }{2}=1$)