The refractive index of water is $4/3$ and that of glass is $5/3$. What will be the critical angle for a ray of light entering water from glass?

Explain internal reflection and total internal reflection.

Assertion: Critical angle is minimum for violet colour.
Reason: Because critical angle $\theta_c = \sin^{-1} \left( \frac{1}{\mu} \right)$ and $\mu \propto \frac{1}{\lambda}$.

$A$ point source of light is placed at the bottom of a vessel,which is filled with water of refractive index $\mu$ to a height $h$. If a floating opaque disc has to be placed exactly above it so that the source is invisible from above,the radius of the disc should be:

Light travels from an optically denser medium $A$ into the optically rarer medium $B$ with speeds $1.8 \times 10^{8} \ m/s$ and $2.7 \times 10^{8} \ m/s$ respectively. The critical angle between them is ($\mu_{A}$ and $\mu_{B}$ are the refractive indices of media $A$ and $B$ respectively).

Assertion $(A)$: Propagation of light through an optical fibre is due to total internal reflection taking place at the core-clad interface.
Reason $(R)$: Refractive index of the material of the core of the optical fibre is greater than that of air.