Critical angle of light passing from glass to air is minimum for

A light beam is travelling from region $\mathrm{I}$ to region $\mathrm{IV}$ (refer figure). The refractive index in regions $\mathrm{I},\mathrm{II},\mathrm{III}$ $\mathrm{IV}$ and are $n_0,\frac{n_0}{2},\frac{n_0}{6}$ and $\frac{n_0}{8},$ respectively. The angle of incidence $\theta$ for which the beam just misses entering region $\mathrm{IV}$ is

Assertion: In optical fibre, the diameter of the core is kept small.

Reason: This smaller diameter of the core ensures that the fibre should have incident angle more than the critical angle required for total internal reflection.

Assertion: There is no dispersion of light refracted through a rectangular glass slab.

Reason: Dispersion of light is the phenomenon of splitting of a beam of white light into its constituent colours.