Why do stars twinkle?

If the refractive index of water with respect to air (${}_{a }\mu _w$)  is $\frac{5}{3}$.  Calculate the refractive index of air with respect to water (${}_{w }\mu _a$).

Light enters from air to glass plate having $1.50$ refractive index. What is the speed of light in glass? (Given-speed of light in vacuum $3\times {10}^8 \mathrm{m} {\mathrm{s}}^{-1}$)

Light enters from air to kerosene having refractive index of $1.47$. What is the speed of light in kerosene, if the speed of light in air is $3\times {10}^8 \mathrm{m} {\mathrm{s}}^{-1}$?

What is the relation between the refractive index of water with respect to air $\left({}_{a }\mu _w\right)$, and the refractive index of air with respect to water $\left({}_{w }\mu _a\right)$.

A fish seems to be at the depth of $75 \mathrm{cm}$ from the surface of a pond, then find out its actual depth from the surface. (Given-refractive index of water $\frac{4}{3}$)