'Mirage' is a phenomenon due to:

Given that the velocity of light in quartz $=1.5\times {10}^8 \mathrm{m} {\mathrm{s}}^{-1}$ and velocity of light in glycerine $=\left(\frac{9}{4}\right)\times {10}^8 \mathrm{m} {\mathrm{s}}^{-1}$. Now a slab made of quartz is placed in glycerine as shown. The shift of the object produced by the slab is

A ray of light passes from vacuum into a medium of refractive index $\mathrm{n}$. If the angle of incidence is twice the angle of refraction, then the angle of incidence is :

A ray of light passes through four transparent media with refractive indices ${\mu }_1,{\mu }_2, {\mu }_3$ & ${\mu }_4$ as shown in the figure. The surfaces of all media are parallel. If the emergent ray $CD$ is parallel to the incident ray $AB$, we must have