A vessel contains water up to a height of $20\ cm$ and above it an oil up to another $20\ cm$. The refractive indices of the water and the oil are $1.33$ and $1.30$ respectively. Find the apparent depth of the vessel when viewed from above.

A fish is a little away below the surface of a table. If the critical angle is $49^o$, then the fish could see things above the water surface within an angular angle of

জলাশয়ের তলদেশ উহার প্রকৃত গভীরতার কতটুকু ? 

ABCD represents a glass slab and PQRS indicates the path of a light ray passing through the slab.If the refractive index of the glass is $\sqrt{3}$, find 'x'.

Three right angled prisms of refractive indices $\mu_1$, $\mu_2$ and $\mu_3$ are joined together ao that the faces of the middle prism are each in contact with one of the outside prisms. If the ray passes through the composite block undeviated, then