The image formed by a lens is due to which optical phenomenon?

When a light ray travels from an optically denser medium to an optically rarer medium, what is the angle of incidence called at which the angle of refraction becomes $90^{\circ}$?

What is the absolute refractive index of common glass?

The figure below shows the refraction and emergence of a ray of light incident on a rectangular glass slab. Copy the diagram and mark the lateral displacement of the incident ray. Name the two factors on which the lateral displacement depends.

$(a)$ What happens to a ray of light when it travels from one medium to another having equal refractive indices?
$(b)$ State the cause of refraction of light.

Light enters from air into diamond,which has a refractive index of $2.42$. Calculate the speed of light in diamond. The speed of light in air is $3.0 \times 10^{8} \text{ m s}^{-1}$.