The angle of a prism is A. One of its refracting surfaces is silvered. Light rays falling at an angle of incidence 2 A on the first surface returns back through the same path after suffering reflection at the silvered surface.

The refractive index $$\mu $$, of the prism is

If the focal length of objective lens is increased then magnifying power of

Two plane mirrors are inclined at 70^o. A ray incident on one mirror at angle, $$\theta $$ after reflection falls on second mirror and is reflected from there parallel to first mirror. The value of $$\theta $$ is

A plano convex lens fits exactly into a plano concave lens. Their plane surfaces are parallel to each other. If lenses are made of different materials of refractive indices $$\mu $$₁ and $$\mu $$₂ and R is the radius of curvature of the curved surface of the lenses, then the focal length of the combination is