Two identical thin plano-convex glass lenses (refractive index 1.5) each having radius of curvature of 20 cm are placed with their convex surfaces in contact at the centre. The intervening space is filled with oil of refractive index 1.7. The focal length of the combination is

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

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

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