A ray of light travelling through rarer medium is incident at a very small angle ' $i$ ' on a glass slab and after refraction its velocity is reduced by $20 \%$. The angle of deviation is
A plano convex lens fits exactly in to a plano concave lens. Their plane surfaces are parallel to each other. Lenses are made up of different materials of refractive indices ' $\mathrm{n}_1$ ' ' $\mathrm{n}_2$ ' and ' R ' is the radius of curvature of the curved surface of lenses. Focal length of the combination is
The phenomenon, which is not due to total internal reflection is
An achromatic convergent doublet of two lenses in contact has a power +2 D . The convex lens has power +5 D . The ratio of the dispersive powers of the convergent and divergent lenses is (in magnitude)