In an equilateral prism the ray undergoes minimum deviation when it is incident at an angle of $50^{\circ}$. The angle of minimum deviation is

A person is observing a bacteria through a compound microscope. For better analysis and to improve the resolving power he should

A ray of light is incident normally on a glass slab to thickness 5 cm and refractive index 1.6. The time taken to travel by a ray from source of light to surface of slab is same as to travel through glass slab. The distance of source from the surface is

Focal length of objective of an astronomical telescope is 1.5 m . Under normal adjustment, length of telescope is 1.56 m . Focal length of the eyepiece is