Some water is filled in a container of height 30 cm . If is is to appear half filled to the observer when viewed from the top of the container, the height upto which water should be filled in it, is [Refractive index of water $=\frac{4}{3}$]

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