A point object kept at P in front of a glass sphere of radius ' $R$ ' has its image formed at $Q$ such that $\mathrm{PO}=\mathrm{QO}$. The refractive index of material of glass sphere is 1.4. The distance PO is equal to

A convex lens of refractive index $\frac{3}{2}$ has a power 2.5. If it is placed in a liquid of refractive index 2, the new power of the lens is

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