When an object is placed infront of a convex mirror at a distance ' $u$ ' from the pole of the mirror such that the size of the image is ' $n$ ' times that of the object. Then, the object distance ' $u=$

The ratio of the focal lengths of a convex lens when kept in air and when it is immersed in a liquid is $1: 2$. If the refractive index of the material of the lens is 1.5 , then the refractive index of the liquid is

If the least distance of distinct vision for a boy is 35 cm , then the lens to be used by the boy for correcting the defect of his eye is

A light ray falls on a rectangular glass slab as shown in the figure. If total internal reflection occurs at the vertical face of the slab at point $B$, the refractive index of glass is