$A$ plano-convex lens is made of refractive index of $1.6$. The radius of curvature of the curved surface is $60 \,cm$. The focal length of the lens is (in $\,cm$)

The refractive index of the material of a double convex lens is $1.5$ and its focal length is $5 \,cm$. If the radii of curvature are equal, the value of the radius of curvature (in $cm$) is

When a plane wavefront is incident on a convex lens,the emerging wavefront becomes . . . . . . .

An object is placed first at infinity and then at $20 \ cm$ from the object-side focal plane of a convex lens. The two images thus formed are $5 \ cm$ apart. The focal length of the lens is $...... \ cm$.

The refractive index of a glass convex lens is $1.5$. The radius of curvature of each of the two surfaces of the lens is $40 \ cm$. The ratio of the power of the lens when immersed in a liquid of refractive index $1.25$ to that when placed in air is:

Explain the first focal centre (first principal focus) and second focal centre (second principal focus) for a biconvex lens and a biconcave lens with the help of diagrams.