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

$A$ symmetric thin biconvex lens is cut into four equal parts by two planes $AB$ and $CD$ as shown in the figure. If the power of the original lens is $4 \text{D}$, then the power of one part of the divided lens is: (in $\text{D}$)

$A$ convex lens forms a real image of a point object placed on its principal axis. If the upper half of the lens is painted black,the image will

When blue light is used instead of red light in a convex lens,its focal length will ......... .

If the aperture of a lens is halved,then the image will be:

When a biconvex lens of glass having refractive index $1.47$ is dipped in a liquid, it acts as a plane sheet of glass. This implies that the liquid must have a refractive index