$A$ 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, then

$A$ turnip sits before a thin converging lens,outside the focal point of the lens. The lens is filled with a transparent gel so that it is flexible; by squeezing its ends toward its center,you can change the curvature of its front and rear sides. Suppose that the image must be formed on a card which is at a certain distance behind the lens,while you move the turnip away from the lens,then you should

The sun subtends an angle of $0.5^o$ at the earth's surface. Its image is formed by a convex lens of focal length $50 cm$. The diameter of the image will be......$mm$.

The power of a biconvex lens is $1.25\,m^{-1}$ in a particular medium. The refractive index of the lens is $1.5$ and the radii of curvature are $20\,cm$ and $40\,cm$ respectively. Find the refractive index of the surrounding medium.

Radii of curvature of a converging lens are in the ratio $1 : 2$. Its focal length is $6 \,cm$ and refractive index is $1.5$. Then its radii of curvature are ......... respectively.

The distance between an object and its $3$ times magnified virtual image produced by a convex lens is $20 \,cm$. The focal length of the lens used is: (in $\,cm$)