$A$ biconvex lens has radii of curvature,$20 \ cm$ each. If the refractive index of the material of the lens is $1.5$,the power of the lens is .... $D$

For a thin convex lens, the radii of curvature are $15 \,cm$ and $30 \,cm$ respectively. The focal length of the lens is $20 \,cm$. The refractive index of the material is:

The focal length of a thin converging lens in air is $20 \ cm$. When the lens is immersed in a liquid,it behaves like a concave lens of power $1 \ D$. If the refractive index of the material of the lens is $1.5$,the refractive index of the liquid is

The sun subtends an angle of $0.5^{\circ}$ at the earth's surface. Its image is formed by a convex lens of focal length $50 \ \text{cm}$. The diameter of the image will be (in $\text{mm}$)

Photographs of the ground are taken from an aircraft flying at an altitude of $2000\; m$ by a camera with a lens of focal length $50\; cm$. The size of the film in the camera is $18\; cm \times 18\; cm$. The area of the ground that can be photographed by the camera is

An object and a screen are fixed on the uprights of an optical bench. The distance between them is $100 \, cm$. $A$ convex lens is placed in between the object and the screen and the position of the lens is so adjusted that the image of the object is formed on the screen at two conjugate positions of the lens. The distance between these conjugate positions of the lens is $40 \, cm$. What is the focal length of the lens in $cm$?