$A$ 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 .... $D$

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 same size images are formed by a convex lens when the object is placed at $20\, cm$ or at $10\, cm$ from the lens. The focal length of the convex lens is ............ $cm$.

Parallel rays are incident on a thick plano-convex lens having radius of curvature $R$,refractive index $\mu$ and thickness $t$. When rays are incident on the plane surface,they converge at a distance $x$ from the plane surface. When rays are incident on the curved surface,they converge at a distance $y$ from the curved surface. Then:

If the radius of curvature of the convex surface of a plano-convex lens is $10 \ cm$ and its focal length is $30 \ cm$,what is the refractive index of the material of the lens?

When light rays from the sun fall on a convex lens along a direction parallel to its axis,which of the following statements is true?