Question

When a spherical lens is held towards the sun and a sharp image is formed on a piece of black paper, a hole gets burnt in the black paper , after some time . 1) what name is given to the distance between the spherical lens and the black paper

Answer 1

The rays coming through the sun to convex lens is said to infinite and the distance from lens to paper is it's Focal length.

Answer 2

As per the question a sharp image of sun is formed on the paper, thus the image must be very small compared to the object which is situated at infinity compared to other parameters.

At the focal point only, this is possible to form an image from an object at infinity, thus the point at which the image is formed is Focus or Focal point, the black paper is Focal plane and the length between the image and the lens is Focal length.