Question

When the light comes from infinite but not parallel to principal axis then where the image will be formed when passing through a concave lens?

Answer 1

Assuming that the light is coming from infinity and the light rays are not parallel to the principal axis then the light after passing through the lens will tend to bend parallel to the principal axis, this means that when the light that is coming from infinity is directed to a specific point beyond the lens all the light rays will be bent exactly parallel to the principal axis, hence this point is called the focal point.

We always assume that any light coming from infinity will be parallel to the principal axis so that it is easy for us to draw the ray diagram, we say it is coming from infinity because we want the rays to be parallel.