How can you prove that a convex lens converges the sun rays to a single point?

When object is at infinity (sun) from the lens then, the image is formed at the focus. Place the convex mirror facing the sun, then light rays coming from the sun are parallel rays converges at the focus, if we keep a screen at the focus of lens then we see a bright spot at the focus.This small bright spot is the image of the sun at the focus shows that all the rays from sun that are falling on the lens are converges at is single point.