Question:

A person is putting effort to determine the characteristics of an image formed by a converging lens by means of a ray tracing diagram. The person has sketched one principal ray. What is incorrect with the principal ray?

Ans:

For a converging lens there is certainly a principal ray that leaves the object and from there travels toward the plane of the lens along a line that passes through the focal point on the same side of the lens as the object. For the case of an object far from the lens than the focal point this ray will indeed pass through the focal point on its way to the plane of the lens.

At the plane of the lens this ray suddenly changes to a path that is parallel to the principal axis of the lens.