Question

A professor is trying to quickly estimate the focal length of a 2.5 cm diameter biconvex lens.  He takes the lens and finds the image of the overhead ceiling lights on his table to be 3 cm below the lens, so he estimates the focal length as about 3 cm.   His student objects that at the focus there should be NO image formed, so this must be wrong.   Why can the prof make this approximation successfully?

Answer 1

Convex lens is a converging lens, so it focuses the rays coming parallel to its principal axis at a point to form a sharp image, which also becomes the focus of the lens. According to the definition of focal length, which is the distance between the lens and its focus, so professor's approximation is correct. We can assume rays coming from ceiling lights as parallel to the principal axis. The ceiling at the top and table below the lens gives a nice setup for forming the image by the convex lens. Distance between lens and table will give focal length.