Ray diagrams?? (lenses)?

Question

why do you need them, how do you use them?

what do they look like?

Answer 1

Ray diagrams trace light rays from a source object to the lens and follow them as they refract (bend) through the lens, and show where they converge to form an image (either real or virtual).

Typically, we represent the source object as a vertical arrow, so that when we trace rays from the top and bottom of the arrow, it will be easy to see whether the resulting image is upright or inverted.