PHYS102: Introduction to Electromagnetism

Lenses work because light rays are refracted once on the way into the glass, and a second time on the way back out. But in the thin-lens approximation, the two surfaces are so close together that you do not have room to draw the two refraction events individually. Instead, one uses a trick.

Watch this video, which details how this trick works. It is based on the idea that rays going through the exact center of a lens do not suffer any refraction at all, whereas light rays coming into the lens parallel to the optical axis must be refracted precisely through the focal point. This is why in the ray diagrams for a lens, we typically draw only two rays (out of the infinitely many rays coming from the object) to completely characterize what the lens does.