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Lesson: Chapter 5 — Optics

5.4.1. Convex lenses 4 of 4

Three useful rays

Three rays through a convex lens are easy to draw, and any two of them find an image.

(a) Through the optical centre

A ray through the optical centre passing straight through.

A ray through the optical centre goes straight through, without being refracted.

(b) Parallel to the axis

A ray parallel to the axis refracted through the focus F on the far side.

A ray entering parallel to the axis passes through the focus on the other side.

(c) Through the near focus

A ray through the focus on the near side refracted parallel to the axis.

A ray that passes through the focus on one side and strikes the lens leaves parallel to the axis.

Same three, like the mirror's

Optical centre → straight on. Parallel → through the far focus. Through the near focus → parallel. These are the rays used on every image page that follows.