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Lesson: Chapter 5 — Optics

5.3.2. Total internal reflection and the critical angle 4 of 4

Turning light with a prism

A glass prism with one 90° angle and two 45° angles can turn a ray of light through a right angle — by total internal reflection.

A ray entering a right-angled prism straight through one face, striking the long face at 45° and being totally internally reflected, then leaving straight through the third face — turned through 90°.
  1. The ray enters along the normal, so it is not refracted.
  2. It strikes the next face with an angle of incidence of 45°.
  3. The critical angle of glass is about 42°. 45° is larger, so the ray is totally internally reflected.
  4. It meets the third face along the normal and leaves without bending, turned through 90°.

Prisms like this are used in cameras, telescopes and binoculars.