Lesson: Chapter 5 — Optics

5.3.2. Total internal reflection and the critical angle 2 of 4

Total internal reflection

Increase the angle of incidence beyond the critical angle, and no light gets out at all: it is all reflected back into the denser medium.

Light in water striking the surface at more than the critical angle, reflected back down into the water with the angle of reflection equal to the angle of incidence.

Total internal reflection

When light travelling from a denser into a rarer medium strikes the boundary at an angle greater than the critical angle, it is reflected entirely back into the denser medium. This is total internal reflection.

i < c

The ray is refracted out into the rarer medium.

i = c

The refracted ray runs along the boundary.

i > c

Total internal reflection: the ray is reflected back into the denser medium.

Two conditions

Total internal reflection needs both: the light must be going from a denser medium towards a rarer one, and the angle of incidence must be greater than the critical angle.