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Lesson: Chapter 5 — Optics

5.3.2. Total internal reflection and the critical angle 1 of 4

The critical angle

Going from a denser into a rarer medium, a ray bends away from the normal. Tilt it further and something new happens.

Light passing from water into air at a small angle of incidence, bending away from the normal.

As the angle of incidence in the denser medium is increased, the refracted ray leans further and further from the normal. At one particular angle, the refracted ray runs along the boundary between the two media: the angle of refraction has become 90°.

Light in water striking the surface at the critical angle c; the refracted ray skims along the surface.

Critical angle

The angle of incidence in the denser medium for which the angle of refraction is 90° is the critical angle (c).

Critical angles of some materials

Material Critical angle
Water 49°
Glass 42°
Diamond 24°