Total internal reflection
Beyond a critical angle, light travelling into a less dense material reflects entirely back rather than refracting through, which is how optical fibres trap light inside them.
Practise this conceptTotal internal reflection can only happen when light tries to leave a denser material for a less dense one — glass to air, or water to air, never the other way around. As the angle of incidence increases, the refracted ray bends further and further from the normal, and at some critical angle, it would have to refract at a full 90°, skimming right along the boundary.
Beyond that critical angle, refraction becomes impossible altogether, and 100% of the light reflects back into the denser material instead — not "most of it," but every last bit. There's no partial refraction lurking past the critical angle; the transition from "some light gets through" to "none does" happens right at that angle.
The critical angle itself isn't a universal constant — it depends on the specific pair of materials involved, since it's set by their refractive indices. This effect is what traps light inside optical fibres: light striking the fibre's inner wall at a shallow angle exceeds the critical angle every time, bouncing along the fibre's length for kilometres with essentially no light escaping.
Key ideas
Requires: Snell's law
Formulas
| Symbol | Name | Unit |
|---|---|---|
| refractive index | 1 (dimensionless) | |
| refractive index | 1 (dimensionless) | |
| angle | deg (degree) |
Common misconceptions
- Total internal reflection can happen when light travels from a less dense into a more dense material.
- Beyond the critical angle, some light still refracts through, just less of it.
- The critical angle is the same for every pair of materials.