The formula
n = c / v
| Symbol | Quantity | Unit |
|---|---|---|
| n | Refractive index | |
| v | Speed in the medium | m/s |
| θ₁ | Angle of incidence | ° |
| θ₂ | Angle of refraction | ° |
What it means
Light travels slower in matter than in a vacuum, and the refractive index is the ratio of the two speeds. That change of speed at a boundary is exactly what bends the ray, which is Snell's law.
How to work it out by hand
- Divide the speed of light in a vacuum by the speed in the medium
- The result has no units
- It is always at least 1 for ordinary materials
What is worth knowing
Water's index of 1.33 is why a straw looks bent in a glass and why a pool is shallower than it appears. Glass is around 1.5 and diamond 2.42, and diamond's exceptionally high value is what gives it its brilliance: it produces total internal reflection at only 24°, so light entering the top bounces around inside before coming back out. The index also varies slightly with wavelength, which is dispersion, and that is the whole reason a prism separates white light into colours.