Dispersion of White Light

Dispersion is the separation of white light into its component colours (the visible spectrum) using a prism.

How dispersion works

White light is a mixture of all visible wavelengths (colours).

When white light enters a glass prism:

1. Different wavelengths travel at different speeds in glass.
2. Different speeds mean different refractive indices for each colour.
3. Different refractive indices mean different angles of refraction.
4. The colours separate because they are refracted by different amounts.

The spectrum

The order from most refracted (bent) to least refracted:

Violet (most refracted, shortest wavelength, slowest in glass) to Red (least refracted, longest wavelength, fastest in glass).

Full order: Violet, Indigo, Blue, Green, Yellow, Orange, Red.

Key physics

• Violet light has the highest refractive index in glass, so it bends the most.
• Red light has the lowest refractive index in glass, so it bends the least.
• In a vacuum, all colours travel at the same speed ($3.0 \times 10^8$ m/s).

Recombining

A second prism, inverted, can recombine the spectrum back into white light. This proves that the prism does not “add” colour; it separates what was already there.

Common errors and how to correct them

• Saying the prism “creates” colours. The colours are already present in white light; the prism separates them.
• Getting the order wrong. Violet is bent most, red is bent least.

How to apply this in an exam

Explain that white light contains all visible wavelengths. State that different wavelengths have different refractive indices in glass. Explain that this causes different amounts of refraction, separating the colours.