Dispersion of Light
Glass bends violet more than red, and a prism fans white light into a spectrum.
Lesson at a glanceShow the key points
- Refractive index depends on wavelength: approximately μ = μ₀ + A/λ², A a small positive constant (Cauchy).
- μ decreases as λ increases: largest for violet, smallest for red.
- So violet is deviated most by a prism, red least: white light is separated into colours. This is dispersion.
- Materials with higher μ (flint glass) also tend to have a larger spread of μ than low-μ ones (fluorite, fused quartz).
Newton let a beam of sunlight through a prism and saw it spread into a band of colours. The colours were already in the white light; the prism only separated them, because glass bends each colour by a slightly different amount.
Refractive index depends on wavelength
So far we have treated a material’s refractive index as a single number. In fact it changes a little with wavelength. A good approximation is
So μ falls as λ rises: in the visible range it is largest for violet and smallest for red. Materials with a larger refractive index, such as flint glass, also tend to have a larger spread of μ across the spectrum than low-index materials like fluorite.
Dispersion by a prism
Since each colour has its own refractive index, each bends by its own angle. In a prism, violet is deviated most and red least, so a beam of white light fans out into its colours. This separation of light into its component colours by a medium is called dispersion.
- 1Refractive index depends on wavelength: approximately μ = μ₀ + A/λ².
- 2μ is largest for violet, smallest for red.
- 3A prism deviates violet most and red least, separating white light: dispersion.
Check yourself
Keep exploring
Hand-picked videos, simulations and readings that explain this idea from a different angle.