Dispersion and chromatic effects

Snell's law describes refraction using a single refractive index, but that index is not quite a fixed property of a material — it depends slightly on the wavelength of the light involved, a phenomenon called dispersion. Blue light typically bends more than red light passing through the same glass, because glass's refractive index is slightly higher at shorter wavelengths. The effect is small for any single wavelength, but it is enough to matter whenever light of many wavelengths passes through the same interface at once.

White light is not a single colour that a prism somehow paints with new hues — it already contains the full range of visible wavelengths mixed together, and a prism does not add anything to it. What the prism does is separate what was already there: each wavelength refracts by a very slightly different angle at both faces of the prism, and after two refractions those small differences add up to a visibly spread fan of colour. Newton's classic demonstration — a second prism recombining the spread spectrum back into white light — makes the point directly: nothing was created or destroyed, only sorted by wavelength and then unsorted again.

Dispersion is not confined to prisms; any refracting element disperses light, including an ordinary lens. Because the lens's focal length depends on its refractive index, and that index varies with wavelength, different colours converge at slightly different focal points — a defect called chromatic aberration that blurs images taken with simple single-element lenses and fringes their edges with colour. Camera and telescope lenses correct for this by combining glasses with different dispersion properties so the colour errors of one element cancel those of another, a technique that would be unnecessary if refractive index really were a single fixed number for a material rather than a function of wavelength.