Dispersa

Meet Dispersa, a prism who fans white light out into a rainbow.

Light slows down in glass, by a factor called the refractive index n. In most transparent materials n is not a single number: it depends on wavelength, usually being larger for shorter wavelengths. This is dispersion. Each colour is therefore bent by a different amount at a glass surface, and a prism, with its angled faces, spreads the colours apart.

In common crown glass n is roughly 1.51 to 1.53 across the visible spectrum, slightly higher for violet than for red. That small difference is enough for violet to bend most and red least.

Isaac Newton used prisms in the 1660s to show that white light is a mixture of colours. Rainbows come from dispersion in raindrops, and spectrometers use prisms or gratings to analyse light from stars and chemical samples.

Dispersion is also a nuisance. It makes simple lenses focus colours at different points, which lens designers correct by combining glasses. In optical fibres it smears out pulses, limiting data rates.