White light, fanned into colour

Dispersion is the splitting of white light into its separate colours, because each colour bends by a slightly different amount as it passes into a material like glass. A prism fans a white beam into a band of colour — a spectrum — running red, orange, yellow, green, blue, to violet. The colours are not made by the glass; white light is already a mixture of them all, and dispersion simply spreads them apart so each becomes visible. The same effect in raindrops paints rainbows and in a cut diamond creates its fire.

Because each colour of light travels at a slightly different speed inside the glass, so each has a slightly different refractive index and bends by a different amount. Violet light slows the most and bends the most; red slows the least and bends the least. As the white beam enters and again as it leaves the prism's two slanted faces, this colour-by-colour difference adds up, fanning the colours apart into a spectrum. A flat sheet of glass disperses too, but its parallel faces nearly cancel the spread, which is why a triangular prism works so well.

Violet bends the most and red the least. The reason is that violet light has the shortest wavelength and is slowed the most by glass, giving it the highest refractive index of the visible colours; red, with the longest wavelength, is slowed the least. So as white light refracts, violet is deflected most strongly and red least, with orange, yellow, green, and blue ranged in between. This fixed order is why a rainbow's colours always appear in the same sequence, red on the outer edge and violet on the inner.

They were already there. White light is a mixture of all the spectrum's colours travelling together; the prism does not add anything, it merely separates them. Newton proved this in 1666 by passing a spectrum through a second, reversed prism, which recombined the colours back into white light. If the glass were creating colour, a second prism could not have undone it. So dispersion reveals the colours hidden in white light rather than manufacturing them.

A rainbow is dispersion in raindrops. Sunlight refracts as it enters each drop, disperses because each colour bends by a slightly different amount, reflects off the back of the drop, and refracts once more on leaving. Violet bends most and red least, so the colours fan out. Each drop sends just one colour toward your eye from a particular angle, and millions of drops together build the full arc. Because the light leaves the drops back toward the direction it came from, the rainbow always appears in the sky opposite the Sun, centred on the shadow of your head.

In empty space there is nothing for light to interact with, so every colour travels at exactly the same speed, the famous speed of light, and no dispersion occurs — starlight reaches us still white. Inside glass, the light waves jostle the material's electrons, which re-radiate the light and effectively slow it. How much they slow it depends slightly on the wave's frequency, so each colour is slowed by a different amount. That tiny, colour-dependent slowing is what gives each colour a different refractive index and causes dispersion.

No — the blue sky is caused by scattering, a different effect. Air molecules scatter the shorter, bluer wavelengths of sunlight far more than the longer red ones, sending blue light in all directions so it reaches your eyes from across the whole sky. Dispersion separates colours by refraction inside a medium like glass or a raindrop; scattering redirects light by wavelength as it passes through air. The same scattering reddens sunrises and sunsets, because near the horizon the blue has been scattered out of the path before the light reaches you.

It is dispersion, here called chromatic aberration. A simple lens refracts each colour by a slightly different amount, so it brings red, green, and violet to a focus at slightly different points. The colours never quite line up, leaving coloured fringes around bright edges in the image. Cheap binoculars and telescopes show this clearly as colour edges on the Moon or a rooftop. Good optics fix it with achromatic lenses, which pair two kinds of glass whose dispersions cancel, bringing the colours back together.

Diamond has an unusually strong dispersion, meaning it separates the colours of light much more than ordinary glass. White light entering a well-cut diamond bounces around inside by total internal reflection and is dispersed along the way, so it emerges fanned into flashes of spectral colour — the prized "fire" of a diamond. Gem cutters shape the facets precisely to maximise both the internal reflection that returns light to the eye and the dispersion that splits it into colour, making the stone blaze.

The visible spectrum, red to violet, is only a thin slice of a far wider range of light. Just past red is infrared, which we feel as heat and which night-vision cameras detect. Just past violet is ultraviolet, which causes sunburn and makes some materials glow. Beyond these, the full electromagnetic spectrum stretches from radio waves and microwaves on the long-wavelength side to X-rays and gamma rays on the short side. All are the same kind of wave as visible light, differing only in wavelength; our eyes simply happen to see one narrow band.

The Lab bends a single disc as it crosses the glass slab, which is plain refraction — the bending of a path as it slows in a new material. Dispersion is that same bending, but happening by a slightly different amount for each colour of light. If the one disc were replaced by separate rays of red, green, and violet, they would each bend a little differently and fan apart into a spectrum, just like the colours in the prism lab. So the Lab demonstrates the mechanism behind dispersion, and the prism shows its colourful result.

A faint second rainbow sometimes appears outside the bright primary one, and its colours run in the opposite order — red on the inside, violet on the outside. It forms from sunlight that reflects twice inside each raindrop instead of once before leaving. The extra internal reflection flips the geometry, so the dispersed colours emerge at a different angle and in reversed order. The second reflection also loses some light, which is why the secondary bow is always dimmer, and the dark band between the two is called Alexander's band.

It only separates them; each colour keeps its own identity throughout. Red light stays red after passing through a prism — it is simply bent a little less than violet, so it lands in a different place. Dispersion does not turn one colour into another or invent new ones; it spreads out the colours that were already mixed in the white light. This is exactly why a second prism can gather them back into white: nothing was changed, only spread apart and then recombined.

By spreading starlight into a spectrum and reading it. An instrument called a spectroscope uses a prism or a fine grating to disperse a star’s light into its colours. Dark lines appear at exact wavelengths where the star’s own atoms have absorbed light, and each chemical element leaves a unique pattern of lines — a fingerprint. From these dispersed spectra, astronomers work out what distant stars are made of, how hot they are, and even how fast they move toward or away from us. Dispersion turns a faint point of starlight into a detailed report, carried across light-years.