Hold a glass of water up to a sunbeam, or catch the light off the back of a CD, and a whole rainbow spills out — red, orange, yellow, green, blue, violet — from what looked like plain, colourless sunlight. Where were all those colours hiding?
The answer is the secret behind this entire page: white light is not a single colour at all. It is a mixture of every colour of the spectrum blended together. Your eyes read that even blend as "white," but the colours are always in there, waiting to be pulled apart. Once you know that one fact, three everyday mysteries fall into place: why a rainbow appears, why a strawberry looks red, and why a red jumper turns black under a stage light. This page unpicks all three.
Send a narrow beam of white light into a triangular glass prism and it comes out the other side fanned into a band of colours — the same seven you see in a rainbow, always in the same order: red, orange, yellow, green, blue, indigo, violet. This spreading-out is called dispersion, and the band of colours is a spectrum.
Why does it happen? Each colour is light of a different
A rainbow is exactly this trick, done by raindrops instead of a glass prism. Sunlight enters each tiny droplet, disperses into its colours, bounces once off the back of the drop, and comes out spread into a fan. Millions of drops each send one colour towards your eyes, and together they paint the giant coloured arc.
That is why a rainbow always sits opposite the Sun (look for your own shadow — the rainbow is centred on the point directly across the sky from it), and why the colours always run in the same order, red on the outside and violet on the inside. It is also why you can never reach the end of a rainbow: it isn't a place, it's just the particular directions the split sunlight happens to reach your eyes from.
If white light already contains every colour, then the colour of an object can't come from the object making that colour — it comes from which colours it sends back to your eyes, and which it swallows.
Shine white light on a red strawberry. The strawberry's surface reflects the red light and absorbs all the other colours — orange, yellow, green, blue, violet — turning their energy into a tiny bit of heat. Only red bounces off and reaches your eyes, so the strawberry looks red. A blue jumper reflects blue and absorbs the rest; a green leaf reflects green and absorbs the rest. The colour you see is simply the colour (or colours) an object fails to absorb.
Here is where it gets clever. If an object can only reflect colours that are present in the light hitting it, then changing the light can change — or destroy — its colour.
Put that red strawberry under a pure green light. The strawberry can only reflect red… but there is no red in the light to reflect, and it absorbs everything else. So nothing bounces back to your eye and the strawberry looks black. The very same strawberry that glowed red a moment ago now sits there as a black lump. Under a red light it looks red and bright again; under blue, black once more.
A white object is the chameleon: because it reflects whatever hits it, a white shirt looks red under a red light, green under a green light, and so on — it simply wears the colour of the light. Play with all of this in the box below.
Choose the colour of the light, the colour of the object, and a filter to hold in front of the eye. Watch the beam: the lamp lights the object, the object reflects only the colours it can, and the filter lets only its colour through before the light finally reaches the eye. Whenever no colour survives the whole trip, the eye sees black. Try a red object in green light; then a red object seen through a blue filter.
A colour filter is a piece of transparent coloured plastic or glass. A red filter transmits (lets through) red light and absorbs every other colour. A green filter transmits green and absorbs the rest. So a filter works on light exactly the way a coloured object works on reflection: it keeps its own colour and swallows the others.
Shine white light through a red filter and pure red comes out the far side. Now stack a red filter and a blue filter together: the red filter first throws away everything except red, then the blue filter throws away that red too (blue filters absorb red). Nothing gets through — you get black. And look through a red filter at a blue object: the object only reflects blue, the red filter absorbs blue, so the object looks black. A filter can only ever remove colours from light — it can never create a colour that wasn't already there.
Dispersion pulls white light apart; you can also add colours together. The three primary colours of light are red, green and blue — mix them in the right amounts and you can make every colour your eye can see. Overlap two of them and you get a brighter secondary colour:
This adding of light is how every screen you own works. Look at a phone or TV under a magnifier and you'll see it is made of nothing but tiny red, green and blue dots. A patch that looks yellow is just red and green dots glowing side by side; a white patch is all three at full brightness; a black patch is all three switched off. Your eye does the mixing.
Both come from the scattering of sunlight by the air. Air molecules are far better at bouncing short-wavelength light (blue and violet) around the sky than long -wavelength red. So when you look up away from the Sun, the blue that's been scattered in every direction is what reaches your eye — the whole sky glows blue.
At sunset the light skims low through a much thicker slab of atmosphere on its way to you. By the time it arrives, almost all the blue has been scattered away sideways, and only the reds and oranges punch straight through — so the Sun and the clouds around it blaze red and gold. Same sunlight, same air; the colour you see just depends on how much atmosphere the light had to cross. Stage-lighting engineers use the same physics on purpose, clipping coloured gels (big filters) over spotlights to wash a scene in exactly the colour they want.