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Your Body · No. 114 of the first 100

How do we see colour?

Your eye has only three colour sensors. Every sunset, painting, and screen you have ever seen is a three-number code, decoded by your brain.

Plate XXXII — Three cones, all of colour S · M · L · metamers
Sweep the rainbow, then run the screen trick: yellow with no yellow light.
400 nm700 nm S M L S · 0% M · 74% L · 98%what you seeyour eye reads colour as the ratio of three cone signals
FIG. XXXII — THREE CONES, ALL OF COLOUR
Light source
Wavelength 580 nm
Sweep the rainbow and watch the three signals trade places.
Your eye has just three colour sensors — bluish, greenish, reddish. Every colour you've ever seen is your brain reading the mix of those three signals. Try the screen trick: red + green light makes your cones fire exactly like yellow light would, so you see yellow — even though no yellow light exists there. Every screen you own runs on this hack.
The short answer

Your eye has three kinds of colour sensors — one likes blue light, one green, one red. Every colour you see is your brain mixing those three signals, like a chef tasting a soup and naming the recipe.

What's actually happening

Light itself has no colour — it has wavelength, a physical length measured in billionths of a metre. Colour is manufactured later, inside your head. The retina carries three types of cone cell, each a broad, sloppy detector: the S cone responds best to short wavelengths, M to middling, L to long. A single wavelength of light wakes all three by different amounts, and that trio of numbers — say, S faint, M strong, L strong — is everything your brain will ever know about it. "Yellow" is not in the light. Yellow is the brain's name for that particular ratio.

This three-number bottleneck has a glorious loophole: completely different mixtures of light can produce the same three cone responses, and when they do, you cannot tell them apart — not because you aren't looking carefully, but because the difference was never recorded. These twins are called metamers, and your screen exploits them billions of times a day. It cannot make yellow light; it makes a little red plus a little green, your L and M cones fire exactly as yellow would have fired them, and your brain — honestly, correctly, by its own rules — reports yellow. Every photo you've ever seen on a screen is a forgery your cones co-signed.

The bottleneck also explains the exceptions. Colour-blindness is usually one cone class shifted or missing — the code drops from three numbers to two, and red/green become the same word. Some birds and reptiles run four cone types and see a dimension of colour we cannot imagine. And magenta? There is no magenta wavelength — it's the brain's invented label for "L and S firing with M silent", a colour that exists nowhere in the rainbow, only in the decoder.

Try it at home Exhaust one cone on purpose
  1. 1Stare at a bright red shape on white paper (or a red square on your screen) for thirty seconds without moving your eyes.
  2. 2Look at a blank white wall: a cyan ghost of the shape floats there.
  3. 3You fatigued your L (red) cones; the white wall now drives M and S harder in that patch, and the ratio reads as cyan — the afterimage is your own decoder, caught mid-recalibration.