Why Do We See Colours?

Colour vision begins when light enters the eye and strikes the retina, which contains about 6 million cone cells and 120 million rod cells. Cones come in three types: S-cones (short wavelength, peak sensitivity around 420 nm — blue), M-cones (medium, ~530 nm — green), and L-cones (long, ~560 nm — red). When light hits an object, certain wavelengths are absorbed and others reflected. The reflected wavelengths enter our eyes and stimulate different combinations of cone types. Our visual cortex interprets the ratio of signals from the three cone types as a specific colour — this trichromatic theory was proposed by Thomas Young in 1802 and refined by Hermann von Helmholtz. The brain can distinguish roughly 10 million colours from just three receptor types. Colour blindness (affecting about 8% of men and 0.5% of women) typically results from absent or abnormal cone types. Some animals have more cone types: mantis shrimp have 16, though this likely aids rapid colour recognition rather than finer colour discrimination.