Your eyes have two different types of sensory cells: rods and cones. These two types of photo-receptors in the retina divide up the work and perform different tasks: the rods enable us to perceive changes in brightness up to a certain light intensity. The rods in your eyes are essential for twilight and night vision. They enable you to see both when it's bright and when it's dark. The cones are in charge of colour perception. They come in three different varieties, each of which reacts to different wavelengths:
- Cones for blue light (S-cones, the S stands for "short"; these react to shorter wavelengths)
- Cones for green light (M-cones, the M stands for "medium"; for medium wavelengths)
- Cones for red light (L-cones, the L stands for "long"; for longer wavelengths)
And how does this affect your colour vision? – If a surface reflects e.g. only short waves, then this surface looks blue to your brain. If only long waves are reflected, then you see red. Medium-length light rays make you see green. You only perceive mixed colours like yellow, purple, orange or violet when a surface reflects waves with different lengths. If these types of cones perceive all waves lengths simultaneously, then your brain sees them as white.
But there's another important factor which affects our colour perception: objects do not only reflect colours, they also absorb them. A ripe cherry, for example, has such a lovely red colour because the surface of the fruit absorbs the green and blue light and only reflects long light waves, i.e. those which appear red. Which colours we perceive thus depends on the proportion and strength of light absorbed by the three colours blue, green and red.
Your eyes usually process a light spectrum between 380 and 780 nanometres. They don't perceive light with shorter (UV) and longer (infrared) waves, i.e. everything which is below and above the visible light spectrum.