Colours

[Alej]

Just read the question about dogs only seeing in black and white - how do we know there arent more colours we cant see - i know there are a couple but how do we know there arent more?

[New Forester]

Yes there are, and also everyone may experience colours differently. We see only a very small proportion of the electromagnetic spectrum. The shortest wavelengths (highest frequencies) of "light" are gamma rays, then there are X-rays, ultraviolet, visible light, infra-red, microwaves, and the very longest ones are radio waves (I may have forgotten some...). We have two main kinds of receptors in our eyes, rods and cones. Rods are receptive to the whole visible spectrum, and to much lower levels of light than cones. This is why moonlight seems grey -- our cones have long since given up. Cones do colour. There are normally three kinds of cones, each sensitive to a band of wavelengths -- blue (shortest), green (medium) and red (longest). Other colours are in-between wavelengths, or mixtures. For example, yellow sodium street lights are one wavelength which stimulates both the red and the green cones about the same. We see yellow. However, if you mixed a pure red wavelength and a pure green one, we'd also see yellow, although there are no "yellow" wavelengths present (you'll notice that light mixes quite differently to paint). (continued...)

[New Forester]

(...continued) If you look very closely at your screen, you'll see that it also uses a blue-green-red system, as does a telly. Insects such as bees and butterflies use a three colour system too, but quite different wavelengths -- one is in the ultra-violet. (Colour printing and photography also use three colours, but the logic is different -- new question please...). People who are red-green colour-blind have only two kinds of cones, and so cannot distinguish reds or browns from greens. About 5 to 10% of men are like this, and a few women. There are other much rarer kinds of colour-blindness. A very few people (I think they are all women) have four kinds of cones. One of the usual ones comes in two kinds, sensitive to slightly different wavelengths. Theoretically this ought to allow many more colours, but more likely it may help them distinguish some similar colours slightly better. All this is the mechanism, not the perception. As the original question suggested, we can't know what someone else experiences, dog or human. When you look at a red object, for all I know you are experiencing what I'd call purple, or something else I can't even imagine. And what do bees experience?

[Alej]

you always give very good answers. thanks