Question: What creates the colour of the eyes?

Well I learnt something trying to answer this question! So it turns out that the colour of the eyes is determined by the density of a substance called melanin, which is a general, all round, useful molecule that comes in a number of different forms. Melanin is responsible for skin colour, and has a very broad absorbance of light – which helps protect you from UV and sunburn! A colleague of mine at the University of Queensland has a research program on melanin, trying to work out why it has such broadband absorbance – with the goal of trying to use it to make environmentally friendly solar cells! (Silicon – which most solar cells are made of – is not particularly friendly to produce).

Anyway – melanin gives you brown -> black eyes. So where does blue and green come from? Well, that is the cool part – it is from the same mechanism that causes the sky to be blue! Something called “Rayleigh scattering” – basically some molecules can scatter shorter wavelength light, but not longer wavelength. White light passing through the atmosphere has the blue light scattered out of it, and it comes down to the ground, making the sky appear blue. Likewise, at sunset, all the blue light is scattered out of the white light before it reaches us, so it can appear red/pink. So the melanin in some peoples eyes is arranged such that it scatters blue/green light back.