Blue eyes aren’t blue, it’s just a trick of the (scattered) light. 

Take a look a those baby blues. That’s my grandson, Casper, taken with a nice 85mm portrait lens. Processed from Sony’s Raw format in Adobe Lightroom, given a little extra punch in Photoshop, and compressed down to a tidy, web-friendly JPEG file for your enjoyment. But it’s just a trick: those eyes ain’t really blue.

OK well of course they are and so is the sky but there’s a difference between blue things like natural-blue-flavoured jelly beans or Picasso’s Blue paintings and Casper’s eyes. The jelly beans appear blue due to the pigments in the food colouring, likewise with blue paint, ink, fabric or paper. The blueness in those cases is a product of the behaviour of the surface when presented with light. The light is absorbed by the surface and, in this case, the shorter wavelengths are reflected back to our eyes and perceived as blue. That explanation cuts a few corners but the basic idea holds ups: blue paint looks blue because it reflects blue light.

Blue eyes, blue skies, and the beautiful blue of opals and sapphires are a different matter. You may have read that the sky appears blue (when you’re not in Beijing, Mumbai or, recently, Sydney) due to a phenomena known as Rayleigh Scattering – after the British, 1904 Nobel Prize winner, Lord Rayleigh, who came up with the idea. That is, light from the sun scatters when it hits the atmosphere creating the diffused illumination we know as daylight. But the shorter wavelengths are scattered more than the longer ones as they interact with particles of a particular size so we perceive a bias towards blue.

There’s some complex maths here (the amount of scattering is inversely proportional to the fourth power of the wavelength) but the short version is the sky is blueish during the day, more intensely blue when the air is drier (thinner), and becomes redder when the sun is lower in the sky and the light has further to travel – through the atmosphere to you, the observer. The crazy red skies over Sydney and elsewhere recently are witness to the same effect: as the air gets denser, and the interfering particles larger, the light appears redder – ghoulishly so in the case of bushfire smoke.

But what about Casper’s blue eyes? Well, there’s a similar principle at work with our eyes (and the wings of certain moths and butterflies). Brown-eyes (and to a less extent green and hazel) we can put down to melanin (like that of your skin) in the fibres of the iris but blue eyes have little or no melanin, the blueness coming almost entirely from light scattering. It’s pretty-much the same principal as the skies over Lord Rayleigh but in the case of eyes and wings and opals, the phenomena is known as the Tyndall Effect

This happens in other materials too, like fluids or gasses, where the particles are substantial enough in size – roughly around the wavelength of the light – to cause backscattering (a kind of light feedback). As with the sky, the scattering takes places to a greater extent with shorter wavelengths – hence the blue – and when there’s less melanin to absorb the light.

PS. The yellowish appearance of the sun is also a product of the atmosphere. Out in space, the sun is white and the rest is, well, black.