Blue is not popular in nature. Aside from the odd bird feather, a few flower petals, or perhaps an ultra-rare lobster, it’s rare to see instances of blue in the natural world. So, it might seem a little bit odd that a fair few humans have blue eyes.
Well, weirdly enough, blue eyes don’t contain any blue pigment at all. Their color is actually caused by the Tyndall effect, a phenomenon similar to the effect that makes the sky look blue.
The iris, the colored donut of the eye, is made up of two layers of cells: the stroma on top and the epithelium beneath. In blue eyes, the stroma is a translucent layer and contains zero pigment due to a genetic mutation. When visible white light hits it, it scatters the lightwaves. Blue is scattered more than other colors because it travels as shorter waves, making the blue reflect most easily and making it more visible. All of this is also why the same set of blue eyes can look more vibrant at certain times than others – their color depends on the quality of light in the room.
The stroma in brown eyes contains high levels of melanin, a black and brown pigment, meaning that a brown color is simply reflected back. For grey eyes, it’s a similar phenomenon to blue eyes except the stroma contains more collagen. This makes the layer less translucent and dampens the blue tinges. For a green eye, the Tyndall effect still occurs. However, the stroma contains low concentrations of melanin pigment, making the blue reflection appear a darker, greener color.
When you look at the sky on a clear day, it also appears blue due to light scattering, but not because the sky contains blue-pigmented particles. This effect is known as Rayleigh scattering. Although largely a similar effect, it differs because the light-scattering particles are far smaller than the wavelength of the light.
Back to blue eyes for a moment, this is what scientists called structural coloring, as opposed to pigment coloring. In fact, a lot of the times when you see blue in the natural world, it is structural coloring. You know that weird blue-green shine that bacon sometimes gets? It’s a similar phenomenon as blue eyes. Many blue-colored birds, such as the blue-and-yellow macaw, don't actually have blue feathers. They only appear blue because of nanochannels within their feather structure that alter their light-reflective properties. Even certain berries have picked up on this trick.
[H/T: Paul Van Slembrouck via Medium]