Have you ever heard of the Tyndall effect? It’s a term that is named after 19th-century physicist John Tyndall and is used to describe the scattering of light by particles that are in a fine suspension. When light becomes scattered, different colored wavelengths are produced, each one having a different frequency and wavelength.
It’s the reason our skies are blue, and also the cause of something else we see almost every day: people with blue eyes.
When it comes to eye color, many people are led to believe that our irises (the colored part of our eyes) naturally have different colored pigment within them, creating all the different colors of eyes that we see. The truth is actually much cooler.
The iris is made up of two layers, the epithelium, which lies at the rear, and the stroma, which sits up front. The epithelium, or the back layer, is about two cells thick, and features black or brown pigments.
The front layer, the stroma, can either be made up of colorless fibers of collagen or a dark pigment called melanin. Folks with dark or brown eyes tend to have more melanin in their stroma than folks with green or blue eyes. Both their front and back layers have dark pigment coloration.
People with blue eyes have what is called “structural coloration,” meaning the color of their eyes is entirely dependent on the fact that their stroma has absolutely no pigment coloration in it--it’s basically translucent or clear. Because the epithelium layer that rests behind the stroma is dark, it absorbs much of the longer wavelengths of light, resulting in only the shorter wavelengths of light reflecting back outward--creating the blue hue that we all see.
Any light that enters an iris with a translucent structure gets scattered back outward because of the fine suspension of particles in the eye, or, the Tyndall effect.
So, if you have blue eyes, your eyes aren’t really blue. They’re colorless, which gives off the illusion that they are blue because of how light interacts with the tiny particles in your stroma.
Paul Van Slembrouck ponders this phenomena in depth:
“Imagine that you could shrink yourself to a microscopic size and then climb through the mesh of fibres in the stroma. That’s where structural colouration is coming from…
… and in the mesh are also strands of smooth muscle tissue that contract to dilate (expand) the pupil, pulling the inner edge of the iris toward the outer edge. When this happens, the stroma fibres slacken and may become wiggly as tension is released. This makes me wonder, does that slightly alter the colour of your eye as well?”
So, almost all eyes will look different based on how much light is present at the time, as well as the specific structure of their stroma. Structural coloration occurs naturally in many things, such as butterflies, beef and certain berries.
h/t Science Alert