Why are our irises colored

Knowledge question

Why are our eyes colored?

In theory, all eyes should be blue. In fact, they are too. The colored ring in our eyes, the iris, contains a cloudy, colorless substance. It is called the stroma and consists of the finest connective tissue fibers.

Cloudy substances react differently to white light than clear water. The cloudy liquid scatters the incident light and it then no longer appears colorless, but blue. If the light now falls into the eye, the short-wave blue light rays are scattered much more strongly on the connective tissue fibers than the long-wave ones. This scattering becomes visible against the dark background and the eye appears blue.

This basic color of the eye can be changed. This is done through pigments that are contained in the stroma and in the front wall of the iris. These pigments consist of the protein melanin, which also determines our skin and hair color.

In low concentrations, the melanin molecules act like a yellow filter. Together with the blue light, the eye appears green. A particularly large number of melanin pigments give the eye a brown color. This is why fair-skinned and blonde people tend to have blue eyes, while the irises of dark-skinned people with dark hair are mostly brown.

Babies usually have blue eyes because they hardly have any melanin. The pigmentation of the iris leaves is not yet complete at birth. This explains the almost uniform dark blue-gray eye color of the newborns. Only after a few months does the storage of brown-coloring melanin molecules form a characteristic eye color that ranges from blue to green to brown.

The copper content in melanin may play a role in the development of some shades of color. But also the interplay between the amount of melanin and the stroma density of the iris. For example, people without melanin with little stroma tend to have anthracite blue eyes.

With a lot of stroma, on the other hand, the iris can have a blue to pale blue-gray color. In the course of our lives, the eye color can "fade" a little. With increasing age, there can be a loss of pigment, which makes the color of the eye appear lighter.

SWR | Status: 10.12.2020, 12:57