The color of the iris, the pigmented part surrounding the pupil, varies among people. Brown is the most common shade. Everyone had brown eyes thousands of years ago. Other colors developed due to later mutations through evolution.
Key points of this article that can be understood in 3 lines in number
- The color of the iris, which is decided by genes and melanin levels, causes different eye colors. The Tyndall effect, a kind of light scattering, is responsible for making blue eyes look blue rather than pigmentation.
- Brown eyes have more melanin in them, while green eyes have less. Blue eyes lack melanin in the stromal layer which makes them look different even though there are no blue pigments.
- Blue eyes appear blue due to the Tyndall effect, just like how the sky looks blue. This occurs because colorless fibers in the stroma scatter light.
Blue eyes are more common than green eyes. Green eyes can look grayish while blue eyes have lighter or darker variations. Only 8-10% of people have blue eyes while only 2-5% have green eyes.
There are three eye colors: brown, green and blue. Each color has many variations. But why do people say that blue eyes don’t exist?
Blue eyes are not blue
Before we proceed, let’s define something. We have eyes that look blue but are not actually blue. The iris color is what makes it appear blue, but there is no real blue pigment in the eyes. To fully grasp this concept, we need to understand how eye color works and what ‘color’ means.
The color of an object is determined by the light it reflects. A white object reflects all colors while a black object absorbs all colors. When an object absorbs some light due to pigment and reflects another part, we see its color. Trees have green leaves because of chlorophyll. Chlorophyll takes in red and blue light and reflects green light that we see.
Eye color is determined by genes, which control melanin production in the iris. Melanin is the pigment that gives color to our skin and hair. The iris has two types of melanin: dark eumelanin that makes brown color and light pheomelanin that creates ocher tones. Both pigments mix in the iris to make our unique eye color.
The level of melanin in a tissue decides how dark the color is. The mixture of eumelanin and pheomelanin decides the shade. Hazel eyes have more eumelanin, giving them their color. If pheomelanin is higher, the eye color looks like honey.
People with blue eyes have less melanin. It was once believed that blue eyes had pigments that made them blue, but there are no blue pigments in the eyes.
The eyes we see as blue are not really blue. The headline is correct, there are no blue eyes. This is because the eyes have no pigments of that color.
Why do blue eyes look blue?
The eye’s iris has two layers. One layer is called the pigment epithelium, which always contains melanin. The other layer is called stroma and is made up of fibers that may or may not contain pigment. The difference in the color of eyes lies in this superficial stromal layer. Brown eyes contain more melanin, while green eyes have less and blue eyes have none. As a result, the fibers in blue eyes show no color.
Blue eyes appear a certain way because of the Tyndall effect, not their color.
The Tyndall effect is when light scatters from colorless particles. White light turns into different colors, and they go different ways. Red light doesn’t scatter much, but blue light does. So the physical structure keeps red radiation, and blue radiation becomes visible. This is like Rayleigh scattering which makes the sky look blue without blue pigments.
The stroma colorless fibers make the Tyndall effect happen, leading to structural coloration. This makes colorless tissue look blue instead of pigmentary coloration. Blue eyes seem blue because of this effect even though there is no actual color in the stromal tissue.