The color of human eyes is one of the most expressive and striking features of human appearance. It is determined by a combination of genetic factors and various biological processes. In this review, we will talk about why and why different people have green or blue eyes, and also discuss the role of anatomical features and processes that play in this manifestation.
What colors and shades of people's eyes are there?
Eye colors among different peoples of humanity vary from almost black to light, green, gray or blue, and are correlated with several factors, including the quantitative volume of color pigment and distribution in the layers of the iris , as well as the refraction of light. The top most popular and most common eye colors in the world are dark brown and brown. Naturally, blue and gray eye colors, as well as green, are the rarest.
Anatomical features of the eye
To understand why some people have gray, green, blue or blue eyes, it is important to consider the anatomy of the visual analyzer. The main parts of the eyeball consist of the cornea, lens, retina and iris. The iris is the ring-shaped part of the eye that surrounds the pupil and regulates the amount of light entering. The color of the iris depends on the type of pigment and its quantity. The pigment melanin is the main one in the human species.
The iris and the refraction of light
The iris consists of two layers. External or front, internal or rear. The outer layer contains the stroma, which contains melanocytes - cells that produce melanin. Due to the way light is refracted through stromal fibers and melanin, the eyes may appear different colors. Blue or light gray eyes, for example, appear this way due to low melanin content and the refraction of light through stromal fibers, which causes the effect of Rayleigh light scattering.
The role of the Melanin pigment
Melanin is a pigment responsible for most of the skin tone, as well as eye and hair color. The amount of melanin in the iris determines how dark or light the eyes will be. An eyeball with a high melanin content will have a dark iris (brown or even black), but an eye with a low melanin content will have a light iris (blue or, even less commonly, green).
And yet, why are the eyes green or blue? Gray or blue?
Eye color is determined by genetics. A person with gray eyes has minimal melanin content in the iris. Light passing through the eyes is scattered, creating a blue or grayish tint. Even rarer, green eyes, green-gray, are the result of a combination of moderate amounts of pigment and unique light scattering, resulting in a blue-green hue.
More interesting facts about the eye
- Heterochromia: It happens that among peoples all over the world there are such “anomalies” as eyes of two colors (each eye has its own color). This condition is called heterochromia. Most often this is a congenital phenomenon, but it can also be acquired.
- Age-related changes:Infants' eyes often appear blue immediately, but with age they can darken due to an increase in the amount of melanin.
- Light plays a role: Sometimes it may seem that the color or shade of the eyes is different. This happens when illuminated at different times of the day or from different light sources. For example, blue or indigo eyes appear gray or greenish when light hits the iris from different angles and with different amounts of light.
- Refractive surgery: Sometimes, medical procedures such as laser vision correction can temporarily or permanently change the color of the eyes due to changes in the structure of the cornea. Also, aesthetic surgery and modern medicine can change eye color from brown to blue.
The color of the iris is determined both by genetic characteristics and the result of the addition of biological processes. Understanding these properties helps us better appreciate human uniqueness and the diversity that makes everyone special in their own way.