A common belief is that light-colored eyes are more susceptible to bright light. Understanding the science behind how the eyes process light and what determines eye color helps clarify this perception.
How the Eye Controls Light
The human eye regulates the amount of light entering it for clear vision. Light first passes through the cornea, which bends and focuses it. It then travels through the pupil, a dark opening in the center of the iris.
The iris, the colored part of the eye, controls the pupil’s size like a camera diaphragm. Muscles within the iris adjust the pupil, constricting it in bright light to reduce incoming light and dilating it in dim conditions to allow more light in. This adjustment occurs in all human eyes, regardless of color, to optimize light for the retina.
The Science of Eye Color
Eye color is determined by the amount and distribution of melanin within the iris. Melanocytes produce melanin, the same pigment responsible for skin and hair color. Higher melanin concentrations result in darker eyes, while lower levels lead to lighter eyes.
Brown eyes, the most common globally, have the highest melanin concentrations. Blue eyes contain very little melanin; their color comes from light scattering within the iris’s stroma, similar to how the sky appears blue. Green eyes result from low melanin levels and light scattering, sometimes with a yellow pigment.
Eye Color and Light Sensitivity
While the pupil and iris control the amount of light entering the eye, eye color influences how light is handled once inside. Lighter-colored eyes, such as blue or green, contain less melanin in the iris than darker eyes. Since melanin absorbs light, its lower concentration in lighter irises means less light is absorbed by the iris itself.
This reduced absorption allows more light to scatter within the eye and reach the retina. Consequently, individuals with lighter eyes may experience more glare or discomfort in bright conditions, a phenomenon known as photophobia. The decreased melanin also means lighter eyes offer less natural protection against harmful ultraviolet (UV) light. Therefore, while the amount of light entering the pupil is regulated independently of eye color, the handling and absorption of that light within the eye differ, which can lead to increased sensitivity and a greater need for UV protection.