Hazel eyes are often described as a complex, multi-tonal blend of brown, green, and gold within a single iris. The color is not determined by a single pigment but by a precise combination of biology and physics. Understanding the science behind this distinctive shade explains why hazel eyes possess such a dynamic appearance. The color is a result of light interacting with a specific, non-uniform arrangement of pigment within the eye’s structure.
The General Science of Eye Color
The color of every human iris is determined by two primary factors: the concentration of melanin and the way light physically scatters within the eye’s tissue. Melanin is the only pigment present in the human iris, and it comes in two forms: eumelanin (dark brown) and pheomelanin (reddish-yellow). The amount of melanin present dictates the resulting eye color.
Eyes with a high concentration of eumelanin absorb most incoming light, resulting in dark brown eyes. Conversely, eyes that contain very little melanin, such as blue eyes, do not have blue pigment at all. Instead, the lack of pigment allows light to penetrate the eye’s stroma, the middle layer of the iris.
This light is then scattered back out through an optical process similar to Rayleigh scattering. Because shorter wavelengths of light (blue) scatter more effectively, the eye appears blue to the observer. Green eyes are the result of a moderate amount of melanin, often including some pheomelanin, combined with this light scattering effect.
The Unique Pigment Distribution in Hazel Irises
Hazel eyes are fundamentally defined by a moderate total amount of melanin distributed in a distinctly non-uniform pattern across the iris. Unlike the uniform concentration found in solid brown or green eyes, the hazel iris exhibits a biphasic, or two-zoned, structure. This uneven placement of pigment is the scientific reason for the multi-colored appearance.
The inner ring, which immediately surrounds the pupil, typically contains a higher concentration of eumelanin and pheomelanin. This dense central zone absorbs more light, causing it to appear as a solid ring of brown, gold, or amber. This central ring is the source of the warm tones that characterize hazel eyes.
Moving outward toward the periphery of the iris, the concentration of melanin drops significantly. In this outer zone, the low pigment level allows the physical phenomenon of light scattering to dominate. As light scatters through the less pigmented tissue, the shorter wavelengths create the visible green or sometimes blue tint. The combination of localized pigment absorption and peripheral light scattering creates the definitive mosaic of colors that makes the eye hazel.
Why Hazel Eyes Appear to Shift in Color
The common observation that hazel eyes seem to “change color” is not due to a physical alteration of the pigment itself, but rather a perceptual effect caused by external factors interacting with the iris’s fixed structure. Ambient lighting is a primary influence, as different light sources can either enhance or mute the scattering effect. For instance, bright natural sunlight will often intensify the light scattering in the periphery, making the green tones appear much more prominent.
Conversely, dim or artificial indoor lighting may reduce the scattering effect, allowing the central, more pigmented brown or gold ring to become the dominant visible color. The contrast created by a person’s clothing or makeup also subtly influences color perception. Wearing colors like green or blue can make the corresponding tones in the eye’s periphery stand out more strongly.
Changes in pupil size further contribute to the perceived shift in color. When the pupil dilates in low light or due to emotion, it contracts the visible area of the iris. This action alters the ratio between the dark inner ring and the lighter, scattering outer ring, which makes the overall color appear different to an observer. The dynamic interplay between the fixed, two-toned pigment distribution and the environment gives hazel eyes their unique, chameleon-like quality.