Brown hair often reveals a surprising reddish tint when bathed in direct sunlight. This article will delve into the biological components of hair color and the physics of light interaction to explain why brown hair can appear red under the sun’s bright gaze.
The Science of Hair Color
Human hair color is primarily determined by pigments called melanin, produced in hair follicles. There are two primary types of melanin that contribute to the vast spectrum of human hair shades. Eumelanin is responsible for the darker hues, ranging from black to brown. Pheomelanin, on the other hand, contributes to red and yellow tones.
Hair color depends on the relative amounts and distribution of these two melanin types. Brown hair contains a significant concentration of eumelanin, providing its dark appearance. However, it also contains a smaller amount of pheomelanin, contributing reddish or yellowish undertones.
How Light Interacts with Hair
Light interacts with objects, including hair, in several fundamental ways: absorption, reflection, and scattering. When light strikes a surface, some wavelengths are absorbed by the material’s pigments. The wavelengths that are not absorbed are either reflected away from the surface or scattered in various directions.
Our eyes perceive the color of an object based on the wavelengths of light it reflects or scatters. Sunlight, which is white light, contains a full spectrum of visible colors, from violet to red. The pigments within hair shafts selectively absorb certain wavelengths of this incoming light, allowing other wavelengths to be reflected or scattered.
Unveiling the Reddish Hue
The reddish hue in brown hair under sunlight results from its pigment composition and the intensity of direct light. Brown hair, rich in dark eumelanin, always contains some reddish-yellow pheomelanin. In typical indoor lighting, the dominant eumelanin absorbs most wavelengths, masking the subtle red tones of the pheomelanin.
When exposed to bright, full-spectrum sunlight, the dynamics change. The strong light can cause a slight “photobleaching” or breakdown of the eumelanin, particularly with prolonged exposure. As some of the darker eumelanin is affected, the underlying pheomelanin’s reddish tones become more prominent and less obscured. The intense light also allows the reddish wavelengths that pheomelanin reflects to scatter more noticeably, making the red undertones visually apparent.
This effect differs from naturally red hair, which possesses a much higher concentration of pheomelanin from the outset. The sun’s energy highlights these naturally occurring, but often hidden, red pigments within the hair shaft.