The observation that brown eyes sometimes appear to shift toward green or a lighter hazel shade during intense emotional moments, such as crying, is a common phenomenon. This temporary visual change is not caused by an alteration in the pigment of the iris itself, but rather by a combination of physiological reactions and optical effects. Understanding this process requires looking closely at how natural eye color is determined and how the body responds to strong emotional signals. The temporary green perception is simply an optical illusion created by contrast and light refraction.
Understanding Baseline Eye Color
The natural color of a person’s eyes is determined by the amount of the pigment melanin present in the front layer of the iris, known as the stroma. Brown eyes are the most common eye color, due to a high concentration of melanin that absorbs most wavelengths of light entering the eye. This absorption prevents light from scattering back out, resulting in a dark brown appearance. Eyes that appear lighter, such as blue, green, or hazel, have lower concentrations of melanin in the stroma. In these lighter eyes, the collagen fibers scatter light, a phenomenon similar to Rayleigh scattering. This light scattering creates the structural color, which, combined with intermediate melanin levels, produces green or hazel hues.
Physiological Changes During Crying
The act of crying triggers several involuntary physiological responses controlled by the nervous system, particularly the autonomic branch. A primary reaction involves the production of emotional tears from the lacrimal glands, stimulated by signals originating in the brain’s limbic system. These tears form a clear, watery film over the cornea, which temporarily changes the surface properties of the eye. Intense emotion also causes vasodilation, or the widening of blood vessels, especially those around the eyes. This increased blood flow brings a noticeable red hue to the tissues surrounding the iris, most visibly in the sclera. This temporary redness is a significant factor in the perceived color alteration of the iris.
The Optical Mechanism of the Color Shift
The temporary shift from brown to green is an illusion resulting from the combined effects of light, liquid, and surrounding color contrast.
Color Contrast
The redness from the dilated blood vessels in the sclera creates a strong, vibrant contrast against the iris. According to color theory, placing red next to a muted color enhances any subtle green tones present, a phenomenon known as simultaneous contrast. The red background visually amplifies the subtle yellow and scattered blue light within the brown iris, making the green appearance more prominent.
The Effect of Tears
The layer of tears acts as a smooth, reflective film over the eye, which alters how light enters and exits the iris. This clear, fluid surface enhances the light scattering within the stroma, making the underlying structural colors more visible. The tears refract light differently, which can briefly highlight the lighter, yellowish flecks often present in a brown iris. This contributes to the overall green or hazel perception.
Pupil Dilation
Emotional states can also trigger a temporary dilation of the pupil, controlled by the nervous system’s response to stress or excitement. When the pupil widens, the dark center of the eye increases in size, changing the amount of colored iris that is visible. This alteration in the ratio of dark pupil to colored iris makes the remaining iris area appear lighter and brighter. This further contributes to the temporary illusion of a color change.