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Ask Your Science Teacher: What causes eyes to change color?
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Ask Your Science Teacher: What causes eyes to change color?

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This week’s question was asked by: a parishioner at our church.

QUESTION: What causes my eyes to change from a blue color to a hazel color?

ANSWER: The pupil is that black center of the eye and it is black for everyone. The iris of the eye is that thin circular structure responsible for controlling the diameter and size of the pupil. The iris of the eye is like a diaphragm allowing more light or less light to strike the retina where the image is formed. It is the iris of the eye that is colored.

Eye color is an inherited trait. There was a once-held view that blue eyes were caused by a recessive trait. Eye color is much more complex, involving as many as 14 different genes. The two main genes associated with eye color are located on Chromosome 15.

Larry Scheckel

Pigmentation of the iris in humans varies depending on the amount of melanin, a natural pigment, located in the back of the iris. Most babies of European ancestry have light colored, or blue eyes, at birth. As a child develops, from age one to three, cells in the iris slowly begin to produce melanin. If sufficient amounts of melanin are produced, the eyes turn brown or green.

Eye color is also dependent on the scattering of light in the stroma, a delicate interlacing of fibers that circle the circumference of the iris and radiate toward the pupil. This fine mesh is interspersed with blood vessels and nerves.

This dispersing of light, referred to as Rayleigh scattering, is the same phenomena that accounts for the blueness of the sky.

Hazel is a mixture of green and brown, sometimes with yellow-gold thrown in. Hazel is tricky because hazel-colored eyes appear to change color, appearing more medium golden-dark green and sometimes a light brown. Hazel eyes are due to a moderate amount of melanin in the iris border layer and that Rayleigh scattering.

Hazel is oft referred to as “mood eyes” because the color depends of the colors a person is wearing and the lighting. Hazel eyes have flecks and ripples of green, gold, and red. So, a color change for hazel-eyed people is quite common.

It turns out that most eye-colored changes occur in Caucasian populations with hazel and amber eyes. There is strong evidence from twins, both fraternal and identical, that the changes are genetically determined.

The question frequently arises: Is there a relationship between eye color and personality? This is probably not a scientific study, but brown eyes have been associated with intelligence, blue eyes with sweetness and kindness, green eyes with being creative. People with hazel eyes are intelligent, sweet, kind and creative!


Email questions and comments to: lscheckel@charter.net.

Larry Scheckel is a retired Tomah High School physics teacher.

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