I鈥檓 told that blue-eyed people find coping with dazzling sun more difficult than brown-eyed people do. Why? Or if it鈥檚 untrue, what difference, if any, does eye colour make?
鈥� The colour of our eyes is determined by the amount of melanin in the iris 鈥� the coloured structure visible through the transparent front of the eye. The function of the melanin is to make the iris opaque and prevent light entering it anywhere except through the pupil. The back surface of the iris contains the most melanin, although this pigment is also found just below its anterior surface and throughout the fibres at its front, called the stroma.
鈥淭he bluer the eyes, the more light is likely to bypass the pupil and enter the eye through the iris鈥�
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It is the amount of melanin in the stroma and anterior surface that determines eye colour.
Brown eyes contain a lot of melanin and so absorb most light, especially long wavelength (red) light. Blue eyes contain less, so they absorb less and scatter some of the light, especially shorter (blue) wavelengths. It is light scattered back to an observer that mainly determines an object鈥檚 colour.
The bluer the eyes, the more light is likely to bypass the pupil and enter the eye through the iris. This may cause light scatter within the eye, which would lead to glare and reduced image contrast. So it is likely that those with blue eyes will be troubled more by bright sunlight than those with brown eyes. Certainly, I have blue eyes and I find it very difficult to see much in bright sunshine. I always have to wear sunglasses. My wife, however, with her lovely hazel brown eyes, rarely has to bother with such eye protection.
She may have other reasons to feel smug: some studies suggest those with blue eyes may be more prone to diseases such as age-related macular degeneration.
Interestingly, the colour of your eyes also affects the drugs used to dilate pupils. Such antimuscarinics bind to melanin, so clinicians may use a 1 per cent concentration of tropicamide or cyclopentolate to dilate the pupil of people with brown eyes, whereas 0.5 per cent may be enough for someone with blue eyes. A colleague informs me that when they punch holes in the iris with a laser to relieve glaucoma, the colour of the iris may have an effect on the success of the treatment.
Ron Douglas, Professor of Visual Science
City, University of London, UK
鈥� My biology master taught me that my grey irises allowed more light through than the brown variety, which meant that I would find the sun鈥檚 light more dazzling. Indeed, I have to wear dark brown tinted spectacles for driving in sunlight most of the year. They give life to the phrase 鈥渞ose tinted spectacles鈥� because they allow more red light through. Flowers and trees are magnificent, and the cloudscapes fantastic.
Steve Swift, Alton, Hampshire, UK
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