Why Do a Cat's Eyes Glow in the Dark?
One of the amazing features of the eye is its ability to see at night. Cat eyes have a special layer called the tapetum which allows them to absorb and reflect more light at night than humans for better nocturnal vision.
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Evolution of the Tapetum
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Different animals, especially deep sea creatures and nocturnal predators like cats, developed the tapetum in order to see at night. This structure allows for light to enter the pupil, bounce out equally and double the amount of dim light available to enhance night vision.
The Pupil
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Because the tapetum magnifies the amount of light, the iris, which is the colorful part of the eye, contracts according to the amount of light currently available, expanding in darker settings. This is why during the day cats have tinier black slits--to reduce the amount of bright light magnification.
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Human Red Eye
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Unlike cats and night-time animals, humans do not have a tapetum. As a result, when you shine light into human eyes or use a camera flash, the blood vessels behind the retina reflect the light with a red glow.
Enhanced Glowing
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The more direct the light source is to the cat's eyes, the more it will bounce back. This is why a cat's eyes do not appear to glow when light hits them at an angle.
The Color of the Glow
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The color of the reflected light depends on the color of the tapetum. For most cats, this color is yellowish, but for black dogs it could be green, for cows it could be blue and for humans who do not have tapetums, it is red.
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References
- Photo Credit cat image by Andrzej Solnica from Fotolia.com
