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Many people wonder why a cat's eyes shine in the dark. This phenomenon is commonly seen at night, though it also may be reflected when a picture of a cat is taken with flash photography. The biology behind this phenomenon lies in a small but vital, portion of the cat's anatomy, according to the "Color Atlas of Veterinary Anatomy Volume 3." The process functions similarly to the way the moon shines, reflecting light from an outside source.

A cat's eyes glow because of a small membrane in the eye called the tapetum lucidum, a Latin phrase meaning "bright tapestry." This membrane is composed of 15 layers of cells directly behind the retina, or inner surface of the eye. There are four distinct types of tapeta lucida, according to "Ophthalmology for Veterinary Practitioners." The structural difference of the different tapeta lucida are apparent, though the functional differences, if any, are not known, as stated by the "Clinical Textbook for Veterinary Technicians." Cats have what is known as a choroidal tapetum cellulosum, a tapetum lucidum made from crystal structures of guanine, according to "Veterinary Neuroanatomy and Clinical Neurology?."

Humans can only visualize the effects of the tapetum lucidum by seeing cat's eyes shine in the dark. A cat's eye can glow different colors at night, depending upon the color of the cat's eye and the color of the tapetum lucidum, as well as the color and manner of the light source. Cats often have a green tapetum lucidum, but since the membrane is iridescent, the reflected color may show up as green, yellow, blue or white. Red eye shine appears only in cats that have one or both eyes of a blue hue. One notable exception to the red-eye rule is found in Siamese cats, whose eyes always shine in hues of red, according to "Ophthalmology for Veterinary Practitioners."

Tapeta lucida are surfaces known as retroreflectors. They reflect light back in the direction of the original source with as little scatter as possible, according to "Veterinary Neuroanatomy and Clinical Neurology?." This means that in reduced light, the cat's vision doesn't lose clarity or contrast. The tapetum lucidum also intensifies the light that passes through the cat's retina, allowing cats to see wavelengths of light not visible to humans.

Eye shine enables humans to detect cats at night. Since large cats like tigers and lions also have tapetum lucidum, this is a particularly beneficial trait. This thin membrane allows cats to see in dim light, enabling them to hunt during their peak hours of activity -- dawn and dusk, according to "The Domestic Cat: The Biology of its Behavior." Because of the enhanced images cats receive, their hunting is quick and efficient.

The amount a cat's eye glows is affected by how dilated the cat's pupils are. The more dilated the pupils, the higher the amount of eye shine. Cat's eyes dilate as light decreases. Cats that have hereditary ocular diseases or a history of trauma to the eye may not exhibit as much or any glowing in their eyes at night, according to the "Clinical Textbook for Veterinary Technicians." Some diseases, like glaucoma, may mimic the eye shine effects of the tapetum lucidum.

The red-eye effect in photography is the common appearance of red pupils in color photographs of eyes. It occurs when using a photographic flash very close to the camera lens (as with most compact cameras), in ambient low light. The effect appears in the eyes of humans and animals that have no tapetum lucidum, hence no eyeshine,[citation needed] and rarely in animals that have a tapetum lucidum. The red-eye effect can also be seen when one human observes another when there is a bright lightsource nearby. This however is very difficult.

Because the light of the flash occurs too fast for the pupil to close, much of the very bright light from the flash passes into the eye through the pupil, reflects off the fundus at the back of the eyeball and out through the pupil. The camera records this reflected light. The main cause of the red color is the ample amount of blood in the choroid which nourishes the back of the eye and is located behind the retina. The blood in the retinal circulation is far less than in the choroid, and plays virtually no role.