The eerie glow that appears in a cat’s eyes when light catches them in the dark is known as eyeshine, or retroreflection. This effect is common across many nocturnal animals, such as deer, raccoons, and cats. The glow is not light produced by the animal, but rather light reflected back out of the eye, similar to how a street sign reflects headlights. This biological adaptation helps these animals navigate and hunt in low-light conditions.
The Tapetum Lucidum
The structure responsible for this reflection is a specialized layer of tissue called the tapetum lucidum, Latin for “bright tapestry.” This layer is situated in the back of the eye, immediately behind the retina. When light enters the cat’s eye and passes through the retina, any unabsorbed light hits the tapetum lucidum.
In cats, the tapetum lucidum is highly reflective and consists of 15 to 20 layers of cells. These cells contain highly organized, parallel crystal rods, often including minerals like zinc and the organic compound riboflavin. These crystalline structures act like a biological mirror, bouncing light back directly through the retina.
Amplifying Light for Night Vision
The primary function of the tapetum lucidum is to maximize the use of ambient light in dark environments. When light enters the eye, photoreceptor cells in the retina absorb some light and convert it into a visual signal. Light that the retina does not absorb on the first pass continues toward the back of the eye.
This unabsorbed light strikes the tapetum lucidum, which reflects it back through the retina for a second time. This mechanism effectively gives the photoreceptors a “second chance” to capture photons, significantly increasing the eye’s overall sensitivity to light. This double-pass system enhances the cat’s light-gathering ability by up to 50%.
The trade-off for superior low-light vision is a slight reduction in visual acuity, as the reflected light causes a degree of light scatter. However, for a predator active during dawn and dusk, the ability to see in near-darkness outweighs the need for extremely sharp daytime vision. Cats can see clearly with only about one-sixth of the light level that humans require. The visible glow is the portion of light reflected back out of the eye after this amplification process.
The Science Behind the Eye Shine Color
The specific color of the eyeshine—which can appear green, yellow, or blue—is determined by the tapetum’s unique composition. In cats, the reflective crystals often contain zinc and riboflavin, a type of B vitamin. Riboflavin absorbs blue light and reflects yellow and red wavelengths, contributing to the common yellow-green glow seen in many adult cats.
The color also varies based on the cat’s age and genetics. Kittens often display a bluish reflection that matures into the adult color, typically yellow or green, by four months of age.
In some blue-eyed cats, the tapetum lucidum may be less developed or lack normal pigmentation. In these cases, light passes through the tapetum and reflects off the blood vessels in the underlying choroid layer. This causes the eyeshine to appear red, similar to the “red-eye” effect seen in human flash photography.