Cats and many other animals can reflect light with their eyes. This is why cat eyes usually glow brightly in photographs taken in a poorly lit room, or when lit in the dark with a flashlight. Animal species whose eyes glow have evolved to see better in low light. They are forced to look out for predators all night, or they spend most of their hunting at dawn and dusk.
Domesticated cats can see in conditions where the brightness is 16% lower than humans require, and cat pupils can become 50% larger than human pupils in dim light. This is because cat pupils, which dilate and contract depending on lighting conditions, are special. They act like windows, with the larger ones allowing more light into the eyes. They also have more of a certain type of light-sensitive cell in the back of the eye. These cells, called rods, pick up weak light.
In addition to large pupils and a large number of rods, cats have an interesting feature that humans do not have – the tapetum (Tapetum lucidum). This is a thin layer of tissue located at the back of the eye behind the retina. It receives light, converts the light into an electrical signal, and sends that signal to the brain to interpret the image. The cat’s tapetum is made up of cells with crystals that reflect light back to the retina like a mirror. This allows the retina to absorb more light.
Tapetum lucidum in cats is special because its reflective compound is riboflavin, a type of vitamin B. Riboflavin has unique properties that amplify light to a specific wavelength that cats can see well, which greatly increases the sensitivity of the eye’s retina to weak light.
In cats, the tapetum most often glows yellow-green or yellow-orange, but the color varies, as does their iris, which can be green, yellow, blue, or gold. Differences in tapetum coloration are not unique to cats and can be found in many species.
Tapetum lucidum is found in many other animals that need to see at night – both predators and their prey. In the inhabitants of the water world, the tapetum occupies most of the eye, and helps to see better in muddy dark water.
Notably, some small domestic dogs lack this trait. Most animals with blue eyes and white or light hair have also lost this trait. So don’t be alarmed if your dog’s or cat’s eyes don’t glow. Other species that do not have Tapetum lucidum include pigs, birds, reptiles, and most rodents and primates, including humans.