Cats’ eyes don’t glow in the dark, but they may appear to have a distinct shine when viewed at the right angle or when you accidentally blast your cat in the face with flash photography. The cause is very well-known and studied in science, and cats aren’t the only ones who experience it! Cats, rodents, and aquatic mammals may seem to have little to nothing in common. Still, they all share one trait called the choroidal tapetum cellulosum, a type of tapetum lucidum that is responsible for the appearance of glowing in the dark.
What is the Tapetum Lucidum?
The term ‘tapetum lucidum’ is Latin, meaning ‘bright tapestry.’ It refers to a reflective layer inside the eye present in many nocturnal animals. This iridescent layer inside the eye reflects the light that passes through the retina. It reflects it through the retina a second time, essentially allowing one beam of light to light the animal’s vision twice! When the light is reflected out of the eye, we get the classic glowing effect of a cat’s eyes, referred to as ‘eyeshine’ in science.
Most creatures with a tapetum lucidum are nocturnal or live in low-light environments, such as deep under the sea. The tapetum lucidum is meant to help them see in the dark and makes their eyes more sensitive to light by reflecting the light that goes in back out of the eye. The tapetum lucidum is observed in numerous species, including all cats.
The tapetum lucidum is a retroreflector, meaning it reflects light back along the exact same path it hit the surface with. The tapetum lucidum uses the scientific property of ‘constructive interference’ to reflect light back through the retina, allowing two light beams to combine into one ‘stronger’ light. This adaptation enables the animal to see in the dark by amplifying the strength of the available lighting.
Humans and most other dry-nosed primates are diurnal, meaning they are awake during the day, so they don’t have a tapetum lucidum since they don’t need one. However, a type of eyeshine can be observed in humans. When you see the red-eye effect while taking photographs, this eyeshine uses the same concepts as eyeshine in cats but from the eye’s interior surface. Without the tapetum lucidum, it’s hard to see the reflection in a low-light environment without harsh light.
How Does the Tapetum Lucidum Work?
The cat’s tapetum lucidum is one of four classifications of tapetum lucidum called the choroidal tapetum cellulosum. This form of the tapetum lucidum is composed of several layers of cells that contain refractive crystals. The crystals’ organization and exact cellular makeup show diverse differences in the several species with this type of tapetum lucidum.
The tapetum lucidum works on any light that passes through the eye, no matter how minimal. Even if it passes through the retina twice, a smaller light won’t provide a significant amount of vision. However, this double-duty that each light particle holds means that a cat’s vision is about 44% more light-sensitive than a human’s. In layman’s terms, cats can “see” light that is entirely outside of human perception.
What Is Eyeshine?
‘Eyeshine’ is the scientific term for the shiny, glowing effect that we see when we shine lights in our cats’ eyes or catch them from the right angle at night. It’s the light reflecting off the tapetum lucidum that we see when we see the eyeshine effect.
The tapetum lucidum has its own color that can influence the color of eyeshine. In tigers, for instance, the eyeshine and tapetum lucidum is generally greenish. However, as eyeshine is a type of iridescence, the color of the eyeshine will change color based on what direction we see the light from.
When captured with flash photography, cats and dogs with blue eyes may show both eye-shine and a red-eye effect like humans. The eye will display eyeshine when viewed in low-light, but when a flash is used when photographing the animal, the pupil will appear to glow red.
Cats’ eyes do not glow in the dark; they reflect light. The refractive crystals inside their tapetum lucidum reflect light giving them that characteristic eyeshine effect that we are used to seeing. This effect is the result of their evolutionary mutation to help them see in the dark. It might look a little scary if you aren’t expecting it, but don’t worry; it means your kitty’s eyes are working precisely as nature intended!
Featured Image Credit: Pixabay