Why do cats eyes glow

Whether you have owned a cat or not, you must have seen a cat’s eyes “glitter” at night or at dusk. Sometimes you can see this in a dark room or under the bed, like this :
Why are cats’ eyes so bright? Is it the cat’s eyes that are shining? The answer is not that cats’ eyes look unusually bright because they are reflecting light.

We know that the eyes can feel light because there are photoreceptor cells on the retina of the eye. When the light entering the eye is projected onto the photoreceptor cells, the human or animal brain can receive visual signals. However, the retina cannot absorb all the light entering the eye, and some light will always be missed. If this part of the missed light can be recycled and reused, it can greatly improve vision, especially night vision – cats have done this.
Compared with the human eye, there is an extra layer of structure called “Tapetum lucidum” behind the retina of the cat’s eye. This layer of structure can reflect the light that passes through the retina back like a mirror and project it onto the retina again. This greatly improves the cat’s visual ability in dim light. It is precisely because of the existence of such a reflective structure that some light is reflected from the cat’s eyes. The bright light we see in the cat’s eyes is these light rays.

Why do cats’ eyes glow?

Cats’ eyes are very special, with particularly large pupils. They can change the shape and size of their pupils under different intensities of light: under the stimulation of bright strong light, the pupils shrink as thin as a line; while in dark places , the pupils will dilate as round and big as light bulbs.
The most interesting thing is that in a relatively dark environment, its pupils will become particularly large, and it can collect very weak light into the pupils, so it can see things clearly in the dim. And you will find that the cat’s eyes will sparkle at this time. Of course, a cat’s eyes are not a source of light, they just reflect external light. Deep in the pupil of a cat’s eye, on the back of the retina, there is a layer of film, which some people call the “illuminator”, which reflects the collected light.
Cat’s eyes sparkle in the dark because the film reflects light. In other words, a cat’s eyes will not glow in the dark, do you understand?

Light reflects on the “bright carpet”

In fact, not just cats, many animals have “bright blankets”, such as fruit bats, crocodiles, dogs, minks, cows, etc. “Bright blankets” can greatly improve their night vision capabilities. Unfortunately, except for a few monkeys, most primates, including humans, do not have this structure.

The reflective layer in the bull’s eye – the “bright blanket”

Since the cells that make up the “bright carpet” are slightly different at the molecular level, the colors of light reflected by various animals are also different. Cat eyes usually reflect yellow, green, and blue colors, while fish reflect white light. etc.

Crocodile eyes at dusk – note that crocodiles are not fish

From the above introduction, you should know that cat eyes do not emit light themselves, they just reflect light. In fact, this is not difficult to verify. As long as the cat is kept in a completely dark environment without any light, he will no longer see the shining cat eyes.
The cat’s eyes we see are glowing because when the bright blanket reflects light, some of the light will be reflected through the pupils, and we see that kind of faint light.
In addition to cats, bright carpets actually exist in the eyes of many vertebrates other than humans, such as dogs, lions, tigers, etc. Some deep-sea animals also have similar organizational structures. Like sharks, although their eyesight is poor, they also have a bright blanket after living in dark water for a long time, so their eyesight in dark places is 10 times better than that of humans.

Although “the sight is like a torch”, it actually needs to be surrounded by light

In addition to the special structure of the “bright blanket”, cats’ eyes are different from humans in many ways. For example, cats can feel fewer colors than humans, and the world they see is not as rich in colors as we see; The pupil of a cat can dilate to account for 90% of the exposed part of the eyeball, which is also an important factor in cats’ excellent night vision ability; in addition, cats’ pupils are upright, while the pupils of tigers and other cats are the same as those of humans. They are all round, so what are the advantages of cats’ vertical pupils compared to the horizontal pupils of sheep and other herbivores?