It's no secret that in some animals the eyes glow in the dark - for many this phenomenon causes, if not fear, then at least goosebumps. Indeed, sometimes it can look very intimidating. Nevertheless, do not be afraid: this is not a demon, but Mother Nature, who took care of the animals. Science explains why eyes glow in the dark.
Bit of biology
In childhood, perhaps, each of us, or at least many, thought that cats and dogs had some kind of “organic” radium in their eyes that made their eyes glow in the dark. Probably many modern children think so too. However, as it turned out, there is no such substance in the eyes of animals.
The thing is that in the back of the eyeballs of animals there is a special reflective layer (called tapetum lucidum), which increases the amount of light absorbed by photoreceptors.
What is tapetum lucidum?
Tapetum lucidum is a reflective layer of tissue found in many vertebrates and invertebrates. In the case of vertebrates (e.g. cats, dogs, etc.), this layer is located on the back of the retina.
The main function of this reflective layer is to increase the amount of light that photoreceptors perceive in the eyes. Photoreceptors are the name of special retinal neurons that convert visible light by absorbing photons into signals that can subsequently initiate biological processes in the body.
Cones and rods in the human eye help us distinguish colors and provide partial visibility at night. They are two of the three types of photoreceptor cells found in the mammalian retina.
In simple words, tapetum lucidum is a kind of mirror in the eyes of some mammals that makes their eyes glow in the dark.
What is the effect?
All vertebrates in whose eyeballs have a layer of tapetum lucidum, as a rule, there is a glow of the eyes at night. But why? How does a layer of cells in the back of the eyeball make animal eyes glow?
In fact, this is elementary optics. Since the aforementioned tapetum layer is a retroreflector (something that reflects incident light toward its source without much scattering) of a transparent form, it reflects the rays that fall on it back along their original path. As a result, the source and reflected light are mixed together, giving the photoreceptors in the eyes a second chance to perceive the rays. This is why animals have eyes that glow in the dark.
Nature took care of her children! This helps the animal see more clearly (especially at night), as photoreceptors absorb more light. This creates a brighter image of the object. However, this enhanced night vision in animals has its own peculiarity: the images that they see, due to the phenomena of reflection and absorption of light, become a little blurry.
Despite the fact that the eyes of animals may have a different color, tapetum lucidum itself may have a color different from the iris of the eye. After all, its shade depends on the minerals that make up the reflecting crystals of the luminous tapetum lucidum. The most common colors with which the eyes of animals shine in the dark are whitish with a blue periphery (for example, in dogs), greenish (in tigers), golden green with a blue edge or pale blue with blue in a gazelle.
For many animals, especially nocturnal animals, this structure of their eyes allows them to see better at night, when they hunt for prey, and helps them avoid predators.
So, tigers, owls, deer, foxes, bears, and many other mammals and birds of the wild have a similar structure to the eye. Interestingly, even some aquatic animals, such as crocodiles and sharks, have this reflective layer at the back of their eyes.
It is interesting
People continue to discover new species of animals with eyes glowing in the dark, looking at their reflections in the pictures. Interestingly, due to such a special structure of the visual organs, trained horses and dogs help us in search and rescue operations carried out at night - so this is another advantage that we get thanks to our smaller brothers. People even used the idea of a tapetum lucidum layer to increase safety on our roads, creating the so-called “cat's eyes” - synthetic retroreflectors that are used on road markings. Do you think it is possible for people to glow in the dark? Let's figure it out!
