Our eyes consist of a layer called the retina, which contains specialized cells called photoreceptors. When light enters the eye, it passes through the retina and is absorbed by the photoreceptor cells. They convert the light into electrical signals and are transmitted to the brain via the optic nerve.
But in some animals especially the ones that go out at night, there is a specialized layer of tissue called the Tapetum lucidum right behind their retinas. It acts like a mirror, reflecting light back through the retina, giving it a second chance to be detected by the photoreceptors.
In animals with a tapetum lucidum, any light that isn't initially absorbed by the photoreceptor cells can pass through the retina and reach the tapetum. The tapetum then reflects this light back to the photoreceptors on the retina, thus giving them a second chance to detect the light. This enhances the the animals' ability to see in low-light conditions such as during night time.
Some of the reflected light also comes out of the eyes creating the appearance of eyeshine, where the animal's eyes appear to glow or shine when light is directed at them. The color of the eyeshine can vary depending on the animal species. For example, some animals have a green eyeshine, while others may exhibit yellow, blue, or red eyeshine.
This phenomenon is particularly noticeable in nocturnal animals that have adapted to low-light environments, such as cats, dogs, raccoons, and deer. It helps these animals to have better night vision and allows them to see more effectively in the dark.