Cats possess a built-in mirror in their eyes that allows them to see in near-total darkness.

The tapetum lucidum is a specialized biological reflector located behind the retina in many nocturnal vertebrates. In domestic cats (Felis catus), this layer is composed of organized layers of riboflavin and zinc crystals. These crystals act like a mirror to reflect photons that weren't absorbed during their first pass through the photoreceptors.By reflecting light back through the retina a second time, the cat's eye maximizes every available photon. This biological adaptation increases the sensitivity of the retina by approximately 44 percent compared to eyes without this layer. Research published in the Journal of Comparative Physiology indicates that while this improves light sensitivity, it slightly reduces visual acuity due to light scattering.The distinctive 'eye shine' or retroreflection seen at night is technically known as the tapetal reflex. The color of the glow often appears green or yellow in cats because of the specific mineral content and structure of their tapetal cells. This mechanism is so efficient that cats can navigate and hunt in light levels as low as 0.125 lux, which is effectively pitch black to a human eye.Evolutionarily, this trait emerged to support crepuscular hunting patterns, where predators are most active during dawn and dusk. Beyond cats, this feature is found in dogs, crocodiles, and even some deep-sea fish. However, humans and most other primates lack a tapetum lucidum, prioritizing high-resolution color vision over extreme low-light sensitivity.