Why do cats’ eyes shine in the dark?
Short answer
Cats’ eyes seem to shine because a reflective layer inside the eye sends some incoming light back out. The eyes do not make their own light.
How it works
Light enters through the pupil and reaches the retina. Behind the retina, cats have a reflective layer called the tapetum lucidum. It sends back some light that the retina did not absorb on its first pass, giving light-sensitive cells another chance to detect it.
Some of that reflected light can leave the eye and reach an observer. When a flashlight, car headlight, or another light source shines toward a cat, its eyes may appear to glow. The effect changes with the position of the light and the viewing angle.
Key point
Widening the pupil to let in more light and reflecting light back across the retina are separate parts of how a cat’s eye works. Without incoming light to reflect, a cat’s eyes do not shine by themselves in complete darkness.