Why do Cat's and Rat's eyes glow in the dark?

While cat’s eyes and other nocturnal animal eyes don’t really glow, there is a layer at the back of their eye’s called the tapetum. This is formed from fibres which are reflective and it’s these which reflect back light out of the eye like a mirror. The result is that cats can see far better in the dark because of the reflective, light enhancing properties of the tapetum, allowing them to operate with one-sixth the illumination we humans require.