Fun fact: the reason cats eyes (and many other animals) light up like this is due to their night vision adaptations. When light enters their eyes it is picked up by the receptors once as it passes through then it hits the back of their eye where it is reflected back through the receptors allowing the animal to pick up more light and therefore see more clearly in low light situations. The side effect of this is that even with this adaptation not all the light is absorbed to a portion of it comes back out their pupil.