Why do my Dog's Eyes Shine When I Point a Light at Him?

Dogs', cats', horses' and cows' eyes glow when you shine a light on them because of a reflective layer of tissue at the back of the eye called the tapetum lucidum which is latin for Bright Carpet. The purpose of this layer is to help animals that come out at night, like dogs and cats, to see better at night. It works by acting as a mirror which reflects light hitting the back of the eye-ball back onto the light-sensitive part of the eye called the retina, giving the eye a second chance to pick up as much information from the light as possible. Some of the light reflected back in this way comes back out of the front of the eye causing the glowing effect that you see when you shine a bright light at your pet. During the daytime there's already too much light around to notice the effect. Humans don't have this reflective layer at the back of the eye and so when a bright light hits our eyes - like a camera flash - you often see people with 'red eyes' which is the camera flash lighting up the blood vessels at the back of the eye.