Under the retina, the back of the eye is covered with shiny, blue-green stuff. This is the tapetum. It reflects light from the back of the eye. Have you ever seen a cat’s eyes shining in the headlights of a car? Cats, like cows, have a tapetum. A cat’s eye seems to glow because the cat’s tapetum is reflecting light.
If you shine a light at a cow at night, the cow’s eyes will shine with a blue-green light because the light reflects from the tapetum.