What causes green eye in photographs is a very common question.
What is Green Eye
Green eye in photographs actually refers to the animal version of red eye
Causes of Green Eye
Green eye occurs when light enters the eye at almost level with the eye. Because light reflects at an angle equal and opposite to its entry (in layman's terms, it makes a "V" shape) if bright light (such as from a flash) enters the eye at almost no angle it will be reflected back, or visible to, the camera. This most often happens when the pupil of the eye is dilated and gives the light more room to enter and exit the eye. This is the same as what happens with red eye in humans.
But Why Are the Eyes Green?
The eyes of animals appear green, or blue, or white, or yellow because of their night vision. Many animals have a membrane called the Tapetum Lucidum in their eyes. Depending on the chemical makeup of this membrane, different light spectrum are reflected. So in animals that have this membrane the light is blocked from striking the blood-rich retina that would show up as red eye. Blue-eyed cats do not have the same membrane in their eyes so they actually show red eye instead of another color.
Green Eye is Most Likely to Occur in These Situations
Built-in camera flash
Camera lens is level with the eye
Preventing Green Eye
Preventing green eye is not as difficult as you might think. To prevent red eye you just have to alter the angle the light enters/exits the eye or change the harshness of the light.
Ways to Prevent Green Eye
Use a flash with a tilt head
Use a reflector to add light
Use a diffuser on your flash
Shoot from a slightly higher or lower position than your subject's eyes