With the cold weather, a large number of mule deer have moved into the canyon. Why are their eyes glowing in this photo?
Many mammals, such as dogs, cats, raccoons, and deer have eye shine. When you shine a light on them at night, their eyes glow. This is due to a layer of tissue called the tapetum lucidum. This layer is located behind the retina, the layer of light sensitive cells in the eye. The tapetum lucidum is reflective. Light enters the eye, and hits the retina. That stimulates the cells to form a visual image. Then the light passes through the retina, reflects from the tapetum lucidum, and hits the light sensitive cells again. This gives the animal much better vision at night. Humans do not have a tapetum lucidum. The red eyes that you sometimes see in flash photos are the result of some light reflecting from your retina.