Science Photo of the Day
Why are this alligator's eyes glowing?
Fantastic job. Its eyes are glowing because of light from my camera flash, reflecting from the back of the gator's eyes. This reflective layer is called the tapetum lucidum, and it is just behind the retina. Light enters the eye, passes over the light sensitive cells of the retina, reflects from the tapetum lucidum, and hits the cells of the retina again. That should make the image look twice as bright, a big advantage for creatures that are active at night, but there is more to it.
When we looked at the oil on water photo, we saw that two waves that meet when they are in sync cause constructive interference, making the light brighter. Constructive interference lets the tapetum lucidum improve night sight as much as 600%.
Many animals, from rabbits to cats, deer to wolves have eyeshine due to a tapetum lucidum, but humans do not. The red eye seen in photographs is light reflected directly from the retina, not a tapetum lucidum.
I took this photo in the Everglades, but I was not worried. Alligators don't usually bother people, unless they are used to being fed by people or the people are bothering them. This one was much more interested in looking for a nice duck to eat.