Eye Shine....What Is It?

Eyeshine is a very common occurence amongst nocturnal animals.  Have you ever tried to take a picture of your dog and cat, but their eyes are glowing a very odd color?  Or you have been watching Animal Planet or another similar channel, and they show a night-vision scene, with the animals eyes glowing?  This is eye shine.  It is caused by a reflective layer in the eyeball, called the tapetum lucidum.

Tapetum lucidum in an armored dog

For nocturnal animals, seeing in the dark of night can be a problem.  Some animals counter this with extra large eyes.  This works well for nocturnal animals, sure: but think about in the morning, when you first wake up.  If someone comes into your room and just opens up the window shade, you might not be very happy, as your eyes generally need some time to adjust to the lighting conditions.  If you had gigantic eyes, this problem would be magnified even more.  So many animals that are active during both the night and the day have evolved something else; little mirrors in their eyes.

You can clearly see that the angle of tilt of the head makes the eyeshine much more intense in the left eye of this chubby cat

Here is essentially what happens.  When light hits our eyes, it is absorbed.  This gives us humans one chance to absorb the light that we can.  When light hits the eyes of, say, a dog or cat, or any other animal active at any time of the day, some of the light is absorbed, while some of the light is reflected back to the object the animal is viewing.  The light hits the object, and then bounces again back to the animal, giving it essentially a second chance to view the object.