It looks creepy when you take a picture of your cat in the dark or shine with a strong flashlight in the jungle. How come animal eyes 'give light'?
A beam of light or a camera flash can show on the savannah at night whether there is a flock of lions in the area.
That the eyes of cats and other animals glow in the dark is due to a layer in their eye, the tapetum lucidum. This layer is behind the light-sensitive retina and reflects the light that passes through it.
The light-sensitive cells in the eye can use the light again. The reflection increases the light sensitivity of the eyes considerably (with cats by around 50 percent) and is therefore especially useful for nocturnal animals.
Cats in particular have such a reflective layer, but it occurs in more species - albeit not in humans or other primates. The red eyes that we sometimes have on photos are caused by reflections from the retina itself. The red color comes from the many blood vessels that provide the eye with oxygen and nutrients.
GO TO THE FASCINATING ANIMALS EVERY MONTH as a subscriber to Science in Focus.
Bright eyes provoke prey
A reflective layer behind the retina, the tapetum lucidum, illuminates animal eyes.© ANDREW MEYERSON
However, some animals have a red tapetum lucidum.
For example, red eyes are common in birds that are active at night, and with some species each eye even lights up in a different color. In fish the reflection can be so powerful that researchers think the animals use it as a lamp to find or lure their prey.
The tapetum lucidum only occurs in vertebrate animals, but moths have similar light-amplifying structures.
With them it is a nano layer on the eyes that prevents the light from being reflected, to protect against predators.
Reflective layer gives eyes color
At the back of certain animal species there is a reflective layer, the tapetum lucidum, that illuminates the eye.