Have you ever walked around a dark corner only to be surprised by glowing eyes staring back at you? The glowing eyes of a cat at night can sometimes be shocking and even a little scary if unexpected. Ancient Egyptians believed cats captured the glow of the setting sun in their eyes and kept it safe until morning. Ancient Greeks believed there was a light source inside the eyes that was like a gleaming fire. We now know that cat’s eyes appear to glow because they, along with the eyes of many other nocturnal animals, reflect light.
All eyes reflect light, but some eyes have a special reflective structure called a tapetum lucidum that create the appearance of glowing at night. The tapetum lucidum (Latin for “shining layer”) is essentially a tiny mirror in the back of many types of nocturnal animals’ eyeballs. It basically helps these animals see super-well at night. It is also what causes the glowing eye phenomenon known as “eyeshine.”
How Does It Work?
When light enters a cat’s eye, it can take a few routes. Some of the light directly hits the retina, a layer at the back of the eyeball containing cells that are sensitive to light. These photoreceptor cells trigger nerve impulses that pass via the optic nerve to the brain, where a visual image is formed.
Some of the light passes through or around the retina and hits the tapetum lucidum. The tapetum lucidum reflects visible light back through the retina, increasing the light available to the photoreceptors. This allows cats to see better in the dark than humans.
In the last route, some of the light that bounces off the tapetum lucidum, misses the retina, and bounces back out of the cat’s eyes. This reflected light, or eyeshine, is what we see when a cat’s eyes appear to be glowing.
Do Humans Have a Tapetum Lucidum?
Though our eyes have much in common with cats’ eyes, humans do not have this tapetum lucidum layer. If you shine a flashlight in a person’s eyes at night, you don’t see any sort of reflection.
The flash on a camera is bright enough, however, to cause a reflection off of the retina itself. This is the infamous “red-eye” in photographs. What you see is the red color from the blood vessels nourishing the eye.
In this two-part activity, you will be able to see how the tapetum lucidum works and then simulate how this reflective layer helps cats see well at night.
- Make about a hole in your paper or cardboard using a pencil or pen. It does not have to be perfect! If using a thinner paper, try folding it a few times before making the hole (the harder it is to see light through, the better!).
- Hold the cardboard about 6 inches away from a blank wall and shine the flashlight through the hole toward the wall.
- Without looking directly into the light, glance at the side of the cardboard facing the wall. Take note of what you see.
- Repeat steps 2 and 3 while using your mirror instead of a wall. Again, avoid looking directly into the light or its reflection in the mirror! Note the difference in light on the side of the cardboard facing the mirror.
Imagine that the cardboard is a retina and the mirror is a reflective layer like the tapetum lucidum. What happened to the “retina” when the mirror was used instead of the wall?
This experiment shows how the amount of light from a singular light source is doubled when a reflective layer is present. Thus, it shows us how having a reflective layer—like a tapetum lucidum—increases the amount of light information available.
1. Place your glass container about 6 inches away from a wall.
2. Shine your flashlight through the glass toward the wall and observe how the light appears on the wall.
3. Fill the container with water and place in the same spot as before.
4. Shine your flashlight through the glass toward the wall and observe how the light now appears on the wall.
Imagine that the wall is the retina and the water is a reflective layer like the tapetum lucidum. How does the reflective layer change the presence of light on the retina?
While this experiment is technically showing how light refraction works in water, it can also show us how having a reflective layer—like a tapetum lucidum—increases the amount of light available to cats’ eyes. Also, that the extra reflective light is not as clear as the original light input.