Why do cats’ eyes glow in the dark? A cat’s eyes glow in the dark because they have a special reflective layer in their eyes called a tapetum lucidum. Cats aren’t the only animals whose eyes glow at night.
The reason for this comes down to the way that we see and why we can’t see well in the dark. We see when light enters our eyes. People used to believe that we had sight because our eyes sent out light rays that touched the objects we looked at, but in the 10th century, Ibn al-Haytham worked out that it was the light coming into our eyes and not the other way around. His evidence for this was that bright light hurts our eyes and if the light was coming out of our eyes, that couldn’t be the case. When the light enters our eye, it is focused on the retina at the back where it activates the cones. We have cones for red, green, and blue, and different wavelengths of light activate them to different amounts, allowing us to see all the colors we can see. The cones need a certain amount of light to function. If there is not enough light for the cones, then we have to rely on our rods. They are sensitive to far lower amounts of light than the cones, but they only register the intensity of light and not the color. That is why we can’t see as clearly at night and why we can’t see in color at night. The rods are also on the edge of your retina, which is why your peripheral vision is often better at night than looking at something straight on.
As humans, we use our eyes to find food either by foraging or hunting. We rely on our ability to see many shades of colors to find animals or food and that is far easier to do during the day. We have evolved eyesight that prioritizes daylight seeing and allows us to see at night, but only the bare minimum. If we were nighttime hunters, we would probably have evolved eyes that saw much better in the dark, which is what happened to cats. Cats rely more on seeing movement than they do on color, so their color vision doesn’t need to be as good.
You need two things to be able to see more clearly at night. You need more rods in your eye and you need to get more light into your eye. Cats have the first thing. They tend to have eight to ten times more rods in their eyes than we do. This means they can see lower levels of light far more reliably than we can. The second thing is to get more light into the eye. When light hits our eyes, some is absorbed and some passes through the retina without hitting any of the cones or rods. This is wasted light. Cats, and several other mammals, have a layer called a tapetum lucidum, which is made from riboflavin and zinc. It is highly reflective and light that passes through the retina the first time, hits this layer and is reflected back out. This gives the rods a second chance to pick up more of the light, basically doubling the cats chance of getting enough light to see. A side effect of this is that even on the second pass, not all of the light is absorbed, and a lot of it reflects back out of the cat’s eye, making them appear to glow. All mammals that need to see at night have a similar layer and if you shine a light on a herd of sheep at night, you will be confronted with many glowing eyes staring back at you. However, while the cats use their night vision for hunting, sheep use it to see predators.
Humans don’t have this reflective layer, but our eyes can sometimes glow when people take photographs of us. Red eye is something that often crops up in photos, although modern cameras can remove it. It happens because the light from the camera flash passes through our eye and reflects off the blood rich retina at the back of the eye. It often happens at night or in a dark setting because our pupil is open to allow more light in and that let more flash in. The person also needs to be looking directly at the camera so the light reflects back to the lens. And this is what I learned today.
Sources
https://panthera.org/blog-post/spooky-science-glowing-eyes-cats
https://www.cats.org.uk/cats-blog/why-do-cats-eyes-glow-in-dark
Photo by Mariya Muschard : https://www.pexels.com/photo/captivating-portrait-of-a-british-shorthair-cat-36376383/