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Why do cats' eyes glow in the dark?

Close up of cat with glowing eyes

Cats' eyes glow in the dark to help them hunt at night.

© PublicDomainPictures - Pixabay

Have you noticed that your cat's eyes glow in the dark? While it can give cats a rather mystical air, have you ever wondered what causes their eyes to glow?
 

By Zoë Monk

Published on the

If you have a cat, you will likely have noticed that its eyes look as if they glow in the dark or have a green glare in photographs.

But while it may make your moggy seem all the more magical, there are actual scientific reasons behind what makes your cat's eyes glow (but this doesn't mean your cat has x-ray vision).

What does it mean when cats' eyes glow in the dark?

Like humans, cats have a layer of tissue at the back of their eyeballs called the retina. This comprises light-sensitive cells that turn light into the eye into electrical signals. These signals are sent to the brain to decode and work out what your cat sees. However, because a cat's most active time is typically at dawn and dusk, there isn't much natural light. They, therefore, need something to improve their night vision. Fortunately for your cat, their eyes are extremely clever. They are large to let more light into the eyes and their pupils can also become up to 50% bigger than human pupils to absorb more light. But it's what's behind a cat's eye that gives it the mystical glow. 

Behind a cat's retina is another layer of tissue called the tapetum lucidum, Latin for "shining layer". This thin layer of tissue acts as a mirror to pick up whatever light enters through the retina and bounces it back towards the light-sensitive cells in your cat's eyeball to provide a second chance at being detected. Not all of the available light is picked up during this second go. Some of it is passed back through the retina again and out through the front of the eyeball. This makes it appear that your cat's eyes are glowing in the dark. The tapetum increases retinal illumination in low light, transforming your cat's eyes into tiny torches that allow them to detect changes in light and motion and help them hunt in the dark.

Is it only cats' eyes that glow in the dark?

Several other animals also have eyes that glow in the dark, including dogs, sheep, goats, nocturnal animals such as foxes, and even dolphins and fish. But their eye "glow" can appear in different colours, depending on what the tapetum lucidum is made from. For example, in a cat's eyes, the tapetum lucidum is made from zinc and riboflavin molecules. How green, yellow or blue the glow appears will depend on how much zinc there is.

What colour do cats' eyes glow in the dark?

Most cats' eyes tend to glow a bright green colour. However, the specific colour will depend on the cat's breed, eye colour, coat colour and amount of riboflavin (type of vitamin B) or zinc in the pigment cells within the tapetum lucidum. Siamese cats, for example, generally have poorly developed tapetum lucidum, which means they often have poor night vision as a result and generally cast a bright yellow glow from their eyes.

Do all cats' eyes glow in the dark?

If you notice that your cat's eyes don't glow in dim light, the light might be entering your cat's eyes differently, or the tapetum is missing or not fully formed in either one or both eyes. However, a problem with a cat's eyes and vision such as conjunctivitis, cataracts or glaucoma can stop their eyes from glowing at night. You can check your cat's vision yourself during daylight hours when your feline friend is at their most alert. For example:

  • Watch how your cat negotiates stairs and their cat tree to see if they move confidently
  • Shine a dim torch into their eyes to see whether the pupil contracts
  • Place soft and non-hazardous objects in your cat's path to their water, food or litter to see if they navigate around them
  • Drop a light, silent object such as a feather and see if your cat hunts it
  • Shine a laser pointer in front of your feline friend and see if they react

If you have noticed a sudden change in the glow of your cat's eyes or you are concerned about their vision or eye health, you must get your cat checked out by the vet as soon as possible.

Why do cats' eyes glow on camera?

If you've noticed that your cat's eyes glow in photographs, this is due to the tapetum lucidum. When you take a photograph of your cat, the light from the flash hits the tapetum lucidum in their eye and bounces back towards the camera. This is what gives the green glare in your photos. There are some easy things you can do to avoid the look of "green eye" when photographing your feline friend:

  • Take the photo from a slightly lower or higher position than your cat's eyes
  • Soften the flash with a semi-transparent white material such as tissue
  • Tilt the flash, so it bounces off the wall or ceiling 
  • Direct your cat's eyes just slightly away from the camera to avoid the light reflecting directly into the camera

So, what colour do your cat's eyes glow?
 

Frequently asked questions

Why do cats' eyes glow green at night?

Why do cats' eyes glow red?

Why do my cats' eyes glow blue?

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