Examining a Cat's Eyes: How Do They Work?

A cat has eyes are designed to operate in low-light conditions, if necessary, the eye itself can see in light approximately one-sixth that of a humans eye. Based on this the cat is considered best suited for hunting at night. In low light situations the cat's eyes will open as wide as possible, and be able to contract it to very small size when the sun is out in order to protect it's retina.

In humans the size variation of our pupils are worked by a circular ancillary muscle, but what distinguishes cat's eyes from humans is that it has two of these ancillary muscles and they work much like a shutter on a camera. A cat's eye is therefore elliptical and this gives the cat a slit like pupil in bright light conditions. Cat's eyes are larger than humans and this allows the pupils to be larger which helps accept light in during low-light occasions.

A cat's eye also has something like a mirror behind it's retinue that is really a "tapeturn lucidum" in the back of the eye. This reflects light back out of the eye towards the source and this is why it appears that a cat's eyes glow at night when they are caught by your flashlight or other light source.

In all eyes there are two types of light receptors, cones and rods. Cones are used for color and are for high levels of light and Rods are most sensitive in low levels. It is no wonder then that a cat has a greater concentration of rods in it's retina. The tapeturn lucidum is designed to give these Rods another chance to see an image, because when it reflects the image back like a mirror, the Rods have another shot at it. A cat is not color blind but it cannot see colors as well as humans.

There seems to be a debate that cats can see in total darkness but this is a myth. Cat's can see in very low light much better than humans but they cannot see in total darkness, and the fact is they cannot see any better than human beings can in the dark.

Cats do have a third eyelid that protects the eye from damage or being too dry, and they occasionally will show this. If a cats third eyelid sticks half open then it is usually a sign that the cat is ill and needs to see a vet as soon as possible.

Cats have very good vision and if perfect or acceptable vision for a human being is 20/20 than a cats "good" and "average" vision would be about 20/100. They have very good vision for things far away which helps in tracking down small critters like mice at a distance (or bugs) but things get fuzzy sometime the closer it gets, this is why some have speculated that a cat will do the bump and sniff technique when something is in front of it's face.

Source:
Orlin Sorenson, Why Do Cat's Eyes Glow in the Dark?, Chet Day
Franny Syufy, In the Eyes of a Cat, About.Com