Without getting too technical, let me try to explain cat eyes. Just like humans, cats have a clear, convex cornea covering the front of the eye, behind the cornea lays the aqueous humor, a clear liquid that nourishes the cornea and the lens behind it. The iris (the colored part of the eye) covers layers of muscle that work like the aperture of a camera. It works by constricting the amount of light that goes through the pupil (the black-appearing opening through the iris) and the lens. The light then passes through the vitreous humor, a viscous material behind the lens that fills up most of the body of the eye. The light eventually reaches the retina, which lines the very back of the eyeball.
The pupil size and shape are controlled by the iris muscles. In cats, (like most nocturnal hunters), the iris contracts into a slit in bright light to limit retinal light exposure. The smaller aperture also improves acuity and depth of field. In dim light, the cat's irises open, allowing the pupils to dilate to almost 90 percent of the irises area to catch all the light it can.
The glow of a cat's eyes at night comes from the reflective cells of the tapetum lucidum ("bright carpet"), which underlies the upper portion of the retina. The tapetal cells contain a crystalline complex of amino acids, minerals and riboflavin, which reflect back any light that passes through the retina, like thousands of tiny mirrors! Light that has not been absorbed by its first pass through the retina is reflected, back through the retina, where it gets a second chance to be absorbed. It is the absorbed light that allows for vision, and so the presence of a tapelum allows cats to see movement and objects in semi-darkness, things that we humans could not possibly see or make out clearly.
Cats' relatively close-set eyes afford them excellent binocular vision. The binocular (or stereoscopic) vision is created by the overlap of the visual fields in each eye. It is crucial for hunters and allows them to judge distance, depth, and size accurately. Cats notice and can focus on the tiniest objects, a dust mote as it floats through the air or a spider the size of a dot crawling half way up the wall across the room.
Do cats see in color? Scientists used to think that cats could not see any colors, some of the specifics about color sensitivity remains unknown. Although under some conditions, cats do have color vision. Cats certainly do not use color vision the way humans do, but they seem to be able to distinguish between colors.
Have you ever heard the term "flicker-fusion"? Look at a cartoon flip-book. Cats can discern separate images at a faster rate, what looks as if smooth movement to us would look jumpy to a cat. A good example would be the television. Cats can become transfixed by the television, especially animal behavior shows. It's not just the sound, but cats see a series of jumpy images rather than the smooth, whole picture, movements that we humans see.
Cats also use their eyes to communicate. Within the cat world, a direct stare, unblinking, is a sign of hostility. The cat may make low pitched noises, a sign that a fight is in the making. Within the cat to human world, direct eye contact can mean that you have the cats' attention, much the way humans use eye contact. When a cat is being dominant, it puts its head down slightly, dilates the pupils and gives the adversary "a dirty look". Cats not only give each other "dirty looks" but we humans get our share from our cats when they are displeased with us!
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