why do cats eyes light up in the dark ???

Unlike human eyes, which work best in day light, the cat’s eyes must function well in extremely low light condition and as such are well suited to an animal that is predominantly nocturnal and crepuscular in activity. In darkness, cats eyes are able to function in approximately one-sixth of the light needed for human vision. However they must also be able to function well in daylight – so just how is this achieved.

In low light levels the cats pupil must be able to open as wide as possible, but also be able to contract to very small size to protect the sensitive retina in bright sunlight. In human eyes, this size variation of the pupil is controlled by a circular ciliary muscle, but this limits the amount of size variation. In cats however, the same process is controlled by two, shutter-like ciliary muscles, which gives the cat it’s characteristic slit-like pupil in bright light conditions. All cats pupils are therefore elliptical, however some, notable the ‘Big Cats’, appear more circular when dilated.

The size of the cats eye is relatively larger than those found in human, this enables a larger pupil and therefore more light to enter the eye. Generally, the lens is more curved enabling sharper focusing even at the edges of the lens. The size of the anterior chamber and the curvature of the cornea is also greater, which helps more light to be refracted onto the light-sensitive retina.

Another feature, which enabled the amount of light hitting the retina to be increased, is the tapetum lucidum. This is positioned at the back of the eye, behind the retina and acts like a mirror, reflecting light back onto the light sensor cells in the retina. This gives the cats eyes the characteristic night-time glow when they are caught in a beam of light.

There are two distinct types of light receptor cell on the retina – Cones, which are sensitive to high levels of light, used in colour vision and Rods, sensitive in low light conditions. In cats, there is a greater concentration of Rods, aiding their night-time vision. As in humans, there is a greater concentration of receptor cells at the centre of the eye, leading to the optic nerve. In cats, however, these a concentrated along a broader, horizontal band. This gives the cat far more sensitivity to movement along the horizontal axis and they are therefore more able to detect prey movement along the ground at greater distances.

A cat's eyesight is 6 times better than a human in lower lighting contitions. When opening (or dilating), a cat's pupils can dilate much faster than yours or mine. They can also dilate three times larger than yours or mine can. This lets in a lot of light, and is one of the reasons cats have their famous ability so see well in low light. They don't see well in total darkness, which is a popular myth, but it is true that their eyes' ability to dilate to such a large degree is part of the reason they have an exceptional ability to see in poorly lit conditions. The one thing everyone seems to notice about a cat's eyes are the pupils. A human pupil is always round, but a cat's pupil can be either round (when it's dilated) or it can shrink in from the sides in stages until it is elliptical. When elliptical, a cat's eyes look like a slit, stretching from top to bottom. This special adaptation allows a cat to squint its eyelids, covering only part of the pupil but not all of it, thus giving it at least some manual control over the amount of light let in. If the pupil was simply reduced to a small circle, closing the eyelids would cover the pupil all together.

really interesting question!

Many animals that hunt at night, including cats, have eyes that appear to glow when a light shines on them. The glow that you see is light reflected from a mirror-like structure, called the tapetum lucidum, which lines the back of the eye. Light enters the eye and passes through the retina where special cells, called rods and cones, absorb the light. Any unabsorbed light hits the tapedum lucidum and is bounced back to the retina where the light has another chance at being absorbed. Because animals with a tapetum lucidum absorb more light, they can see better in the dark than those lacking the special structure, like humans. For example, cats can see in about one sixth the light that humans require. The ability to make the most use of any available light is an important advantage for those animals which must catch their next meal at night.

also this website is intresting and goes into more detail

http://dspace.dial.pipex.com/agarman/bco/fact4.htm

If you mean the ones in the animal: cats and various other creatures are designed for hunting at night. The inner surface of their eyes is reflective so that light passes through the retina twice, allowing them to see in much weaker light. The light passes back through the lens of the eye in the direction it came. Because cats' retinas absorb predominately green light, they reflect red. Foxes eyes reflect green.

The ones in the road are lumps of glass (more modern ones are polycarbonate with a reflective coating on the back so that they reflect the light in exactly the same way as nature's cats' eyes.

Reflection off the retina, did you know that cats eyes in the road were invented by a yorkshire man? A cat jumped out in front of his headlamps and he thought that would be good to light up the middle of the road :D Had the cat been walking the other way he would have invented the pencil sharpener lol

cats' eyes open and close much faster than do ours. And cats have a special membrane on the back of their eyes (called the tapetum lucidum, literally meaning "bright carpet") that increases the quantity of light caught by the retina. The tapetum collects and re-emits light back to the retina, giving the rods a second chance to absorb the image, thus maximizing their sensitivity to low light levels. As this light is reflected off the tapetum, the animal's eyes appear to glow

The back of their eyes are reflective and they can reflect the light that's in the area. Even though though it's dark, there is still some light.

"Cats, like dogs and many other animals, have a tapetum lucidum, which is a reflective layer behind the retina that reflects light that passes through the retina back into the eye."

Glow in the Dark

Human beings are not like tigers, or housecats for that matter, whose eyes are superbly well adapted to seeing in the dark. One reason is that cats have more rods than cones in their retinas, unlike humans, making kitty's night and motion vision superior. (Rods are the receptors that the eye uses for nighttime viewing and sudden movement; cones are used during the daytime and process color information.)

Also, cats' pupils are shaped differently than those of humans (they are elliptical rather than round), which allows for a much larger pupil size. In fact, the most notable feature of nocturnal animals is the size of their eyes. The reason for this is that large eyes can collect more ambient light.

They don't. Cat's have the ability to catch light and reflect it within their eyes which gives them a sort of night vision.

I think it has to do with retina in their eye that allows them to be more sensitive to the dark. They're night creatures so they're able to reflect light better.