It is sometimes thought unthinkingly that the eyes of a cat shine in the dark - that they are autophanous as some have called it. But it is not so. What cats' eyes do have, however, is the property of reflecting a beam of light in a very specific kind of way. Light rays from a distant source entering an animal's eye are brought to a focus on the retina - the surface at the rear of the eye which is, as it were, a screen on to which the images of what is seen are projected.
Now in the case of some animals, and in particular the cat, the spot illuminated on the retina by each light-ray acts itself as a secondary source of light; light emanating from that bright-spot passes back through the lens of the eye and heads off in the direction whence it came - back towards the distant source. The eye, when it behaves like this, is not acting as a mirror. There are important differences. A mirror reflects light in a certain direction which depends on how the mirror is orientated; but in the case of the cat's eye, its spherical construction gives it the unusual property of sending the light back directly towards the source, regardless of the orientation of the eye - or indeed the cat itself. This "retro-direction" is not perfect; the returning light spreads out a little - but not much. You can observe the process at work by shining a flashlight at your feline friend from a distance of some yards; you will notice that when the flashlamp is next to your head the cat's eyes light up - as it happens, with a rather greenish tinge - but if you hold the lamp at arm's length to the side, the glow will disappear.
Cats, of course, are not the only animals whose eyes shine brightly when illuminated. Rabbits and horses have this characteristic; so do sheep, and likewise dogs, whose eyes shine with a distinctly reddish hue. And even human eyes, although they do not show the effect as strongly as those of many other animals, can be seen to react this way in certain circumstances. The phenomenon is well known to camera manufacturers. When a photograph is taken with the flash bulb mounted very close to the camera lens, even the most angelic human subject can appear with devilish red eyes. One solution to the problem is to have the bulb sufficiently far away from the camera so that the "retro-directed" light from the subject's eyes will miss the camera lens on its return journey.