Why does the eyes of some animals glow in the dark???

Question

Why does the eyes of some animals glow in the dark???
Why doesn't the eyes of human being glow in dark? What 's the theory behind it?

Answer 1

It is because of a reflective layer present in retina called TAPETUM LUCIDUM

Answer 2

The truth is that the glow is the only reflection of light from some other source It is probably caused by our car headlights, or it may be caused by a flash light. The reason that reflection takes place is that there is a layer of crystalline substance in the eyes of many animals. This substance has the ability to reflect light. Man has almost none of this substance in his eyes.
This reflecting layer also helps the animals see in the dark, which is why they can see better at night than man can. The differences in colour of the light reflected from the eyes of animals are due to the different number of blood vessels in their eyes. An animal that has many blood vessels in its eyes will reflect a reddish glow. If it has fewer blood vessels, it will have a whiter glow.

Answer 3

I will speak here about cats, but this things is happening to a lot of other animals.

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.
I hope this will help.

Answer 4

these animals have a thick layer of a special crystalline substance behind the retina called luminous tapetum which reflects the light falling on it. the colour of eyes depend upon the amount of blood vessels present in eyes.

Answer 5

They have a special reflective layer in the eye called a tapetum lucidum, that reflects any available light....read more here:
http://en.wikipedia.org/wiki/Tapetum_lucidum

Answer 6

they seem to glow simply because of a reflection from the light.

Answer 7

It is just the light that reflects off their pupils. Kind of like a reflector.

Answer 8

It's just that their pupils hve less melanin. So, light is greatly reflected, compared to most humans.