Why do the eyes of some animals shine in the dark?

Have you ever seen a cat sitting in the dark? Though its body is not visible, yet its yellow eyes shine in the darkness. The eyes of tiger, cheetah, leopard, lion etc. also shine in the darkness of the night just like the eyes of a cat. Besides these, there are many other animals whose eyes shine in the dark, but our eyes do not. Do you know why?

The eyes of these animals have a thin layer of some special crystalline substance which reflects the light falling on it. This reflected light is the cause of the shine. In the night, even the smallest amount of light falling on this crystalline surface gets reflected and we see the shining eyes. Animals with such eyes can clearly see things in darkness. They are called nocturnal animals.
Studies mad on cat's eyes have shown that behind their retina, there is a crystalline layer called the 'luminous tapetum', which reflects light. Due to this layer the cat can see clearly in dim light and its eyes shine in the dark when the light is directed on them. There are many animals whose eyes do not shine because their eyes are not gifted with this crystalline layer.
It has been observed that the colour of the shine is different in different animals. It depends upon the number of blood vessels present in their eyes. If the number of blood vessels is large the colour of the shine is red and if the number of blood vessels is less the colour of the shine is white or pale yellow.