Why do eyes of some animals shine in the dark?

Have you ever seen a cat sitting in the dark? Though its body may not be visible, its yellow eyes shine in darkness. The eyes of many other animals such as tiger, leopard, and lion also shine in the night, just like the eyes of a cat. The eyes of these animals have a thin layer of a 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 thus we see the shining eyes. Animals with such eyes who can see things clearly in darkness are called nocturnal animals. Studies made on cats have shown that behind the retina of their eyes, there is a crystalline layer called the ‘luminous tapetum’, which reflects light. Due to this layer, a cat can see clearly in dim light and its eyes shine in the dark when light is directed on them. Many animals do not have this crystalline layer; hence their eyes do not shine in the dark. It has been observed that the colour of the shine varies 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.