Like most human traits, blue eyes are determined by genes, and no two people have exactly the same hue. It works a lot like individual fingerprints, making our eyes two of the most distinctive parts ...

The condition may also be present from birth. Pupils, or the black parts at the center of the eyes, change size to regulate the amount of light entering the eye. Pupils are large in the dark to ...

That coloured circle is called the iris and the black spot in the middle is the pupil. This is the part of the eye that lets the ... green and blue, and all the millions of shades in between.

Nocturnal animals tend to have proportionally bigger eyes than humans do. They also tend to have pupils that open more widely in low light. So, at the outset, nocturnal eyes gather more light than ...