Catlin Cook, of Balclutha Primary School asks :-

Can eye colour change with age?

Gordon Sanderson, an ophthalmogist at the University of Otago, responded.

The first thing to understand is why peoples’ eyes are different colours. Some people have blue eyes whereas others are a very dark brown, some are green and others grey. These colours although called eye colour, actually refer to a part of the eye called the iris.

The main function of the iris is to control the amount of light entering the eye and it does this by adjusting the pupil size. The pupil is actually a hole; it looks black because normally no light comes out of it. If light ever does come out of the pupil it looks red, this can often be seen in a flash photograph.

The iris needs pigment to block the light; it actually has two layers of pigment (pigment is a dark brown powdery substance that we see colouring peoples’ hair and freckles). One layer is at the back and can’t be seen by looking into the eye without special equipment, the other layer is at the front of the iris and that is the part we normally see when we look at somebody’s eye.

People with blue eyes have no pigment in the front layer; only the back layer is present, blocking the light around the pupil. The front layer of the iris in somebody with blue eyes is actually white, it looks blue for the same reason the sky looks blue: the scattering effect. If there is a little bit of pigment in the front layer our eyes look grey, a bit more and they can look green and even more pigment makes them look brown.

Does the colour change with age? Yes, it does a little; some eyes get lighter, but the majority get darker, particularly up to the age of six, only small changes occur through to adulthood.

Although your genes, that is to say your parents, usually determine your eye colour, some eyes do change colour for other reasons. There are some eye diseases that can make your eyes go darker and some that can make them lighter. This can result in people with different coloured eyes; this is called heterochromia, from the Greek meaning different colours.

Some changes are caused by old age. If an old person will allow you to look closely into their eyes you might see a pale line around the outside of their iris, this is one of the colour changes associated with old age. I will leave it to you to ask how old the person with this pale ring actually is.