Making sense of how our eyes work

Last updated 15:54 19/10/2009

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Laura Moore, of Balclutha Primary School, asked: How do eyes work?

Gordon Sanderson, an ophthalmologist at Otago Medical School, responded as follows:

Our eyes need light in order to work, that is why we can't see so well in the dark.

Light gets into our eyes through the pupil. This is the small round black bit in the middle of the coloured part of your eye.

The pupil is actually a hole covered by a curved, clear part called the cornea. This curved surface helps focus light towards the back of the eye. This focusing is completed by a lens just inside the eye.

The shape of the lens is able to be varied by muscles so that we can change our focus from a hill, which is a long way away, to a book, which is up close.

The lens is clear and looks like a magnifying glass about the same size and shape as a Smartie.

The focused light forms a small upside-down image on a special layer, called the retina, at the back of the eye.

Inside the retina a special chemical sensitive to light turns this picture into lots of tiny electric currents.

These currents, called impulses, send messages from your eye down the optic nerve.

The optic nerve contains more than one million little wires called nerve fibres. These fibres carry the messages through your head to the part of the brain that you use for seeing. If you put your hand on the very back of your head your hand is actually covering that part.

The messages reach your brain almost immediately. By the time they get there the picture is turned the right way up and you start to notice shapes, colour and movement.

This is what we mean when we say we can see.

Send questions to Ask-A-Scientist, PO Box 31-035, Christchurch 8444. Or you can email them to

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