Shannan Mortimer, of Wellington, asks :-

If the eye is a single lens shouldn't the image be up-side down on the retina?

John Campbell, a physicist at the University of Canterbury, responded.

It is. First let us consider how the image is formed. Contray to what most people think, the majority of the focussing of light takes place at the surface of our eyeball. You will notice this whenever you have rubbed or bumped an eye. The image goes out of focus because the curvature of the surface of the eyeball is changed. It will relax back to normal after a short time.

The relaxed eye focusses the image of distant objects onto the retina, the layer inside the back of the eyeball which contains light sensitive detectors. If we want to see closer objects, e.g. when reading a book, we have a thin lens just behind the front of the eye. To do so, muscles strain to alter the shape of this lens to give us a variable focal length thus keeping the image in focus on the retina. That allows us to keep in focus when we move from looking at a distant object to looking at a book. Because the muscles are strained to do this we get eye-strain if looking at anything that is close. We evolved to search the distance for preditors so that is why the relaxed eye is focused on distant objects and can continue to do so for hours.

Many people have eyeballs which are not quite the right curvature or length so they have to permanently wear glasses to avoid eye-strain. Some have curvatures that vary, eg different curvatures horizontally compared with vertically. Their glasses cost more because they have to be ground just for that person.

When a wave enters a different medium, say the aqueous eyeball after travelling in air, the wave slows down. If it meets the interface of the two surfaces at right-angles it doesn't change direction. If not, its direction changes. This is what gives us our focussing ability. Note that there is not much change in light speed in the lens compared to the eyeball, because we are water-based, so the lens is just for fine-tuning of the focal length.

Consider looking at a tree. Light from the trunk at eye level meets the surface of the eyeball at right angles and passes through the centre of the eyeball. The light scattered from higher up branches and meeting the eye's surface at right angles also passes through the centre of the eyeball. So the image of the tree is upside down on the back of our eye.

We are used to this and the eye-brain system inverts the image for us. If we put an inverting prism in front of our eyes we see the image upside down, which is most disconcerting. However, after a few days the eye-brain system once again inverts the image so it seems the right way up to us.