University of Edinburgh

BSL Physics Glossary - eye defect - definition

That's really blurred! I'm going to talk more about the eye: what it's made of, how light gets in, and what we can do to see better. We will discuss why some people need glasses and find out what can go wrong with the eye.

This flask is an imaginary eye. On the front it's got a lens, just as in your eye there is a lens. In our eye when we look round at different things our lens automatically adjusts its shape. This helps us see things close up or a long way off. If you try for yourself focusing on your finger and bringing it towards your nose you can feel those eye muscles adjusting the lens shape! It might feel a bit stretched. Just the next moment you can switch to looking at something a long way away, and your eye can do it easily. So your lens is working perfectly. But some people can't see clearly. The muscles that control the lens in the eye don't always work properly. Glasses can help see more clearly.

Let's look at this with our model of the eye. The light is going to go through into the eye. In your eye you have a black hole, called a pupil, which lets light into the eye. The light coming into your eye is in parallel rays. This is what always happens. As the light goes through the lens, it focuses to a point. That's where the image can be seen. Now really this image should focus on the back of the eyeball.

Here's a diagram to explain what happens.

(normal eye)

Now look at the diagram for a short-sighted person.

Now here is the diagram for a long-sighted person.

A long-sighted person needs a converging lens. This will bring the focal point back from beyond the eyeball to the retina. Look at this diagram:

(long sight - adjusted diagram. Then short-sighted adjusted diagram)

Did you see in the diagram how the parallel rays of light were focused exactly on the retina now? The image on the retina will be clear.

I'm going to turn the light off now so we can see better.

Could you see how the light focuses in the middle of the 'eye'? It should focus at the back on the retina. That means the image at the retina will be blurred. This person is short-sighted. So this person needs glasses - but which sort of lenses are needed: diverging or converging? We'll find out.

This is a converging lens. It's good for bringing the focal point nearer the lens. Have a look. Did you see how the light focused really near the lens? It's way off the retina. This is not the right sort of lens.

Now I will try a diverging lens. Have a look.

That lens is much better at bringing the focal point to the retina at the back of the eye. So with these glasses the person will be able to see the image clearly. The light energy falls on the retina, and is changed to electrical energy. The electrical signal goes up a nerve to your brain. Your brain interprets the signal and that's how you can see what's going on around you.

If the focal point is not on the retina, then the image at the retina will be blurred. That's why you need a lens to help focus the image accurately at the back of the eye.

With a diverging lens the light coming in, with parallel rays, is spread out more so the focal point is moved back through the eye to the retina.