Rights: University of Waikato. All Rights Reserved. Published 14 March 2012 Download

Using the example of using a magnifying lens to focus light rays onto a piece of paper, ophthalmologist Associate Professor Gordon Sanderson from the University of Otago describes how the lens in the eye functions. The convex lens takes light entering the eye and focuses it to one tiny spot on the retina at the back of the eye.


Well we need to form an image, and the only way you’re going to form an image is with a convex lens. Now you recall when you were a child playing with magnifying glasses, and if you played with it on a sunny day, you would possibly find one location where you could actually form a little bright spot of light, and after a few minutes the paper would start to smoke and you’d burn a hole in the paper. Well what it’s actually done is it’s formed an image of the Sun, and because the rays of the Sun which have been essentially spread over quite a large area have now been brought to a focus at one point, not only has the light been concentrated but the heat from the Sun has also been concentrated as well, and that’s why it gets very hot and that’s why it starts to burn.

The convex lens takes a collection of light and focuses it in this one tiny spot, and that of course, if we had multiple sources of light, would form us an image, whereas a diverging lens or a concave lens does the opposite. It takes parallel light and sends it off in a divergent direction so it would actually reduce the illumination on the page.

Well the convex lens in general has one major role and that is to converge light. Now if we look at this white piece of paper here, light is falling on that from all over the place, but part of it is falling on it from a light immediately above my head. Now you can’t see where that light is because the light from that light above my head is going off in all different directions. If I put the lens in place and get it to its focal point, now I’m starting to form an image of the actual light itself.

Let’s pretend for a minute that this lens here is a combination of our cornea and our crystalline lens. Obviously in this case, they’re rolled into one – in the eye, they’re actually two separate lenses but they’re doing the same job, one is just adding to the effect of the other. So that’s the convex lens, if you like, which we’re going to pretend is our cornea. Now back here somewhere is going to be the retina, and the retina is going to need an image that is a) in focus and b) able to be transmitted to the brain. So that image there, which is the image in this case of the light, is going to be seen by the receptors in that area – when I say seen, that’s going to stimulate receptors in that area as opposed to receptors in this area which won’t be stimulated – and the brain will be able to interpret that there’s light there and there’s not light there.

Obviously if we had two sources of light, one image might appear over here, one might appear over there, and the brain would realise that were two separate sources of light and there was a dark space between them and it could then interpret that as an image.