Modelling the working of the eye with a webcam

What the Activity is for

This demonstration is to present parallels between the structure and function of a webcam and the structure and function of an eye. Small webcams are about the size of the eye and provide a live shareable image of how the world appears to them. This can be manipulated and used to promote discussion about the working of the eye. At this early stage we aim to keep the discussions simple. There is much that can be done, but probably only some that should be done.

What to Prepare

• a webcam connected to a computer, and a means of sharing a large view of a computer screen
• some objects to place in the field of view of the webcam, both luminous and not – a bright lamp is very useful

You'll need to practice with the webcam beforehand, as each comes with its own particular brand of driving software, some of which may need a tweak before you get the results that you want. In general, for the uses suggested here, it is worth having a webcam with a small number of pixels (say 640 by 480), but a reasonable colour depth (at least 8 bits/pixel).

What Happens During this Activity

Start with the webcam seeing the class. You'll see some delay in the refreshing of the picture – just like the eye. Pick up the webcam, talking your way around it, from the hole at the front, to the light sensitive bit at the back. Draw parallels with the simplest possible model of the eye, used in this episode.

Look at some non-luminous objects, emphasising the reflecting of the light off the object and then the trip from the object to the webcam. Put some objects out of the field of view of the webcam and ask why they cannot be seen.

Choose a bright, luminous object and point it straight at the webcam. For a while the image will be burnt out, but then the webcam will adjust, allowing less light in. The eye does this by making the hole at the front smaller, the webcam probably does not.

Choose a fair image and blow it up, until you can see the individual blocks of colour that make up the image. Smooth shadings are only apparent – like our eye the webcam has discrete sensing elements. Seeing may be one of the most natural things in the world for almost all of us, but that doesn't make it simple. One outcome from this brief exploration of the similarities is that pupils should feel that they have a simple model of the eye, but that there are plenty of phenomena left to explore and model.

Long sightedness, short sightedness and depth of field can all be modelled with the webcam. But we suggest leaving these until later and keeping it both short and light, for now.