Euclid and his visual rays

What the Activity is for

The case of Euclid and his visual rays allows pupils to see that some ideas from the history of science are similar to existing common sense ways of thinking about natural phenomena. In addition the pupils are asked to challenge a scientific idea or theory (in this case the idea of visual rays) by presenting new pieces of evidence.

What to Prepare

• printed copies of the support sheet (see below)

What Happens During this Activity

There are various ways in which you might draw on this story from the ancient history of science. You might simply tell it as a story and engage the pupils in discussion around the kinds of questions listed below. Alternatively you might use the text for a class reading and comprehension exercise. First of all the class read through the passage, then the pupils work in pairs to talk through the questions and make notes about their responses. Pairs of pupils then report back in whole-class discussion.

Euclid's two main arguments in favour of emission by the eyes were as follows.

In searching for a small object like a needle, or in looking at a page of a book, you do not immediately see the needle, or all of the letters on the page. Euclid argued that this could not happen if the images of these things were reaching the eye and making an impression. It must therefore be the eye which is the active emitting organ such that eventually one's eye rays might land on the missing needle or particular words on the page.

The second argument relates to the shape of the eye. Here, Euclid argued that the ear is hollow and is obviously a receiving organ, such that sound enters the ear. Conversely, since the eye has a protruding shape, it must be an emitting organ, thereby giving out light.

Questions for pupils to consider:

• What idea did Euclid have to explain how we are able to see things around us?
• What two arguments did Euclid propose to support this idea? Outline each argument in your own words.
• Supposing you met Euclid. What evidence would you offer to try to persuade him that we see things when light enters the eye, rather than when light is given out by the eye?
• How would you counter (argue against) the first of Euclid's arguments relating to problems experienced in searching for a small object such as a pin?