New trends in physics teaching, v.4; The ... - unesdoc - Unesco

Childrens’ ideas about light
Figure 3.
‘It [the light] bounces off the mirror [gesture of the child going from the torch to the mirror, then towards
herself]. . . . When the light falls on the paper, that makes a screen. . . . It stays there . . . whereas the mirror
sends the light back.’ [El 1,14 years.]
This idea derives directly from what is perceived: with the mirror, it is possible to light up
something else or to dazzle someone, whereas, in the case of the sheet of paper, the most apparent
effect is seen to be on the paper. A few children thought that the light must disappear rather than
remain, on the paper:
‘It surely doesn’t remain. If it did, you’d only have to turn it off for it to stay there. So it can’t stay there. . . . It
must light up and then disappear . . . since it can’t stay there.’ [E17, 13 years.]
In the previous interpretations, light was seen as an entity in movement in space (as far as the
paper). We were also presented with interpretations in which the light was equated with its source
or with its effects, as with children of 10 and 1 1 [ 31 , but in a smaller proportion: in this case, the
child is unable to reason in terms of a path of light; he merely notes the presence of the light spot
on the sheet of paper (‘What does the light do?’ ‘It makes, for example, a sun.’ E4, 15 years, 2
months); again, with the mirror, he merely describes what he sees (‘You see the light. .. in the
mirror. . .. [You see] the lamp.’ E4). It is important not to overlook the case of such children.
For them, a first step wil be to recognize the existence of the entity ‘light in space’.
However, the dominant fact to emerge is that practically no child suspects that ordinary
objects send back the light. Now this concept is of fundamental importance for the entire field
of optics. It is not possible to understand the formation of images of any object whatsoever
(that is not in itself luminous), as in photography for instance, unless this concept is first of all
grasped. We shall see, similarly, that it is decisive for an understanding of how we see. Hence
it is a basic aim in the teaching of optics at that age to get across this concept. Use can be made
of the example of the mirror, already recognized by most children as reflecting light. The children
refer to the fact that with a mirror it is possible to light up another object or to dazzle someone.
With a sheet of light-coloured paper similar effects can be produced. At noon, in the middle of
summer, a piece of white paper lit up by sunlight has a dazzling effect; in a dark room, the fact
that an object (of a light colour) is lit up by the light reflected on a sheet of light-coloured paper
is clearly perceptible. We see here how, once we know what ideas children hold, we are in a good
position to decide relevant objectives for the age-group considered and to know how to achieve
them.
The role of the magnifying glass
Children know that they can set things on fire with a magnifying glass on a sunny day; many have
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