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

Childrens’ ideas about light
We wanted to know more specifically how they explained the phenomenon of sight and how
they thought that light contributed to it. We accordingly asked them two questions: one concerned
the visual perception of a primary source, a lighted stick of incense; the other the visual
perception of an ordinary object, a multicoloured cardboard box. The two objects perhaps
represented different situations for the children.
Is light thought to be received by the eye?
We first presented the incense stick, asking the children whether he could see its (glowing) tip
so that he would be fully aware of seeing it. Then we asked him: ‘Does it send out light?’ in
order to‘ find out whether, for him, the fact of his seeing it was connected with the idea that the
eye receives light.
Most of the children thought that it did not send out ‘any light at all’ [E2, 14 years, 9 months]
or ‘not very far’, and in any case not as far as them, at a distance of about 1 metre from the stick.
El 1 [ 14 years] :‘It doesn’t send out any, it stays where it is.’
E8 [14 years, 10 months] : ‘It sends out very little. ... It’s chiefly the colour that gives .. . it’s chiefly the
colour of. .. of the stick that gives the effect of light. ...’
I: ‘Does it send out light all the way to you?’
E8: ‘I don’t think so. No, it stays around the stick. It doesn’t go any further.’
I: ‘Is this end sending out light?’
E17 [ 13 years] :‘Yes ... But much less than that [he indicates the torch which had just been used] .’
I: ‘Much less strong. ... How far does it go?’
E17: ‘I don’t know. ... Well,much less far, only a little. [He holds the incense stick close to a sheet of paper.]
Oh, even like that, it’s not sending out any light. ... You can see it, because it’s red, otherwise. ...’
Here again, E 17 connects the presence of light with the manifestation of an effect of sufficient
intensity for it to be perceptible, namely, the lighting up of the sheet of paper. However, the fact
of seeing an object is not accompanied by any violent physical sensation - one is seldom dazzled.
Recognizing the light only when it produces a highly perceptible effect, the children do not think
that the eye can receive light. Light is needed for an object to be seen, but the light does not
necessarily go as far as the eye. This was made explict by El 4 [ 13 years, 4 months] :
I: ‘Is it sending out light?’
E14: ‘Yes.’
I: ‘Yes. Why?’
E14: ‘Because you can see it in the dark. . .. You can see it in the dark, so it gives out light, otherwise you
couldn’t see it. ... Unless there’s another light. ...’
I: ‘Where does it send out light to?’
E14: ‘There[he indicates a point a few centimetres from the stick] .’
............................................................................
I: ‘When you see, does your eye receive light? Is there any light which goes in your eye?’
E14: ‘Not necessarily.’
I: ‘Not necessarily, no ...’
E14: ‘No, because with the incense stick, for instance, if I had binoculars, it could be one kilometre away, and I
would see it just the same.’
Only a few children answered the question, ‘Does it send out light all the way to you?’ with
the reply: ‘Yes . . . otherwise I wouldn’t see it’ [E6, 14 years, 10 months]. Such answers remain
very vague. One or two, however, were more explict:
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