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

Childrens’ ideas about light
some days it’s brighter than others’. [Lionel, 13 years, 7 months.] This way of identifying light
with a state is similar to identifying it with its effects, but not quite the same.
We have thus seen, in connection with the interpretation of shadows, that two different conceptions
of light emerge: (i) light equated with its source, with its effects or with a state; (ii) light
recognized as a separate entity, situated in space between its source and the effects that it produces.
These two different conceptions reappeared in a variety of situations, and it transpired that the
same child can draw on either one of these conceptions, according to the situation, and sometimes
even when explaining the same phenomenon. Thus Lionel first explains the formation of a
shadow on the table using terms that suggest the idea of a movement of light in space: ‘When
you take away the paper, the light comes back on the table. If you put it back, the light won’t
be able topass through the paper and the table is bound to be in shadow.’ And, after this explanation,
he immediately adds an interpretation of the type ‘light = effect’, ‘light = state’: ‘[The light]
is hidden under the shadow. It becomes the shadow of the paper. ’ It is hence impossible to judge
a child on the basis of a single answer. To know how he stands in relation to the concept of ‘light
as an entity in space’, it is necessary to determine the range of situations in which the child uses
this concept by setting him face to face with a variety of examples.
A g k eat deal of variety is then seen in the answers given by children aged 13 and 14. The two
extreme levels of interpretation are represented as are all the intermediate levels in which the
child uses, to a varying degree and in a varying number of different situations, dynamic terms
(set off, meet, go through, rebound, etc.) that suggest a movement of light in space, and in which
he loqates the light, more or less invariably, ‘everywhere in space’ rather than ‘in the bulbs’ or
‘on the ceiling, in the chandelier’. However, if all the children of 13 and 14 are compared with
those of 10 and 1 1 [3], it is seen that the older children make greater use of dynamic terms. At
the ages of 13 and 14, very few fail to use such a term in some situation. Correlatively, a majority
of them has recourse to the concept of ‘light as an entity in space’ to interpret the formation
of shadows, in contrast with children of 10 and 11. Overall, then, there is a distinct evolution
in children of 13 and 14 in relation to younger children: from identifying light with its source
or with its effects, children go on to recognize light as a separate entity, in movement in space.
The fact that they have understood the concept of ‘light as an entity in space’ does not mean,
however, that their ideas about light are altogether satisfactory from the physicist’s point of
view. The limits of this concept in the minds of children of 13 and 14 wil be seen subsequently.
THE PROPAGATION OF LIGHT
We have just seen that most children of 13 and 14 use, to a varying degree, dynamic terms in
connection with light (set off, go through, rebound, etc.). These terms suggest the idea of a movement
of light in space. However, the children never refer explicitly to a movement of light in
space, except in the case of very great distances, concerning which some say to us, for instance:
‘Well I know, for instance, that if all at once the sun . . . went out, well, you know, if the sun’s flames stopped
burning, you could say . . . well that . . . you’d have some . . . some light for quite a time, because the sun would
already have sent out some . . . some rays . . . and the rays, well, they, they wouldn’t go out . . . only the sun’s
core would . . . so during this time . . . during the . . . p’raps four months, I don’t know how long it takes a ray
to get to earth.. .it . . .it goes fast, okay, but it takes . . . takes time even so. . . . It can’t come just like that. . . .’
[F4, 13 years, 2 months.]
This does not mean that they apply this concept of the time needed for light to be propagated in
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