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

Childrens’ ideas about light
‘light ray’ is marked out by means of pins stuck in a sheet of paper in such a way that the one
closest to the eye exactly masks the following ones [6]. The significance of these experiments
for the children is not that which is assigned to them by their authors. They cannot be interpreted
in terms of a path of light going from the object to the eye when the visual perception of the
flame or of the pins is not connected with the fact that light is received by the eye. In the process
of teaching, one must arrive at a more satisfactory model of vision; one should not take such a
model as a starting point, in the mistaken belief that the children have mastered it already.
CONCLUSION: THE IDEAS THAT ARE HELD ABOUT LIGHT
In the foregoing paragraphs, two quite different ideas about light were seen to emerge. On the
one hand, light was equated with its source or with its effects; on the other, light was seen, as
by the physicist, to be a separate entity, situated in space. In the first case, the children have no
means of interpreting related phenomena: they can only note the similarity in form between
an object and its shadow or the presence of an image of the source in the mirror. The second
view represents a definite step forward: the children can then interpret shadows and raise the
question of the reflection of light by objects, a question which has no meaning so long as the
concept of light as an entity in space, differentiated from its source and from its effects, has not
been mastered. However, this is but a first step towards f0rming.a picture consistent with the
physicist’s model. We saw the limits of this view in the minds of 13 and 14 year old children:
(i) the movement of light is not clearly recognized: the children often speak of light as an entity
in movement (it sets off, goes through, rebounds, etc.) but they recoil from explicitly envisaging
the movement of light except, for some, in the case of great distances; (ii) light exists for those
children only when it is intense, and sufficiently intense to produce perceptible effects; they are
thus led to think that, in contrast with the mirror, the sheet of paper does not reflect light and
that their eyes do not necessarily receive light when they look at an object; and (iii) the children
do not think that light is necessarily conserved; for many of them it may vanish (irrespective of
any interaction with matter when it is no longer sufficiently intense to produce perceptible
effects) or, on the contrary, be intensified (when it passes through the magnifying glass).
The children sometimes conceive of light as an entity in space, but they always do so in
material terms. Thus El7 [13 years] said to us, in connection with the light projected by the
lamp on to the mirror: ‘The light. . . it’s solid . . . so it should come back. . . .’ When it is a piece
of cottonwool that is lit up by the lamp, he explains that the light wil not be sent back by it
‘because it’s not hard’. Such a ‘material’ view may help the children to interpret certain phenomena,
such as the interaction of light with a mirror; but in other cases it prevents him from
understanding, as in the case of El 2 with the magnifying glass:
‘My idea is that the rays don’t go through. It’s the light that goes through. What I mean is that it doesn’t go
through, but it ...,how can I put it, it. ... [I: ... the rays aren’t the same thing as the light?] Yes they are,
but. .. . The rays and the light are the same thing. They don’t go straight through, there are no little holes
for them to go through’.
In relation to the identification of light with its source and with its effects, the idea that light
is an entity in space, possessing properties to which the physicist would be more or less ready to
give his assent, clearly represents an advance and not merely a better alternative idea. Inde-d
children seem to subscribe successively first to one and then to the other; we saw that at 10
and 11, most children hold the first idea, whereas at 13 and 14, the second idea is dominant.
While, however, the concept of light as an entity in space is drawn upon by most children
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