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

New Trends in Physics Teaching IV
the case of their immediate environment where this time interval is not perceptible. The child
who spoke the above words concerning the sun continued by expanding on his views as follows:
‘(For the lamp) . . . it’s not the light which takes time there, it’s the electricity which is coming to the lamp . . .
so as soon as the electricity gets there . . . well, the light . . . anyway the lamp reacts and starts . . . shining.’
We also tested the concept of the rectilinear path of light. This concept can be wholly dissociated
from the notion of propagation time. Children are in fact able to conceive of light in terms
of rectilinear rays without realizing that there is any movement of light along those straight lines.
About one-third of the children drew correctly on the concept of the rectilinear path of light
in order to predict the size of the shadow cast by the stick (figure 1) or to predict the position
of the light spot on a screen, placed behind a piece of cardboard with a hole in it higher than
the bulb (figure 2). This finding was confirmed by a written questionnaire, submitted to 250
13 and 14 year olds.
Figure 2.
A number of children also had the idea of a straight line, but envisaged only a horizontal
plane. This being so, they predicted that the screen in figure 2 would not receive any light as the
hole was not ‘opposite’ the lamp (i.e. on the same horizontal line). In all, the idea of a straight
line is thus seen to be present in the minds of half of the children of this age.
LIGHT AND ITS INTERACTIONS WITH MATTER: LENSES, MIRRORS AND OTHER
OBJECTS
The reflection of light by objects
Turning our attention to the reflection of light by objects, we set before the children a sheet of
white paper, then a mirror, placed opposite an electric torch (figure 3), and asked them: ‘When I
turn on the torch, what does the light do?’
Most of the children thought that the light, sent out by the torch, remained on the paper,
while the mirror sent it back:
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