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

Childrens’ ideas about light
connected with the psychological aspects, is reflected in the idea subscribed to by Plato and the
Pythagorean school that there exists a ’visual fire’.For Plato, ‘. . . the Gods caused the pure fire
within us, which is akin to that of day, to flow through the eyes in a smooth and dense stream. ...
So whenever the stream of vision is surrounded by midday light, it flows out like unto like, and
coalescing therewith it forms one kindred substance along the path of the eyes’ vision. ... And
this substance distributes the motions of every object it touches, or whereby it is touched,
throughout all the body even unto the Soul, and brings about that sensation which we now term
“seeing” ’ (Plato’s Dialogues: the Tirnaeus). The Pythagoreans considered that sight was due /
exclusively to an invisible fire coming out of the eyes. As this fire (or, according to Euclid, this
cluster of rays) touched objects, it made their forms and colours known. At the end of the fourth
century, Theon of Alexandria thought that the rays of light sent out by the eyes must be at some
distance from one another for ‘a thing cannot be entirely seen at a single glance: sometimes,
searching for a small object on the ground, a needle for instance, one does not see it although it
is concealed by no obstacle; but when one has directed one’s gaze to where it actually is, it is
seen without difficulty; likewise, one does not see simultaneously all the letters on a written
page’ [41.
For children, the movement that goes from the eyes to the object remains abstract. It is thus
clearly differentiated from the ‘visual fire’ of early theories, from the ‘fluid’ emitted by the eyes
of witches in fairytales or from the red rays that are beamed from Superman’s eyes. Only the
idea that the subject is at the origin of a process, instead of being at the receiving end, is common
to these various ways of portraying sight. The idea is an important one, substantively speaking.
We found evidence of it, however, in only very few children. Consequently, it should not be
given more importance than it actually has from the quantitative standpoint, despite the temptation
that always exists to find historical parallels.
The dominant viewpoint
Most of the children did not point to any form of link between the eye and the object.
I
I: ‘How is it that you see this box at this particular moment?’
E9: [ 14 years, 3 months] :‘Because it’s in front of my eyes, here, I see it.’
I: ‘... Yes. ... How is it that you see it?’
E9: ‘I can see it on account of daylight, because in the dark I wouldn’t see it ... because in the dark there’s no
light ... there’s no daylight. ... The eyes need daylight, need light, in order to see clearly.’
E2 [ 14 years, 9 months] :‘It’s thanks to the light that we see the box. ... [It’s role is] to light up objects so that
we see them.’
Some drew a diagram of the inverted image formed on the retina. This was but the memory
of something learned at school, reproduced without any sort of explanation.
What do the two intersecting lines represent? The child who had drawn them [E17, 13 years]
did not know. All recognized that nothing could be seen without light. But most of them referred
to light only in so far as it lit up the object or as the ambient medium (daylight) surrounding
object and observer without distinguishing any specific linking function. The eye ‘sees’ without
anything connecting it to the object.
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