Polaroid colours

Tom and John

The so-called polarisation of light, and the colour phenomena which appear in connection with polarised light, constitute a fascinating field of study. The phenomenon of polarisation has been known since the beginning of the 19th century. Formerly quite complicated experimental arrangements were necessary to produce the effect. Nowadays, however, one can use plates of so-called polaroid (used, for instance, in sunglasses).

These plates have the following property. If they are held one in front of the other, in a certain relative position they look quite transparent. But if one of the plates is then turned ninety degrees in its own plane, no light can pass through the two plates. In this position the area where they overlap looks black and opaque. If now a piece of torn plastic-bag, a plastic ruler or a piece of glass is held in between the two polaroids, multicoloured patterns appear, which make visible the stresses in the material.

In the same way certain crystals, i.e. thin slices of mica, can give rise to colours. If one or both of the plates are turned, the colours transform in a most surprising way. Some weeks ago, two bright boys of eleven years of age, Tom and John, were visiting me and I showed them the polarisation colours. As expected, these aroused their curiosity. The boys were soon occupied with testing different objects, turning the plates and making friends with the variegated phenomena. They accepted without hesitation my explanation, that the colours are stresses in the material that are made visible by help of the polaroid. - "Sort of an X-ray," they thought.

John, however, was a bit puzzled about the polaroid plates themselves. There is certainly something curious about the fact that two plates, which singly look quite transparent, when put together can give rise to complete opacity, or any degree of opacity, depending of how they are turned in relation to each other. You put them together so that the common area is black, he suggested to me. Then carefully draw away one of the plates, and the black area successively becomes smaller and smaller until at last it disappears, and there you are, with two transparent plates. There is something unnatural about this. Where does the blackness come from, and where does it go?

Thus, while John in an unconscious, innocent way was confused by the peculiarity of the phenomenon, psychologically the situation was quite different for Tom. That is to say, he knew "the explanation," I suppose by some visual memory from a textbook of physics. ---- "The light sort of makes waves and can pass the first plate only this way about, and then, if you turn the other so, the light can't pass through." Understandably, he had difficulties in presenting the explanation verbally. But it was obvious that he was clear about the thing and that to him it was quite uncomplicated. To John, on the other hand, the phenomenon was still a hit puzzling, and he made suggestions for further experiments. If you put a thin slice of mica between the two plates, the area where the mica is, becomes white. This means: putting more material in the way of the light gives you back transparency, where previously complete blackness reigned! Such a conditional absorption of light also seems to be something quite unnatural. At the first moment both boys were equally astonished. Addressing Tom, I gave an explanation: the mica turns the plane of oscillation of the light, so that it can pass also the second polaroid. At once he nodded with an air of comprehension. Why, that is evident. Nothing curious about it any more. However, John continued stubbornly to make his observations. There was to be seen a black spot on the white area, and it turned out that it corresponded to a small hole in the mica. A hole makes opacity! He himself presented an explanation to this: "Where the hole is, the situation is exactly the same as before, when we had just the polaroid plates and no mica." Logically, this is quite another sort of explanation from the one Tom accepted. An explanation not based on a model or some concept about the nature of light, but on a connection to a previously established matter of fact. You go from one experiment to the nest, finding the connections between the different observations. This is also a way to attain understanding!

This incident caused me to consider the question about different kinds of knowledge of reality. What is it, in fact, that school-aged children want and need, when they come with their "why" and "how do you explain that"? Often, one gets the impression, that they are content with -- or plainly prefer to get -- a simplified thought model, appealing to the sense for the concrete, or quite simply a verbal explanation. They are pleased when they are able to say that matters stand so and so. But the example above makes us realize that there is also another mode of explaining things. I mean, the kind of explanation that the ambitious teacher of physics gives when he answers: - "Well, that will become clear when we recall the experience we had the other day, in that experiment, you remember. . ." Understanding might be something that grows inside a person, when he begins to be able to survey and grasp the connections in all the diverse phenomena he is confronted with. To understand, need not mean to have learnt a wording, or to have assimilated some sort of conception of a mechanism behind the phenomena.

It seems that we are faced here with a question of great importance: Whom ought we to support? 'Tom -- quick-witted, sharp and confident, skilful in technical things. Or John --- tentatively searching, imaginative and carefully observing. Evidently there are two ways of bringing young people to the study of the laws of nature. And those who are today shaping the school of tomorrow must take the responsibility of making a choice at this point. Unfortunately, it is a matter of course that one has to decide upon one or the other of the two alternatives. If we choose the way that Tom represents, to some degree we irretrievably make impossible the unfolding of the "John-side." The way that hom takes is efficient and is the simpler one to control and design. John's way is toilsome and requires much more time. We know about Tom's way of acquiring scientific knowledge, that it gives, a 'good yield, in the form of technical applications. We know less about John's way. Yet, after all, in his uncertainty, John stands nearer to the actual state of things. The nature of light and colour is something that science is as yet far from being sure of. The question about the nature of light has in the course of time got several answers, which have all shortly been found to be only partial answers: light as a stream of particles, as a wave motion, as electromagnetic radiation, as quanta of energy, as the fastest possible signal in the theory of relativity . . . Without doubt a decision with consequences: it concerns laying the foundations for the direction and aim of scientific research, the effect of which belongs to the future.

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