What difference does shifting energy in chunks make?(Teaching tip)



Why don't lorry drivers get sunburned arms?

It is tempting to argue that whether the energy of a light beam arrives in a steady continuous stream, or in a steady line of photons, it all adds up to the same thing (in the same way that a series of tiny dots very close together looks very much like a continuous line). This is not the case for many phenomena, which can only be explained in terms of the existence of light photons of different sizes. The following example is a useful one for teaching.

Thinking about the teaching

Lorry drivers spend long hours sat in their cabs, often with full exposure to the Sun. But it is well known that lorry drivers don't get sunburned arms. Why should that be?

The first point to bear in mind is that sunburn is caused by ultraviolet light being absorbed by the skin. As it happens, ultraviolet light is also absorbed by glass, so in the case of the lorry driver, the ultraviolet light arriving from the Sun will be stopped by the lorry window. Hence no ultraviolet light lands on the skin of the driver and no sunburn results. But, again, why should that be?

It is clear that although ultraviolet light is absorbed by the glass of the window, other visible and infrared frequencies are transmitted through the glass. Why don't these radiations cause sunburn? The answer to this question lies with the light-as-photons model.

Ultraviolet light produces sunburn simply because the individual photons of energy shift sufficient energy to trigger the process when they are absorbed by the skin. In the case of infrared or visible light, it doesn't matter for how long the radiation falls on the skin, there will be no sunburn, simply because the individual photons don't shift enough energy.

The picture to have in mind is that of a continuous deluge of photons arriving from the Sun, with a range of different values of energy. The high-energy ultraviolet photons are stopped by the lorry window, while the lower energy photons pass through. Each individual lower energy photon contributes to filling an energy store somewhere, but none of them are able to brown (or pink in more extreme cases) the lorry driver's white arms.

So the process of sunburning involves an energy threshold. In other words, it can only be triggered if chunks of energy of sufficient size are involved.



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