Physics Narrative for 14-16
Photons are only detected when they are destroyed
There is a perplexing thing about photons that follows directly from these same photons being something used to model lighting. You may remember the experiment strongly recommended in the SPT: Light topic where chalk dust is used to scatter the light from the beam to reveal where the beam was going. Notice the past tense. Even in that simple experiment, and with the simple 11–14 model to hand, careful thought reveals that there is a significant philosophical tussle to be had if you insist on asking the question
What is light? or even
Where is the light?
This arises because you cannot
see light: in an explanation we now have access to, you have to destroy the photons before you can know where they
are (whoops –
were). It was the same with the light beam, only more difficult to explain and picture exactly what was happening: as each segment of the chalk dust is traversed, some of the intensity of the beam is transmitted, some is reflected, and some is absorbed.
After 11–14 we could say that some of the amplitude of the vibrations travelled on and some of the amplitude was reflected by the dust. This is unsatisfactory as a model to think about unless we have a rather sophisticated mathematical model of the amplitude splitting. It is certainly quite hard to picture what is happening. We think that the photon account is easier to deal with here: some photons go on; some are absorbed; and some are reflected. Thinking in terms of a stream of individuals turns out to be easier than imagining what is happening with a continuous wave.