Hello yet again Alan,
The ultimate problem in understanding photons is that, as implied by Andrew and sort of stated in the Muthukrishnan paper, they don’t actually exist! Not as usually understood in explanations of optical phenomena, anyway. They are just one way of looking at light, which can be useful in some situations but not in others. In the end, as is also shown in the paper, everything depends on a) the maths and b) what you are trying to deduce from your observations. However, mere mortals who are not quantum physicists need models on which to base their understanding, so as long as you don’t think too hard about the ultimate nature of light and carefully pick the scenarios in which you use one or the other, the wave and particle (i.e. photon) models provide reasonable, if not exactly rigorous, explanations of such things as the excitation of molecules involved in retinal detection or photosynthesis. So don’t despair entirely!
But yes, as you say, often the closer you look the less definite things become – an example of the Heisenberg Uncertainty Principle!
Please do pass on anything I have said to your grandson. I believe it to be accurate at a “non-specialist” level so hopefully it won’t lead him down incorrect paths. However, I doubt whether a more detailed discussion than that in this forum thread would be considered appropriate as a Tutorial, as it is really in the realm of physics rather than astronomy.
As to the status of the observer in the observing process, I have read articles discussing the effect of variation of colour discrimination between observers on the observation of coloured binary stars but, as you say, little else. Still, now that most observations are done with electronic devices perhaps the poor old human observer will become a seriously endangered species!
Best regards,
Steve