Your posts are really good and you certainly understand me, thanks
The fact of indirect evidence, I suppose the answer is yes, that is what I am getting at. If I said I had discovered a dragon, you would ask for observation evidence to prove the Dragon existed, without the observation evidence it is in my ''eyes'' no more than a ''Godly'' nature or a pink fluffy Unicorn nature.
I prefer axioms that I know are 100% fact and agreeable by observation.
Gravitation , magnetic fields and likes are ''invisible'' to our perception, only the imagination can imagine these things to be such as a curvature of space etc.
However I am open minded and willing to listen to ''evidence'', whether direct or indirect.
So what do you consider is evidence that a ''Photon'' exists?
Added - I also consider in reply to your Photo-electric effect that Kmax=hf/S where S is entropy.
>hf/S=W where W is work?
In the photo-electric effect it is found that shining light on a metal can cause electrons to be ejected from the surface which can then be accelerated towards a +ve charged electrode , causing a current to flow. It take energy to eject an electron, as the attraction between the electron and the +ve charged ions in the metal has to be overcome to do this. The amount of energy required is called the "work function", as it is the amount of work that has to be done on an electron to pull it out against the force of electrostatic attraction.
According to the classical (Maxwell-type) theory of light as waves, the energy from a light source depends on both the intensity of the beam and the frequency of the light. So what people expected to find was that if you had a beam of light of any frequency, by turning up the intensity, the rate of electron flow would increase, because more electrons would be ejected as a result of the the greater energy input from the light. In fact what was found was that no electrons at all were ejected if the frequency of the light was below a certain threshhold value, which is characteristic of each metal. Furthermore light above the threshhold frequency could still eject electrons, though not so many, even if its intensity was reduced to very low levels. Classical theory could not account for these observations. However Einstein realised they could be accounted for, if the light did not come in a continuous wavelike stream but in "quanta" of energy, the size of which depended on the frequency (E=hν). Then, at low frequencies the quanta would be too small to give an electron enough impulse to be ejected, but once the frequency was high enough for E to exceed the work function value, they would start to appear.
This idea of E=hν, known by various names, one of which is the Planck-Einstein relation, could be fitted very nicely with the observations at the time about spectroscopy, by which it was apparent that atoms absorbed light at characteristic frequencies only. Using this idea one could explain spectra in terms of electrons occupying orbits at various distances from the atomic nucleus and being promoted from lower energy ones closer in to more energetic ones further out as a result of absorbing only those "quanta" of light exactly matching the energy gap between orbits.
So a lot of things started to click into place.
This is an example of indirect evidence in science. Note that there was never any "proof" of the existence of quanta of light - later called "photons". It was just that this hypothesis started to make sense of more and more previously separate phenomena, so the the quantum physicists and chemists of the time thought they must be on the right track.
In conclusion it is time for one of my hobby horse statements about physical science. Scientific theories never claim to "prove" a theory "true". That is for logic, maths and law. In science, all "truth" is provisional, because something can always come along tomorrow and show us our ideas about the world are wrong or at least incomplete. The photon is part of the Quantum Theory model we have to explain certain phenomena. This model successfully accounts for many things and successfully predicts many more and that is why we think it is a good model. But in the end, like all scientific theories, it is just a model of reality, which is probably imperfect, just as all its predecessors have been.
Edited by exchemist, 17 June 2016 - 11:15 AM.