Consider a typical optical focusing system: A small light source, then a collimating lens, then a focussing lens, and then a detector (e.g. CCD).
Assume that source intensity is so low that only one photon enter collimating lens per second. Today's modern technology is able to produce single-photon light sources. Assume that the dark room (in which the experiment is done) is 100% dark, i.e. detector detects only the photons coming from the source.
A photon that originated from the source is detected by the CCD. Is the detected photon the same as the one which originated from source?
Between source and detector, there are two lenses. When the photon enters the first collimating lens, it will interact with the electrons inside the molecules of the material from which the lens is made, but it will not interact with nuclei of different elements present in the molecules. Is the photon that came out of this collimating lens the same as the photon which entered the collimating lens?
What is the reason that the photon falling on the edge of the focussing lens gets deviated by some angle (i.e. gets focussed) and falls on the detector?