What subset of the infinitely long wave is a photon? I see diagrams of light stretching from source to infinity.  Is the entire thing the wave form of a photon? Or is it each peak and trough?
 A: An electromagnetic wave has energy density proportional to the square of the field amplitude. As such, an electromagnetic wave that has a uniform amplitude and infinite extent would have an infinite amount of energy and would not be a single photon.
Let us talk about a physically realistic scenario. If you set up a resonant cavity with a "classical" standing electromagnetic wave with frequency $\omega$, the quantum-mechanical description of that wave is that it contains a large number of photons at the frequency $\omega$, each of which carries energy $\hbar \omega$. Those photons are all indistinguishable from each other, which implies that all of them are in the same place, which in turn implies that each photon is everywhere in the wave at once.
A: Light is almost always many many photons. A single photon is much different than that.
Air exerts a smooth continuous pressure on the walls. Air is many many molecules. If you had just one molecule, there would be discrete clicks as it hit the walls. Light and a single photon are something like that.
When you deal with light as a continuous wave, you are smoothing out the discrete nature of photons by looking at the cumulative effect of many of them. Where there are many photons, the light is intense. When a light beam extends from a source to infinity, it continually emits photons. You can find them anywhere along the beam.
Typically if the nature of photons becomes evident, it is because light interacts with individual atoms. Perhaps you shine a light on photographic film, and dye molecules change their state. To understand what happens, you need to understand how individual photons interact with individual molecules.
