How are photons arranged in a light ray? Title, or more specifically, if a constant light source is directed onto a flat surface, would the photons spread equally on said surface? In addition, say, at the beginning, 10 photons reached the surface, would it still be 10 photons reaching the surface after a split second?
I just came this up when after watching a photoelectric effect experiment presented by Khan Academy. Any help is very appreciated.
 A: Well, I think your question is in fact about flux of photons, namely, a number of photons reaching a certain cross section during a certain time interval. Say, million of photons reaching one square meter surface in one second.
Back to your question, the answer actually depends on how intense this flux of photons is. If we are talking about a laser beam falling on a spot behind a focusing lens, then yes, a number of photons reaching this spot for one second (or one millisecond, or even one femtosecond) will be (more or less) the same. Meaning that fluctuations of this number are much lower than the number itself.
This is about photons reaching a whole spot on the surface. The distribution of photons across the spot is defined by the properties of the light source. Most lasers produce so called Gaussian laser beams, which means that intensity distribution (and hence photon flux distribution) follows Gauss law (https://en.wikipedia.org/wiki/Gaussian_beam).
The above said becomes less and less applicable when we turn to weaker light sources and shorter times. This is something that statistical optics is about, and yes, under certain conditions a (very small) numbers of photons reaching a certain spot during some (very short) time interval may vary significantly.
Hope that helps.
