system
Given the following post, respond as if you're commenting on Reddit:
human
Would a one-photon-thick beam of light be diffused after reflecting off a convex mirror?
gpt
There's no such thing as a "one-photon-thick beam of light". Photons are not little balls that can be perfectly lined up into a one-wide beam. Photons are quantum wave/particles that act mostly like waves when traveling and mostly like particles when interacting. So, to make sense, your question should be changed to: "Would a very thin beam of light experience divergence after reflecting off a convex mirror?" The answer is yes. But if the beam width is small compared to the radius of curvature of the mirror, the mirror is going to look very close to flat to the light beam, and the divergence will be small. As the beam width gets thinner and thinner , and the mirror stays constant, the beam divergence after reflection decreases. 

Something you also have to keep in mind is that light beams diverge on their own because of diffraction even before reflecting off of a convex mirror. There is no such thing as a perfectly collimated beam of light. So if you tried to make the light beam infinitely thin in order to get exactly zero divergence after reflection from a convex mirror, you would get complete natural divergence of the beam due to diffraction.