Nick on Nostr: It's of course correct that, as [@j_bertolotti](https://mathstodon.xyz/@j_bertolotti ...
It's of course correct that, as [@j_bertolotti](https://mathstodon.xyz/@j_bertolotti ) says, to fully understand what's going on you have to learn electrodynamics. That being said, you can understand a ton of things with simpler models like ray optics or Huygens' principle. It's just that if you want to connect them to what's going on with atoms and electrons, then you have to involve electrodynamics.
If we go back to your question in the original post, it was "how does a reflected ... photon know the _angle_ at which it should go?" If you want to understand it through a simple model, first of all I'd avoid any talk of photons, to really understand those you need quantum electrodynamics and that really makes it complicated. I'd stick to thinking of light as rays or as waves.
As rays, there's just a rule that the reflected angle equals the incident angle, but that model isn't detailed enough to really offer an explanation of why. However, note that if you were to, say, bounce a tennis ball off a hard wall it would follow the same law (because it's an kinetic-energy-conserving collision that doesn't impact the component of the motion parallel to the surface, so it's not too far fetched at least.
In a wave model, the basic answer is that the incident wave front comes into the mirror at an angle, so one side of the wave front hits the mirror first, followed by the point next to it, and so on, as shown in the video, and it's this timing that causes the specific angle of the outgoing light. Huygens' principle shows geometrically that the angle should match the law we assume in ray optics.
But once you start asking why Huygens' principle is true or works the way it does, that's when you have to delve into more detailed wave physics or even electrodynamics (especially if you want to link it back to what atoms are doing).
If we go back to your question in the original post, it was "how does a reflected ... photon know the _angle_ at which it should go?" If you want to understand it through a simple model, first of all I'd avoid any talk of photons, to really understand those you need quantum electrodynamics and that really makes it complicated. I'd stick to thinking of light as rays or as waves.
As rays, there's just a rule that the reflected angle equals the incident angle, but that model isn't detailed enough to really offer an explanation of why. However, note that if you were to, say, bounce a tennis ball off a hard wall it would follow the same law (because it's an kinetic-energy-conserving collision that doesn't impact the component of the motion parallel to the surface, so it's not too far fetched at least.
In a wave model, the basic answer is that the incident wave front comes into the mirror at an angle, so one side of the wave front hits the mirror first, followed by the point next to it, and so on, as shown in the video, and it's this timing that causes the specific angle of the outgoing light. Huygens' principle shows geometrically that the angle should match the law we assume in ray optics.
But once you start asking why Huygens' principle is true or works the way it does, that's when you have to delve into more detailed wave physics or even electrodynamics (especially if you want to link it back to what atoms are doing).