I think when we shine a laser through a vacuum, we should not see the laser beam until it hit the material containing the the vacuum. We can see visible-region lased photons as a beam of light because they interact with air molecules; therefore, they (some of the lased photons) are no longer laser photons right? but what about the remaining photons that have not interacted with the air?
A) SInce the laser photons are coherent and the beam is colimated in one direction, when we shine a visible region laser, let's say, at 45° angle onto a smooth surface, why do our eyes detect that visible light on the surface? Are the following the results after laser hit the surface? 1. Depending on the material, some of the photons reflected and travel at 45° the other direction still coherently. 2. Some photons were absorbed and the electrons in the atoms of that surface interact with the lased photons and emit the visible photons of the same wave length spontaneously in all directions, thus to our eyes?
B) Another similar question is; how could a lidar laser speed detector (I assume that it's an infrared camera) receive the reflected laser photons when the angle of the beam to the license plate (a usual target) surface isn't 90° (for ex, when the police is standing on the side of the road rather than in front the on coming car)?