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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)?

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License plates are printed with a special kind of paint that retro-reflects light. The paint contains glass micro-spheres that reflect light back toward the source. If you hold a flashlight next to your face and shine it at a license plate, you will see that the license plate reflects brightly over a wide range of angles. If someone else holds the flashlight, the license plate will not appear as bright because the brightest refelction will be straight back at the flashlight.

Most surfaces, including specular (mirror-like) surfaces that are dirty or scratched, scatter light in all directions. A sensitive camera can detect even the small amount of light that scatters back toward the source from a typical surface. If you can see the spot of light on a surface when you shine a laser pointer at it, probably a camera can see the light too.

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  • $\begingroup$ Thank you S. McGrew, so no matter where a lidar gun is projected on a car, diffuse reflection will always happen right? $\endgroup$ – Kobe Kobe Kobe Jan 29 '20 at 10:18
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    $\begingroup$ Yes, some amount of diffuse reflection will always occur. $\endgroup$ – S. McGrew Jan 29 '20 at 15:14

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