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A laser produces a coherent beam of photons with particular $\mathbf{k}$ and $\mathbf{\omega}$.

So, if there is no particular $\mathbf{k}_0$ directed toward our eyes, why do we see laser beams?

I think this is related to the Huygens–Fresnel principle, but I wasn't sure.

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Ever notice how you can't really see the beam of a laser pointer? –  Marcin Apr 16 at 14:56

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up vote 14 down vote accepted

You see it because it travels through air, dust, and a lot of other molecules and particles that can reflect and diffuse it. This, together with focussing, is also the reason for why it cannot travel arbitrary long distances. If you go to vacuum then the laser beam has much less losses, and it can travel much farther as happens in the LIGO interferometers where it goes 75 times through the 2.5 miles arms.

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So, in vacuum with a few particles in a unit of volume the laser beam would be almost invisible? (haven't found any photos of laser beam in space) –  m0nhawk Apr 16 at 6:51
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@m0nhawk You won't find anyone, simply because it cannot be seen ...unless you look at sci-fi movies, of course! –  DarioP Apr 16 at 6:57
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Thanks! There is also a Boom-Boom in Star Wars, so... the sci-fi isn't give much help in learning physics. –  m0nhawk Apr 16 at 7:03
    
Michio Kaku will disagree: en.wikipedia.org/wiki/Physics_of_the_Impossible –  Davidmh Apr 16 at 8:37

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