[Physics] Why do we see laser beams


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.

Best Answer

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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