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Light travels in straight path, and our eyes detects an object's reflected light then we see the object. So if laser light is going in a straight path, how come we can see laser light?

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You either see it because it is aimed straight into your eyes (in which case it might be the last thing you see with that eye) or because it scatters off (minute) particles in the air. Sometimes people will deliberately blow a bit of smoke to make the laser light more visible.

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In ideal conditions, you cannot see the beam from a laser, even if it is within the visible spectrum. This is because the light is coherent, and no photons are scattered by obstacles which would direct them to your eye.

This is why, for laser shows such as in a planetarium or dance club, fog machines are used to add particles to the air, which cause scattering of the light and make the beams visible.

USU Atmospheric laser

Lasers used for atmospheric study, such as the ones used at Utah State University (near me), are often visible because of haze or fog naturally present in the atmosphere.

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