Why is light visible?

Specifically lasers.

I think the question has come up before, but I don’t know that it was satisfactorally answered.

To “see light,” the little photons have to be reflecting off something.

What is that “something” with a laser?

I assume you’re referring to the beam itself. I think it’s tiny dust particles and so forth in the air.

Dust and water vapor in the air along the path of the beam. In space, lasers are invisible, Sci-Fi movies not withstanding.

The key is to have a bright highly focused beam, with a well defined ‘edge’. Bright searchlights are visible in air as well. The primary fluorescence is going to be water vapor (humidity) in most locations. Smoke, dust, rain, etc. can also help. Lasers aren’t always going to be visible. In extremely dry weather and without smoke or dust, you’ll only see the beam if it strikes a cloud, or some other object.

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For that matter, it’s pretty hard to see laser beams in air unless it’s very dark and/or the air is dirty (fog, smoke, dust, etc.) There are some applications that use beams strong enough to be seen without turning off the room lights, but I haven’t seen any myself.

Whenever they wanted to show us lasers on the educational science programs on TV when I was a kid they would always have a fog machine going so you could see it. I was disappointed - I wanted it to be like a star wars blaster.

Caution - Do not look directly into laser with remaining good eye

More of the same: Laser pointers only show the red dot when it hits something, and they are to weak to interact with normal atmospheric effects. Somewhat fun in the fog, however.


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