Ok, see counselor Troi firing her phaser?
You see this kind of thing all the time on film in scifi. Whether it’s Star Trek, Star Wars, or pretty much anything else, energy beams fired from future weapons are visible. Usually someone will point out that in fact laser beams are not visible in this manner. To see light, it has to reach your eyes. This is clearly not possible when all the light is actually traveling down the beam path. You can see this in action with laser pointers – only the spot where the light hits and diffusely reflects is visible. The path is not.
Writers of TV shows usually explain this by saying that the beam is not strictly light, but some stream of particles that slightly emits to the sides along its main path. While this has its own problems, at least it acknowledges the issue.
But what’s even more interesting is that in fact there are already automatically particles present along the beam path in the atmosphere. Some of them are sizable particles like dust, others are individual atoms and molecules. Generally they don’t scatter much light, but if the light is intense enough then the small amount they do scatter is enough to see. And so you have a visible laser beam. Here’s one in my lab:
The beam scatters off the air and you can actually see it as a straight line. Apologies for the terrible camera phone picture, I really need to get a classy camera that can take nice pictures. This is not actually a laser I’m working on, so honestly I’m not sure which variant of frequency-doubled Nd:something laser this is. Probably Nd:YLF.
This is used to pump an infrared ultrashort-pulse laser with a repetition rate of 1 kHz. This can itself be focused to a point in the air, which becomes visible as a little stationary spark as the intense beam ionizes the air. This produces a 1 kHz buzz which can easily be heard by the unassisted ear.
I have to say it’s a nice job perk that I can see old science fiction tropes come to life pretty much every day. 🙂