You shine a laser beam in some direction in order to create an image/flashbang/whatever twenty meters away. Why does it ionize the air twenty meters away but not ten meters away (along the same ray from the laser to the projection)?
This page linked earlier [0] relates one possibility to the 'Bethe formula', where depending on the energy of the beam (proton in their example) you can control where the burst appears.
Maybe. But that article is talking about beams of massive particles -- not beams of massless photons, which is what lasers are.
The fact that the particles have mass and decelerate is what produces the sharp drop in velocity: the slower it goes the more it slows down per unit of time or of distance traveled. Like a bullet travelling through water. So you get that nice sharp asymptote in the position-vs-time graph.
I don't think that webpage is applicable to light or other photon beams. Photons can't slow down unless they encounter a change of medium -- but that's basically projecting onto a physical "screen"... the laser operator can't influence the depth position of the screen. But thanks for the link, it's the first time I've seen a "3D projection technology" that had a coherent answer to the depth-control question. Maybe the Navy is using massive-particle beams instead of lasers and the Forbes journalist just doesn't know the difference. Or maybe it's deliberate disinformation, like how fusion weapons were named "hydrogen" bombs.
There are two ways off the top of my head (assuredly there are more): (1) use two lasers; or, (2) the geometry of a laser is hour-glassed shaped, so only the “neck” may have sufficient energy density.
