Jerome Kasparian in American Scientist:
Next time you give a presentation about your research, take a close look at the laser pointer you’re holding in your hand. How big is the beam coming out of it? And how large is the spot that it forms? The answers will, of course, hinge on the particular laser pointer you’re wielding and the distance between podium and screen. Typical values might be a few millimeters for the beam as it exits the aperture of the pointer and a centimeter or so for the circle of light it casts across the auditorium. It takes only a smattering of physical intuition to guess the reason: Diffraction causes the beam to diverge. The actual cause may be a little more complicated, because some laser pointers include a lens that makes the light converge at a fixed distance from the tip, which leads the beam to spread out beyond this focal point—more so than if only diffraction had operated.
Imagine now that your laser pen packed a more powerful punch—say that the intensity of the beam was a whopping 1012 times that of a typical pointer. What then would the beam do as it crossed the room? (It’s clear enough what it’ll do when it hits the screen—quickly burn a hole). The answer, it turns out, is anything but intuitive. A laser of sufficient intensity traveling through air will—all by itself—engineer a narrow channel, one perhaps a tenth of a millimeter wide, over which light will propagate for tens or even hundreds of meters.
More here.
