In article ,
Larry Jaques wrote:

On Sat, 23 Aug 2008 00:18:25 -0400, with neither quill nor qualm,
Joseph Gwinn quickly quoth:

In article
,
Bob wrote:

OK, but it is fair to note that the constant of proportionality is
very small for a well-collimated laser. As opposed to 1 for point
source incoherent light. It is more accurate to say "laser beams are
subject to **an** inverse square law", rather than **the** inverse
square law (constant of proportionality = 1) that applies to point-
sources.

No, it's really *the* inverse square law. All forms of radiation follow
this; the effect is geometric. Lasers focus their energy into a narrow
beam, but that beam nonetheless spreads.

Think of this example. Starlight is almost parallel when it reaches us,
yet it is incoherent and most certainly follows the inverse square law.

Stars are just possibly a _wee_ bit farther than the vulnerable 200
mile (or 22,000, geo) satellite orbits we were discussing, Joe.

True enough, but it butresses the point that just because the divergence
is low doesn't mean that inverse square doesn't apply. Close to the
laser, inverse square is not followed. Far away, it is followed. For
use as a weapon (versus an industrial tool), the behavoir far from the
laser is the key.

http://www.stats.uwo.ca/faculty/aim/...ng/WriteUP.pdf


Radar systems work the same way, as do optical systems not involving
lasers.

The most striking example is a burning lens being used to focus
sunlight. In the region from the Sun to the lens, inverse square
applies. By contrast, in the region between lens and focal point,
inverse square law is *not* observed. However, far from that lens,
inverse square reasserts itself.

Joe Gwinn