Re: IR filters

[quk@isham-research.demon.co.uk (Phil Payne)]

In message <19980918210429.9841.qmail@hotmail.com> "scott miller" writes:

> >The same inverse square law that also applies to the reflected
> >radiation.  The ratio between their strengths is independent of
> >distance.

> Wouldn't a very directional reflector maintian the laser's tight sine
> wave, and thus maintain the return beam focus much better than normal
> light?  The little spherical reflective beads in license plate paint are
> very directional reflectors, that's why they're the preferred targets
> for laser.  Scattered reflections from other surfaces ought to be much
> less directional, and lose strength as rapidly with distance as
> non-laser light.

Nah.  Sorry - the inverse square law _ALWAYS_ applies.  A laser beam is
more tightly collimated than a non-coherent beam, but it has divergence
and the inverse square law applies.  A truly parallel non-divergent
beam requires an infinite aperture width, and your average cop has
around a quarter of an inch.

If you're interested in bending your brain on this one, I can recommend
a classic conundrum known as "Olber's Paradox".  Any decent cosmology
textbook will cover it.  Odd but true (and the math is fairly simple)
that absolute luminosity as perceived by an observer is independent
of distance.  Explains why lights are just as bright, even if they're
far away.

I _DON'T_ recommend explaining Olber's Paradox to a seven-year-old
child.  The result of this is the said child's teacher saying that it's
obvious the sky is dark at night, and a seven-year-old child at the
back of the room sticking up his hand to say: "Please, Miss - Olber
says it shouldn't be."

--
 Phil Payne
 Phone: 0385 302803   Fax: 01536 723021
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