Re: inverse square law and "perfect" laser

Hi Gary,

I do not agree that laser light obeys the inverse square law.
Basically, the decrease in intensity as the inverse square of the
distance is a property of point sources. These sources emit uniformly
into all angles (hence the argument with spheres of increasing
diameters). However, a laser is essentially the farthest from a point
source that any light source could be (in the sense that its angular
divergence is the minimum possible consistent with diffraction, at
least for lasers with nice transverse beams). Said another way, lasers
emit into a very narrow angle range. So if you measure the intensity
of a beam with a photodiode (say) 1 inch from the laser and then 10
inches from the laser, you will not find 1/100th of the light with your
fixed area detector. If you do this same experiment with an
incandescent light bulb and make your closest measurement far enough
away (much, much greater than the size of the filament), you will
observe this square law decrease.

Also, there is nothing in the perfect vacuum that will prevent all the
energy from reaching your distant sphere. Interference can
redistribute energy (in space and time) but it will not make the energy
go away.

Hope this is helpful.

John


Gary Jefferson wrote:
Laser light obeys the inverse square law, but is this *only* because
the diameter of the laser beam increases linearly (at distance), or is
there something else fundamental going on to decrease light energy as
it travels?

I suppose another way to ask the same question is, in a perfect vacuum,
do all the photons that a central source emits make it to the edge of
some distant sphere, or are there other effects that cause the photon
waves to dissipate or interfere (e.g, cancel out) as they travel?

.

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