RE: inverse square law

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erobkin@uwc.edu
Date: 04/27/00-02:50:20 PM Z


The Inverse Square Law only holds for point sources. Each little tiny bit
of the surface of a flourescent is effectively a tiny point source and the
light from it follows the inverse square law. The tube is a whole line of
such tiny bits and when you get through the integration (addition) process
for all of these tiny bits you discover that the tube considered as a single
object and considerd over "small" distances from the tube emits light
following an Inverse Linear Law.

If you put a bunch of tubes next to each other and integrate all of those
inverse square contributions then you find that, again for "small" distances
the array follows an constant illumination law. What "small" means is
subject to experimental determination but it is not paper thin nor is it
several meters for the kinds of light arrays that have been discussed here.
Actually the tube will follow a constant illumination law if you are no
further from it than some moderate fraction of its diameter. All of this
needs experimental determination or moderately complicated computations to
get the distance details close to right.

Incandescent lamps have tiny glowing filaments and the "small" in this case
to account for their physical dimensions is small enough so that you would
have to be inside the glass bulb before you'd see much dparture from Inverse
Square.

Newton's Law deals with the inverse square relationship of gravity. He got
to it by considering Kepler's Laws of Planetary Motion and inventing the
calculus to deal with it. If you want to argue about Leibnitz's role in all
of this we should take it off list.

You don't need Newton, calculus, or gravity to get to the inverse square law
for light simple geometry will do. Light illuminates the inner surfaces of
a surrounding sphere and the area of that surface varies as the square of
the radius. Increase the size of the sphere and you have the same light
over a bigger area and hence inverse square for light. This requires only
that the radius be "big" relative to the physical dimensions of the light
source. True point sources being really hard to come by in nature.

A single clear glass incandescent light in a room is a good approxamation to
a point source. Set one up and get out a light meter to see.

ER

-----Original Message-----
From: Ed Stander [mailto:glassact@compuserve.com]
Sent: Thursday, April 27, 2000 2:46 PM
To: INTERNET:alt-photo-process-l@skyway.usask.ca
Subject: Re: inverse square law

Sandy:
  The Inverse Square law DOES hold for fluorescent lights. It would be
rather sad if the laws of physics only held true for incandescents. The
trick here lies in defining the distance correctly. The light received
from a bank of lights does not come only from the center of the bulb. It
bounces around a lot and effectively comes from everywhere the light
bounces from. So what distance do you measure? As long as you are within
the enclosure of the box, a move away from the lights may simply bring new
rays into play. Once you move far enough away, however, the inverse square
law works fine. If you wish to test this, simply make white paper blinders
for each bulb. Wrap the paper over the top of each bulb, and have it open
towards the print. This will limit the amount of scatter. Do this and I
think you'll find tat Newton's law works just fine.


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