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[[email protected]: Re: How to find the focal length of a lens]
----- Forwarded message from "Michael S. Sipior" <[email protected]> -----
Date: Fri, 22 Jan 1999 16:44:10 -0500
From: "Michael S. Sipior" <[email protected]>
To: [email protected]
Subject: Re: How to find the focal length of a lens
On Fri, Jan 22, 1999 at 12:38:44PM -0500, Michael A. Weinstein wrote:
> A person from the theater department has given me a large plano-convex
> lens, several centimeters thick. He needs to know the exact focal length
> so that he can order another one just like it. Does anyone have a
> practical way to figure out the focal length? If so please reply to me.
Hey Mike,
I'm not sure about practical, but I have a notion. You'll need a few
things, though:
--The lens in question
--1 high-quality mirror
--15-20 Meter sticks (alternatively, you could use a tape measure)
--Boxes and boxes of Sugar cubes
--Some ants, preferably of an angry, agressive variety
--Clear weather
--Plenty of beer
Lay out the sugar cubes at regularly spaced intervals, corresponding
to the precision in focal length you need. If you have lots of sugar cubes,
try making a continuous "sugar line", say around fifteen or twenty meters
long. Now, set up the lens so that it sits at one end of the sugar-cube
line. Position the mirror so that it catches sunlight and redirects it full
onto the lens face. This may require some adjustment over time, depending on
the ants. Release your ant army at the other end of the sugar-cube array.
Drink the beer.
The ants should progress at some steady rate across the sugar-cube
array, drawing ever closer to the lens which we are investigating. At some
point, actually, the focal point in question, the ants will begin to notice
a certain change in environmental conditions. Some of them may explode or
catch fire. When all the ants have combusted or run away, measure the
distance from the lens to the mess of burned and twisted ant corpses that
have recently manifested themselves. This should be the very focal length
you are trying to measure! Easy, huh?
There are certain problems, based on whether the ants are, in fact,
carnivorous or not, so be certain to take all reasonable precautions against
insect bites. If possible, try to arrange your experiment in the vicinity of
a number of picnics---these other food sources should distract the ant mob
should something go awry, giving you a chance to make your escape. A
oil-drum of Raid is probably a sensible purchase as well, pumped out through
a garden hose attachment. With domestic ants there should be fewer problems,
though the experiment is likely to take longer.
There is also the question of where, among the mass of burned and
exploded ant bodies, one should measure to obtain the proper focal length. I
have found that it is best to measure to the point closest to the lens, and
then repeat the experiment a number of times, beating down the Gaussian
error until below the tolerances you require.
Another nice feature of this experiment is that it is one of the few
scientific procedures approved by PETA, as ants are neither cute nor fuzzy.
--
There is in the land of Mnar a vast still lake that is fed by no stream, and
out of which no stream flows. Ten thousand years ago there stood by its shore
the mighty city of Sarnath, but Sarnath stands there no more...
---H. P. Lovecraft, "The Doom that Came to Sarnath"
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