---------- Forwarded message ----------
Date: Mon, 11 Aug 1997 23:50:30 -0700 (PDT)
From: William Beaty <billb@eskimo.com>
To: DCJohnson@auburn.mec.edu, billb@eskimo.com
Subject: Re: The Wavelength of Light
Newsgroups: sci.skeptic,sci.physics
In article <34EA7336A95DD011AABA00A0245B7F18500701@auburn.mec.edu> you wrote:
: While I was a young physics undergrad, a professor of mine once told our
: class that if you looked up into a sunlit sky between the slit created
: between your index and middle fingers, and if you constricted this slit
: just right, you would see small black bands that were created by light
: waves coming through the slit.
This doesn't work. It requires a slightly-coherent light source such as a
star or distant streetlight at night. The sun is an extended source, and
it totally blurs out any diffraction lines. I've heard this incorrect
description from other people too, which leads me to believe that it is a
"mutated mind virus" of physics demos, being passed from instructor to
student without reference to the real world.
When I follow your instructions, what I see is some pinhole physics (from
blurry edges of fingers) and sometimes the "scar" created by eyelids on
the moisture on my cornea.
I've played with the proper demo at night. It works well, but you need to
cross your eyes at a distant streetlight in order to blur it and make it
appear as a disk of light. Then hold up your fingers to cast their shadow
upon the disk, pinch them slowly closed, and you'll see fine frilly stuff
which is caused by light's waves. Or forget the fingers, and just squint
your eyelids while crossing your eyes at distant streetlights in the dark.
Fresnel diffraction fringes appear on eyelid shadows and eyelash shadows.
Even better, on a rainy night inside a car, place your eye behind a single
raindrop on the glass, as close as is comfortable for your eye, and look
at a distant streetlight through the drop. You'll see all sorts of
diffraction fringe patterns caused by the complex lensing of the water
droplet.
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