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Re: streaked windows and lights



On Wed, 17 Jan 2001, SSHS KPHOX wrote:

I made a mess of my windshield this morning when the fluid was frozen and
the wipers made a streaked smear before my very eyes. Oncoming cars and
taillights ahead created a wonderful pattern of elongated "rays" that ran
to the center of the arc. I tried to explain this phenomenon to myself but
was never satisfied with my answer. I was thinking some kind of
diffraction effect but can not put all together. So..Can anyone help me
out? Please.

Nope, not diffraction, instead it's little lenses. Sci. Am. has an old
"The Amateur Scientist" column about just this effect.

Jearl Walker, WHAT DO PHONOGRAPH RECORDS HAVE IN COMMON WITH WINDSHIELD
WIPERS? The Amateur Scientist, Scientific American. July 1989 Vol261
No1. pp106-109

Each streak on the windshield acts like a long cylinder lens which
scatters light in the plane which is perpendicular to the streak, while
passing it through unaffected in the plane perpendicular to the first. (I
guess I need a drawing for this to make sense!) Because each streak is
curved, it creates a (very astigmatic) virtual image of the distant light
source, where the 'aerial' location of the virtual image depends on the
radius of each windshield streak. If a point source of light shines upon
the array of concentric streaks on the windshield, it creates a glowing 3D
line which floats in the air outside.

A few years back I stumbled upon a way to harness this effect in drawing
'holograms' by hand, by scratching the surface of black plastic with a
compass/scriber. See:

Hand-drawn holograms
http://www.amasci.com/amateur/holo1.html

Are they really 'holograms'?
http://www.amasci.com/amateur/holo3.txt


The effect is rigorously investigated in this article:

W. Plummer & L. Gardner, Applied Optics, V.31 No.31, Nov. 1992,
pp. 6585-6588, "A mechanically generated hologram?"




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