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If you want to really get into it, the "group velocity" of the speed of
light can actually go faster through some materials than light travels in a
vacuum. (Google faster than speed of light cesium - the most famous
experiment verifying this was done in a cesium gas I believe).
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"The Magic of Learning, the Science of Fun"
-----Original Message-----
From: Informal Science Education Network
[mailto:[log in to unmask]] On Behalf Of Eric Siegel
Sent: Friday, July 18, 2008 3:16 PM
To: [log in to unmask]
Subject: Re: Doppler Shift
ISEN-ASTC-L is a service of the Association of Science-Technology Centers
Incorporated, a worldwide network of science museums and related
institutions.
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*
>
>
>
> William and Wayne are right: the frequency changes, not the speed of
> light. The light from distant, redshifted galaxies has always been
> moving at the same speed ever since it started out on its trip to us.
>
Assuming that it has traveled in a vacuum all that time, right?
Passing through gas, our atmosphere, water changes the speed
significantly...how do I know? Google said so...
"
If light travels not through empty space but rather through a gas
(air) or a transparent fluid or solid, then the speed of the light in
that substance is less than the speed of light in empty space. The
ratio of the speed of light in empty space and the speed of light in a
substance is equal to the index of refraction of that substance. The
index of refraction of water is about 1.4 (depending on the color of
the light), so the speed of light in water is equal to about c/1.4
which is about 0.7c, which is about 210,000,000 m/s or about 130,000
miles per second."
<http://www.astro.uu.nl/~strous/AA/en/antwoorden/licht.html>
Eric Siegel
Director and Chief Content Officer
New York Hall of Science
47-01 111th Street
Queens, NY 11368
www.nyscience.org
718.699.0005 x 317
esiegel at nyscience dot org
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