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Re: Another student question



-----Original Message-----
From: Daniel L. MacIsaac <Dan.MacIsaac@nau.edu>
To: phys-l@atlantis.uwf.edu <phys-l@atlantis.uwf.edu>
Cc: danmac@bohr.phy.nau.edu <danmac@bohr.phy.nau.edu>
Date: Thursday, November 13, 1997 6:09 PM
Subject: Another student question


I've been asked yet another question I am at a loss to explain. I think
it's
of the "Why isn't torque measured in Joules?" calibre.

The Joule is reserved as an energy unit. It could be used as a unit for
torque but it would not be respected as such.

In optics, when a ray of light changes media its frequency is said to be
unaffected, but its wavelength is changed to reflect the change in velocity
in the new medium and /nu = /lambda * f.

Why is frequency unchanged and wavelength changed? Why not both, or a
frequency shift? My handwaving response is that frequency is more directly
related to energy (a more fundamental measurement) and energy conservation
is the reason. I also did a little handwaving through the Xray scattering
treatment of an optical medium as a crystalline array of scattering objects
retarding total velocity but not changing frequency.

BUT I don't have any real confidence in this explanation, and I told my
students I would pose this question to my peers. So here it is.

I like this one. When I explain this to my students, I say that the rate at
which the waves impinge upon the new medium is set by the rate at which they
are propogating in the first medium. Therefore, if the new medium is
slower, the wavelength will be shorter in the amount of time it take to
transmit one wave (pulse) across the interface.

I use a cute analogy to illustrate this. Imagine a string of students arm
in arm and a number of equally spaced rows of students. They begin marching
from the beach into the water at normal incidence. The rate at which they
enter the water is fixed but as they enter the water, the rows are slowed
down. This same analogy does a nice job of illustrating the refractive
effect of a wave entering at an arbitrary angle.

A nice counter question (situation) to go along with this.

If a cavity is heated, what will happen to the speed of sound, the wavelenth
of the resonant wave, its amplitude, and its frequency.



Dan MacIsaac, Assistant Professor of Physics and Astronomy, Northern AZ
Univ
danmac@nau.edu http://www.phy.nau.edu/~danmac/homepage.html