Re: Another student question

[Jim Riley <jriley@lib.drury.edu>]

My first reaction is that we don't decide which is changed "nature" does.
That is, if I actually watch a wave as it passes from one medium to another
(easy to do with water waves, not so easy with light) what I observe at a
single point is that the frequency, i.e. the number of waves passing per
second, is the same as it was in the prior medium.  The wavelength, however,
as measured in the new medium is different.  If I try carrying this argument
over to light I find that measuring the wavelength is much easier than
measuring the frequency and the wavelength (as determined by interference
experiments for instance) does change.  If I can also manage to measure the
speed in the new medium what I discover is that by calculation, v=f x
lambda, the frequency does not change.


At 03:47 PM 11/13/97 -0700, you wrote:
> 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.  
>
> 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.
>
> Awaiting enlightenment,
>
> Dan M
>
> Dan MacIsaac, Assistant Professor of Physics and Astronomy, Northern AZ Univ
> danmac@nau.edu  http://www.phy.nau.edu/~danmac/homepage.html
Jim Riley
Department of Physics
Drury College
Springfield Missouri  65802
(417) 873 7233
e-mail:  jriley@lib.drury.edu
fax: (417) 873 7432