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.