/================/
What about an interferometer placed in the water. That too would show no
change in the fringes because both beams would be effected the same. If
only the experimental beam is passed through water and compared to a
reference beam in the air, then you see a change in wave length of the
experimental beam. Is this not evidence that wavelength does indeed
change as the light changes medium?
/================/
Yes, indeed. I haven't done these experiments with light in water, but I've done them with light in various glass and plastic fibers, and with sound in various liquids. The wavelengths change just as you'd expect in each of these media, based on c = lambda*f. They *have* to, for the wave equation to apply.
There is at least one case I can think of where the wavelength plays a big role, and that's where nonlinearities have to be taken into account. For example, the production of harmonics in nonlinear acoustic media is based on, in part, the failings of the assumption of adiabaticity, because temperature gradients can become too great for heat to dissipate "gently" enough; this seems to be a wavelength-dependence (compared to thermal dissipation length scales) moreso than a _frequency_ dependence.
/================/
Your point that Physics should use frequency to describe the color of
light is well taken. I am of the opinion that if you are measuring color
(keep in mind I am teaching at the High School level) you are almost
always doing so in air - in the lab - where it does not make any
difference whether you use frequency or wavelength. Knowing that it is
the frequency that remains constant as the light changes medium and
results in the wavelength and velocity changing is interesting but not
going to help the understanding of color or light or any of their
properties. When they shine their laser pointer into the water tank to
see the path of the light - the path in the water is still red. Opening a
can of (it's really green) worms is not helpful.
/================/
Agreed. And I never did (rather, I hope I never did) suggest that it's really green!
Our perception of color, as has been discussed on this list many times, is fantastically complicated, and depends on a lot of things other than frequency or wavelength. There are a whole lot of other worm cans which can be opened to pique students' interests, especially when they start to see that even people who have worked tremendously hard on these questions can't yet give a comprehensive "theory of color" which always works. Maybe this will turn students towards being interested in physiology or neurology or psychology, which is great.