Okay, thanks :\
I meant the formula:
v=fλ
where a change in the medium creates a change in speed and correspondingly changes the wavelength, but the frequency remains the same.
(About that I said I'd heard an explanation that uses the idea that the time axis is fixed, but that sounds really weird.)

To K,

You're mixing up two things. The formula you mentioned is always correct. It's just a simple relation between the position of a particle (or of a wave crest, say) and time via the speed. That is, x=vt. This is a formula that can't change from one medium to another or from one time to another, because it simply defines the concept of speed.

The frequency of the wave in the other medium doesn't change for a simple reason: if you generate some periodic phenomenon (say, by completing two cycles per second), and that same phenomenon generates (as a cause) another phenomenon (and nothing else in the world changes), then the other phenomenon will also occur at the same frequency, namely two cycles per second. Think of a cannon that fires three shells per second, and a shell fired from that cannon hits some wall, and with each shot nothing else in the world changes—the wind speed hasn't changed, etc.—how many times per second will a shell fired from that cannon hit that wall?

Thanks, I'm not sure one couldn't still raise difficulties, but it's not a bad explanation.
Isaac,
I wanted to ask how you understand the whole topic of waves, and diffraction in particular.
For example,
why is it that in diffraction through a wide slit or past a short barrier, we don't see the wave spreading out in all directions the way we do with a narrow slit?
How do you understand the whole topic of waves בכלל? I mean, the topic of particles is more intuitive.