Changing the frequency of the wave entering the material

Okay thanks :
I meant the formula:
v=fλ
when a change in range creates a change in speed and changes the wavelength accordingly but the frequency remains.
(I said that I heard an explanation that uses the fact that the time axis is fixed but that sounds really strange).

L – K

You are mixing up two things. The formula you mentioned is always correct. It is just a simple relationship between the position of a particle (or the crest of a wave, for example) and time through the velocity. That is, x=vt. This is a formula that cannot change from one medium to another or from one time to another because it simply defines the concept of velocity.

The frequency of the wave in the other medium does not change for the simple reason that 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), the other phenomenon will also occur at the same frequency, that is, about two cycles per second. Think of a cannon that fires three shells per second and a shell fired from this cannon hits some wall, and with each shot nothing else in the world changes – the wind speed has not changed, etc.’ – How many times per second will a shell fired from a cannon hit this wall?

Thanks, not sure it can't be made more difficult, but it gives a good explanation.
Yitzhak,
I wanted to ask how you understand the subject of waves and in particular diffraction.
For example,
Why doesn't diffraction through a wide slit or through a short barrier show the propagation of the wave in all directions compared to a narrow slit?
How do you even understand the subject of waves? As if the subject of particles is more intuitive.