About wave and nuclei

[dttom]

It is known that refraction occurs when wave pass from, say, deep water region to shollow water region, and wavelength of the wave decreases and hence the speed of the wave. I would like to know why this would occur in detail. I can't find any expanation in text book, and I was just told that it would occur.

 

And beside, we all know that nuclei of atoms are composed of protons and neutrons, I just know that the reason of why neutrons and protons would be packed together is related to somewhat about nuclear force though I don't know the detail = =...