# Demonstrate diffraction in a ripple tank

When we demonstrate diffraction in a ripple tank, why does shallower water slow the passage of the water wave down?

Also, are there any other ways to demonstrate when refraction happens without light nor water waves?

Thanks.

I was thinking that when the water gets more shallow that the frequency increases, and the wavelength decreases. but the equation $$v = \lambda f$$, wouldn't this show that velocity stays the same?