Demonstrate diffraction in a ripple tank

[Cheman]

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.

 

[compute_a_nerd]

according to my physics book. When the water is coming into shore, or becoming more shallow it slows down because the water is actually dragging on the bottom. Is this correct?
I was thinking that when the water gets more shallow that the frequency increases, and the wavelength decreases. but the equation <br /> v = \lambda f <br />, wouldn't this show that velocity stays the same?

Thanks