Why does a wave not go straight through an opening in a barrier like a ball would?

Question

A ball would go straight through, but a wave will come out circular (depends on how wide the opening is). Why does this happen?

Answer 1

The wave is not an object. It is pressure moving. The water in waves does not actually move much. When the pressure comes to an opening then the effect is like the opening is the starting point for the wave on the second side. If the opening is wide it is more like a flat wave generator. If it is small then it is like a point wave generator.
The difference is like starting a wave with a wide board or creating the wave by throwing a stone. The smaller the opening the rounder the wave and the more noticeable its apparent bending. But it really is not bending as much as it is the effect of being a new wave on the second side of the obstacle.

To get the idea of how little the water is actually moving look at how something floating in the water just bobs up and down as the waves pass by. If the water was moving forward or back and forth then the object would travel with the wave.

Answer 2

This is due to a characteristic of waves called diffraction.