Why the water waves bend in diffraction?

[Aladin]

Waves bend when they pass throungh a narrow gap.It is the definition of diffraction.
what is the reason that when the water waves pass through a narrow gap in ripple tank the wavefronts become circular?
what is the reason that when the water waves pass through a wide slit in ripple tank the wavefronts remain straight?
please explain.

[ObsessiveMathsFreak]

This can be explained using Huygen's Principle (http://en.wikipedia.org/wiki/Huygens'_principle), where the wavefront at any point is considered to generate a circular wave at every point to advance itself. I didn't say that very well, but the Link may explain better. Be sure to check out the java applets link at the bottom of the Wikipedia page.

I like to think of it in this way. Whenever a wave encounters a "disturbance", it will generate a circular "ripple" centered at that disturbance. This sort of explains reflection, refraction and diffusion.