Why does diffraction depend on wavelength? [closed]

We've all heard of the diffraction of radio waves over a mountain and the diffraction of water waves through a gap, but why does this effect depend on wavelength?

I'm looking for as simple answer as possible - if it's a bit hand-wavey that might be OK.

Another thing that troubles me is when I read the effect of diffraction depends on the size of the wavelength compared to the gap/ obstacle. But, how do you define the size of the obstacle if it's an edge, e.g. the tip of a mountain, the edge of a wall, the edge of a razor blade?

• That's an awfully broad question. You're basically asking us to explain diffraction to you. I would start by browsing the Hyperphysics pages on diffraction and come back to us with specific questions. Apr 12, 2014 at 18:29
• @JohnRennie I'm not interested in the complex interference patterns that occur, just the basic principle as to why it should depend on wavelength. I can't think of a simple, intuitive reason. Apr 12, 2014 at 18:37
• @JohnRennie Could you answer my second question - about defining size? Apr 12, 2014 at 18:38
• @JohnRennie Thanks for providing the link. I've now read all of the pages therein, and I can't find the answer to my question. Fancy giving it a bash? Apr 12, 2014 at 18:52