[Physics] Why does diffraction depend on wavelength?


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?

Best Answer

For larger objects the radio wave gets reflected. Compare this to a water wave hitting a wall. For smaller objects the radio gets diffracted. Compare this to a stick placed in the path of water wave. This stick bends the water wave which is similar to diffraction

A light wave consists of larger number of smaller waves. A mountain reflects most amount of these smaller waves but the tip is small compared to the wavelength. It diffracts the smaller waves which are incident on it

edit 1: http://www.acoustics.salford.ac.uk/feschools/waves/diffract.php