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?

  • 1
    $\begingroup$ 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. $\endgroup$ Apr 12, 2014 at 18:29
  • $\begingroup$ @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. $\endgroup$
    – User 17670
    Apr 12, 2014 at 18:37
  • $\begingroup$ @JohnRennie Could you answer my second question - about defining size? $\endgroup$
    – User 17670
    Apr 12, 2014 at 18:38
  • $\begingroup$ @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? $\endgroup$
    – User 17670
    Apr 12, 2014 at 18:52

1 Answer 1


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

  • $\begingroup$ By "the tip is small compared to the wavelength" do you mean, like, a few atoms on the top? $\endgroup$
    – User 17670
    Apr 12, 2014 at 18:58
  • $\begingroup$ no. radio wave has wavelength in the range of few meters. Even a chair can diffract the radio waves. So the upper part of the mountain ,like about few meters below the tip, can diffract the wave $\endgroup$
    – anuraag
    Apr 12, 2014 at 19:03
  • $\begingroup$ Yeah, but does the same not apply to light. It's a smaller wavelength, so it would be diffracted only by a smaller object, like a few atoms on the top? If not, why not? Cheers $\endgroup$
    – User 17670
    Apr 12, 2014 at 19:10
  • $\begingroup$ radio wave is also a light wave. If you consider light as visible light then it's wavelength is in the order of nano meters. Even if diffraction occurs at that level it is too negligible for us to observe diffraction due to mountain tip. Instead you can observe diffraction due to a pen tip. The image behind the pen tip when magnified seems distorted. $\endgroup$
    – anuraag
    Apr 12, 2014 at 19:16
  • $\begingroup$ Why are radio waves not diffracted by a pen tip? $\endgroup$
    – User 17670
    Apr 12, 2014 at 19:49

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