What causes waves to bend around objects?

  1. I've heard that a property of waves (water/sound/light...) is that they can bend or refract around an object as long as the sides of the object are shorter than the wavelength.

    I'm trying to picture what would cause this 'bending', but its not coming to me. Can anyone help to explain how the geometry and dimensions of an object will effect the interaction of waves with that object?
  2. jcsd
  3. chroot

    chroot 10,427
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  4. I take it the opaque barrier is the one that applies to my question...? If so, that really doesn't give a good general explanation.

    I'll ask my question slightly differently:
    Let's say I position a full orchestra in front of a building. This building has a sharp corner on one side and a more obtuse corner on the other side. If I have the orchestra play, and I stand first around the sharp corner, I will probably hear a full range of instruments. If I then stand around the obtuse corner, I will probably only hear the lower frequecny instruments. What causes this?

    I know sound is just pressure waves in the air. How do these propagating pressure waves 'feel' the corner. What about the lower frequency waves allows them to bend more readily?
  5. chroot

    chroot 10,427
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  6. jimmy p

    jimmy p 580
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    Ahh i have been asking similar questions but about the equations of this (see my thread!!!) and stuff! however i havent really found much to help me in these sites...are there any sites with a bit more continuous prose explaining this instead of showin pretty pictures that i have in my text books?

    p.s the text books dont have what i want in them!!
  7. Claude Bile

    Claude Bile 1,479
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    Consider the surface waves on a body of water. Imagine that a plane wave (wave whose wavefront is a straight line) encounters an aperture, say an inlet into a bay or something. The wave will diffract.

    Why will it diffract? Well, imagine for a moment the wave didn't diffract, that the wavefronts continued to propagate as a straight line. What would this look like to an observer? An observer will see a wave of some amplitude A, with a finite width which then suddenly drops down to zero at the end of the wavefront. It would be like viewing a wave that has been severed at either end to leave a perfectly flat surface.

    Now (hopefully) you can begin to see the ludicrousness in this situation and how unrealistic it is. Consider a point on the surface of the water. If energy travelling through the water (in the form of a surface transverse wave) causes that point to rise say a metre up into the air, the surface of the surrounding points must be pulled up with it like the membrane of a drum. When the wave's oscillatory motion causes the wave to return to equilibrium, the surrounding points are also pulled down. That point essentially acts as a point source for spherical waves.

    Now consider that this happens on every point along the wavefront. The resultant wavefront will be the sum of many spherical point sources (This is Huygen's principle). An infinite plane wave will continue to propagate as an infinite plane wave until it meets an aperture. Upon leaving the aperture the ends of the wavefront will be curved. The degree of curvature depends on how far apart the wavefronts are (Wavelength) compared to the size of the aperture.

    This does not apply only to water waves, electromagnetic waves and sound waves also behave in this fashion. DIffraction basically occurs because Electromagnetic fields etc are continuous in nature. If they are continuous then a finite wavefront (A wavefront that does not end on a boundary) must diffract to some degree.

    Well that's the end to my long winded explanation, no doubt there are other people who could explain in better, however people were crying out for a discription, so I felt compelled to act :smile: . Cheers.

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