Fraunhofer diffraction

  1. In single slit fraunhofer diffraction, only the light which goes near the slit is diffracted... If it goes just through the middle it will not be... If the slit is really small would light be diffracted even if it goes through middle?And do all light waves that go near the slit diffract? Does all light that goes through the middle go undiffracted and contribute to central maximum?

    What would be the classical and quantum interpretation of this question?
  2. jcsd
  3. Drakkith

    Staff: Mentor

    The EM wave cannot be divided into parts that "go through the middle" or not. When focused down to a spot, the resulting airy disk is the result of the EM wave interfering with itself. Contrary to popular belief, no light is being "diffracted off the edges". The pattern of the airy disk is a result of missing part of the wavefront. For example, a larger aperture in a camera or telescope allows more of the EM wave in which results in more of the wave interfering with itself, with the end result being a smaller airy disk. I cannot explain it well. See the following link for a MUCH more detailed look:
  4. jtbell

    Staff: Mentor

    Where did you get this from? I think you must have misinterpreted something that you read. Each point in the diffraction pattern receives light from all parts of the slit.
    Last edited: Nov 16, 2012
  5. Drakkith Thank you very much for the link... But what do you mean by 'no light is being "diffracted off the edges'... If there's no diffraction then why the pattern is like it is? You said for the missing wave fronts... Then is the proof wrong where the pattern is explained using

    'some rays.who change their direction at an angle.then we pair them & explain minima.'
    Though I didn't see the explanation for maxima there?

    Jtbell I was asking what happens for which rays? But I should've left these lines and asked the ques... "do all light waves that go near the slit diffract? Does all light that goes through the middle go undiffracted and contribute to central maximum?"
  6. Drakkith

    Staff: Mentor

    I imagine you are thinking of diffraction like photons bouncing off the sides of the slit. This is not true at all. Light is an EM wave. When it passes through a slit and emerges on the other side it wants to expand outwards from the slit. Per the Huygens–Fresnel principle, we can consider the wave to consist of an infinite amount of waves of infinitesimal size. (Not wavelengths) These infinitesimal waves all interfere with each other as they propagate, with the resulting diffraction pattern depending on the size of the slit and the wavelength of the light.

    See here:–Fresnel_principle

    Do not think of light as "rays" or you WILL be confused. Rays do not explain diffraction and interference.

    The entire wavefront is affected by going through the slit.
  7. Thank you...

    I knew the huygen's principle and the explanation given by wave theory of light... Trying to think about photon in it was a wrong approach... However,I should say waves that hit the screen at an angle(angle that is created from their origin with the axis)...

    btw what does quantum mechanics say abt diffraction?
  8. Drakkith

    Staff: Mentor

    The math is different, but the end result is the same.
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