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B A Question about Diffraction...

  1. Jul 18, 2017 #1
    Okay...I have a question that, I have often read the statement in various topics on diffraction that "Diffraction is maximum when slit is comparable to wavelength of wave". Can someone point out what is exactly meant by ''greater or maximum diffraction'' here?...Like if we are going to talk about in the sense that "diffraction is greater when water waves, after passing through the slit, bend upto the "outer surface" of the slit", then is'nt that what happens in ''every'' diffraction, if we are to consider Hyugens Principle???...Any help is appreciated...
    P.S: I know my question does not make much sense, so i will try my best to elaborate it in my replies...
     
  2. jcsd
  3. Jul 18, 2017 #2
    In the first image, the water waves after passing through the slit, form an almost semi-circular shape...However, in the second image, the water waves form the same almost semi-circular shape, even though gap between obstacles in the two images is not same...So what is this "greater diffraction"???...I hope my question makes some sense now...
     

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  4. Jul 18, 2017 #3
    The more light is diverted from the straight-through horizontal direction, the greater is the diffraction.
     
  5. Jul 19, 2017 #4
    If you look very closely at the two images i posted, you can see that there is the same semi-circular shape of water waves in both images....So where is this "more bending of light in greater diffraction"???
     
  6. Jul 19, 2017 #5

    Nugatory

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    However, the intensity of the deflected light is much greater in one image than the other. If you were to place a screen behind the slit, in one case you'd see a small bright spot directly behind the slit and in the other you'd see a larger bright spot.
     
  7. Jul 19, 2017 #6
    I can only describe it simply as "When the waves create a more perfect semi-circle around the barrier, diffraction is considered greater (the gap being equal to the wavelength of the wave)".
     
  8. Jul 19, 2017 #7
    But in the second case it would not be as bright as less light (due to smaller opening) is distributed over a wider range of angles.
     
  9. Jul 19, 2017 #8
    The equation for the angle, θ, of the first minimum with respect to the horizontal is sinθ = λ/w. This defines the extent of the main area of light on a screen. As you decrease w, the angle increases, i.e., more diffraction.
     
  10. Jul 19, 2017 #9

    Nugatory

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    Staff: Mentor

    That is true. However, the off-axis areas are still brighter than they otherwise would be, a situation which is reasonably described by the informal "more bending of the light" that OP was asking about.
     
  11. Jul 20, 2017 #10

    sophiecentaur

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    The shape of the wave front is basically to do with the overall (radial) path length and it looks pretty circular. That doesn't change much as the slot is widened - in fact. at infinity, the wave fronts are all circular. What does change A LOT is the width of the region of maximum wave power. The wider the slot, the narrow is the region. That is what Diffraction is all about.
     
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