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How a very short λ enable light, not obliterate all shadow? (diffraction)

  1. Nov 2, 2015 #1
    Hello all,

    Guys, I just pointed out that I actually lacking in somewhere in the understanding of the phenomenon of diffraction of light, of which I am aware from no less than 10 years.

    Recently I revised Huygens wave theory of Light and studied in what ways Newton criticize it.

    At one of the place I found this:

    Newton wrote: 'If light consists of undulations in an elastic medium it should diverge in every direction from each new centre of disturbance, and so, like sound, bend round all obstacles and obliterate all shadow.'

    Newton did not know that in fact light does do this, but the effects are exceedingly small due to the very short wavelength of light.

    Now after this the thing that I am unable to comprehend is that, How does a small wavelength of light prevent it to enter into all region of geometrical shadow? How does the wavelength magnitude related with the size of the obstacle, in order that diffraction may occur?

    Hope I stated my question clearly.

    Any help will be highly appreciated.

    Thanks a bunch. :)

    Regards
     
  2. jcsd
  3. Nov 2, 2015 #2

    BvU

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    It doesn't: light really deflects in all directions. But all the light from an aperture does that -- due to the short wavelength of the light the net result is that all these light waves interfere with each other and thereby degrade the intensity to virtually zero. Only under special conditions (small aperture, laser light -- so that the light waves are in phase with each other) you can make visible diffraction patterns that differ from trivial 'shadows'.
     
  4. Nov 2, 2015 #3
    Would like to know how does this(degradation of intensity to virtually zero) become significant due to short wavelength of light?

    How does the same may not achieve when wavelength is larger. May be a picture or some description from your side may help me.
     
  5. Nov 2, 2015 #4

    BvU

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    Last edited: Nov 3, 2015
  6. Nov 3, 2015 #5
    Yes. It is after that, that I could appreciate the Genuineness in Newton's crtisim for huygens wave theory.

    I want to know how exactly the fact that "light has small wavelength" enable light wave not to behave like sound. We noticed sound (can) bend to a very high degree and we are able to listen even when their is a sufficiently big obstacle between sound source and listener. But how a short wavelength of light reduces this effect appreciably, is where I am not clear and need help.


    I am sorry for my weak mind and poor understanding standard. :(
     
  7. Nov 3, 2015 #6

    BvU

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    Seen as waves, light, water and sound and even electrons and a lot more things do behave alike. One or two wavelengths "into the shadow" for light is very little, for sound it is a considerable distance !

    The diffraction pattern intensity falls off quickly over a width that is proportional to the wavelength. So longer wavelengths have a wider pattern, i.e. can be observed at wider angles.
     
  8. Nov 3, 2015 #7
    Sadly I am not able to find my answer in this.

    Let me state my question in more detailed form.

    Consider a large (opaque) thick wooden plank, in front of it are placed a small light source and a small sound source on side and an observer on the other side.

    As a matter of fact the observer on the other side will observe sound waves are well reaching him but light waves do not get diffraction much.

    The reason that is explained to us in text books and in also repeated in the previous replies is that light has lesser wavelengths that is why diffraction effect is lesser.


    What is wanted to know:(1)how exactly a lesser wavelength of light is causing a lesser diffraction effect and(2) how does wavelength and size of obstacle is even related?

    Any help from from anybody will highly appreciated.

    Regards
     
  9. Nov 3, 2015 #8

    BvU

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    I tried to make it simple by talking about the shadow.

    The opening in the wood is many many wavelengths wide. Most of the light doesn't even see the obstruction and goes straight on. The transition from light to dark at the observer is over a few wavelengths -- so a fairly sharp shadow.

    The hole in the wood is much much smaller than the wavelength of the sound. So the sound after the hole seems to come from a point source and propagates in all directions.
     
  10. Nov 3, 2015 #9
    I don't know what are you talking. I have not used a wood with any "hole" or holes.

    :nb)

    I just wanted a descriptive explanation that tells me how the "short" wavelength of light wave is enabling it not to bend it like sound and obliterate all shadow.
     
  11. Nov 3, 2015 #10
    If you think that everything can be explained "as simple as to be understandable by your grandmother" withoute using mathematics, this is a quite good example that you're wrong.
    In physics there are things that cannot be explained without mathematics.

    --
    lightarrow
     
  12. Nov 3, 2015 #11
    I would like to look up at all the mathematical and geometrical details that needed to address the solution of the problem.

    It would be very kind of you guys if you help me understand may be by giving some pictures one showing larger wavelength and other shorted wavelength meeting separately on an edge for diffraction. In that it could be explained as to what is happening on wavelength level with or without mathematics or geometry.
    I remember one of my thread in which i took to long to understand, "why don't the nearby atmosphere look blue?"

    I hope physicsforums is still has people with such nice nature :)

    Regards
     
  13. Nov 3, 2015 #12

    BvU

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  14. Nov 3, 2015 #13

    sophiecentaur

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    I suggest you start with the most simple case of diffraction. That would be the 'two slit' experiment. There are dozens of hits from a Google search, using the terms Youngs Slits or Two point source interference. You can take your pick about how much Maths you can or can't accept.
    With just two points, the main features (maxes and mins) of an interference pattern can be drawn from first principles. When you extend this to the effect of , firstly, many point sources (diffraction grating) and then to an infinite number of points across a slit of finite width. The same basic principle applies for all diffraction. When an obstruction or a gap (aperture) is many wavelengths wide, you get 'hard edged' shadows with very little 'diffraction effects' round the edges. That corresponds exactly with the very tight set of fringes that two wide slits produce and the very broad fringes that closely spaced slits produce.
    As with most of Physics, the Maths helps you get to grips much better with this than trying to do it by hand waving.
     
  15. Nov 3, 2015 #14
    I suggest starting with Huygen's principle. See if you can explain how a plane wave travels in a straight line. It's because all the non-straight paths have some destructive interference. When the wave front is big or infinite, the destructive interference cancels out almost or all the intensity along non-straight paths.
     
  16. Nov 3, 2015 #15

    sophiecentaur

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    The amount of light that's diffracted around the edges will be the same - it's just that so much more light energy ends going forward, when the slit is wide. The effects at each edge can be calculated for a single knife edge..
    Actually, trying to draw Huygen's secondary sources on a diagram is pretty difficult. It is far easier, ime, to consider path lengths and path differences (as in the link I suggested) to find where the maxes and mins will occur in simple situations.
     
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