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Interference Pattern Properties

  1. Jul 1, 2010 #1

    jaketodd

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    Why does the distance from a light source to the two slits make a difference in the interference pattern?

    Thanks,

    Jake
     
  2. jcsd
  3. Jul 1, 2010 #2
    The bands will become wider or more narrow with distance because distance changes the possible paths (or probability densities) of the particle/wave.

    A simpler explanation is refraction. By way of simple refraction principles, moving the source in relation to the slits creates different focal points.

    Imagine bouncing a ball off a wall at a given angle. If you throw the ball at the same angle every time, the ball will follow a given trajectory. However, if the wall suddenly moves away from you, a different point on the wall will be struck and the ball will take a different path.

    A wave is much like throwing billions of balls at every possible angle at the same time, so when all of these possible angles and paths add up, the result will be different if the wall is a bit farther away.

    Thus, if you change the path of one ball by moving the wall away, you will also change ALL possible paths... You then see a different pattern.
     
  4. Jul 1, 2010 #3

    jaketodd

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    But wouldn't the waves come out of the slits, spread out and interfere the same regardless of the distance from the source and the slits?
     
  5. Jul 1, 2010 #4
    Does it?At a greater distance the intensity of the light reaching the slits will be reduced and this will reduce the contrast across the pattern,but are you suggesting that the fringe separation will change?
     
  6. Jul 1, 2010 #5

    jaketodd

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    Yes, doesn't the fringe separation change when the distance between the source and slits changes? This happens in my own little double slit setup I have.
     
  7. Jul 1, 2010 #6
    You need to distinguish between near-field and far-field effects

    http://scripts.mit.edu/~raskar/lightfields/index.php?title=Near-field_and_far-field

    In the extreme case, where the detectors are placed right in front of the slits (as close as possible) there is obviously no interference

    edit oops, that's varying distance of detectors from slits not source (In the case of source varying then the scattering angles will be less oblique I guess)
     
    Last edited by a moderator: Apr 25, 2017
  8. Jul 1, 2010 #7

    Cthugha

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    Generally speaking, a larger distance between the light source and the slits increases spatial coherence of the light and the coherence volume becomes larger. Therefore a larger distance will usually improve the visibility of your interference pattern (as long as the total intensity does not decrease drastically). If both slits are well inside the coherence volume, your interference pattern visibility will be good. If they are not, the visibillity will decrease. However, the Fringe separation is a measure of the wavelength of your light. What kind of light source do you use?
     
  9. Jul 1, 2010 #8

    jaketodd

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    A light bulb.
     
  10. Jul 3, 2010 #9

    jaketodd

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    Is that the wrong thing to use?
     
  11. Jul 3, 2010 #10

    Cthugha

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    No, a light bulb producing white light is fine, but leads to a rather complicated interference pattern. The distance between the fringes depends on the wavelength of the light used. If you use white light, you would therefore expect to see overlapping interference patterns of different colors, which makes a clear definition of fringe separation somewhat complicated.

    See for example the following link to see what a white light interference pattern should look like.

    http://www.thespectroscopynet.com/Index.php?Physical_Background:Optics:Interference"
     
    Last edited by a moderator: Apr 25, 2017
  12. Jul 4, 2010 #11

    jaketodd

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    Thanks. I am using white light but my interference pattern is made of light and dark fringes without any colors. Could this be because the pattern is on a white wall? If so, what color wall should I use? Also, when I change the distance from the light source to the slits, the sizes of the fringes change. Any ideas?

    Thanks,

    Jake
     
    Last edited by a moderator: Apr 25, 2017
  13. Jul 4, 2010 #12
    Hello Jake.Each separate wavelength will produce its own interference pattern and all of the separate patterns will overlap.There is a maxima for all wavelengths at the centre of the pattern so the middle of the central fringe will be white.As you proceed from the centre outwards violet will be the colour closest to the central fringe and red will be furthest away.As you proceed further outwards from the centre, the overlapping becomes more complex but there will still be coloured fringing.A white screen would be ideal to show these colours so you should be able to see them on your wall.May I suggest that you set up your experiment again and look more carefully at the fringes.
    In what way does the fringe size change with light separation?
     
    Last edited: Jul 4, 2010
  14. Jul 6, 2010 #13

    jaketodd

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    I've looked pretty closely at the fringes when it's totally dark except for my light source and there are more than two fringes so I know there's interference happening. My setup is a lamp with a piece of cardboard that has two vertical slits cut in it. The cardboard can be made to completely block the light (except for the slits) from the lamp (the lamp is a light bulb inside a half-sphere) and I can see multiple fringes. When I pull the lamp away from the cardboard the fringes uniformly get smaller vertically and horizontally. What is happening?

    Thanks all,

    Jake
     
  15. Jul 6, 2010 #14
    Hello Jake.From what you have written above I suspect that your experiment has not been set up properly and that on your wall you are seeing diffraction pattern images of the two slits.I say this because it is hard to cut two narrow enough and close enough slits in cardboard.The slit separation needs to be of the order of a fraction of a mm.Also, are you using a third slit so as to form a diffracted beam incident on your two slits or are you placing your lamp at a great enough distance from the double slit?
     
    Last edited: Jul 6, 2010
  16. Jul 13, 2010 #15

    jaketodd

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    Thanks, [blush], well I didn't know any better so I've stopped blushing =)
     
  17. Jul 13, 2010 #16
    Nothing to be embarrassed about jake and if you did have it set up wrong then why not try again?A good way to make the slits is to take a thin piece of glass(a microscope slide would be ideal) and paint it black.Next,with a sharp blade scratch a thin slit.Now comes the fiddly bit,move the blade over and scratch a second slit which is very close to and parallel to the first slit.If you do a search you should be able to find detailed setting up instructions.I am now wondering how Thomas Young made his slits back in eighteen hundred and whenever it was.
     
  18. Jul 13, 2010 #17

    jaketodd

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    Thanks man.
     
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