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Double Slit Experiment inquiry

  1. Dec 30, 2009 #1
    Does the distance from the photon/electron gun to the slit matter at all? Or even the distance from the slit to the photographic plate?
  2. jcsd
  3. Dec 30, 2009 #2
    Well the distance of the source from the slilts will affect the intensity of the light when it comes out of the two slits, so I guess it does matter. As for the distance from the slit to the screen, again intensity will be affected. If the screen is too far away, the image will appear rather faint.
  4. Dec 30, 2009 #3
    Say if we had the gun at a distance A from the slit, and the slit a distance B from the plate and we ran the experiment to obtain our first pattern. We then proceed to run the experiment again with both A and B increased by a particular factor.
    I guess my real question then, is whether or not the interference pattern created by the light is affected or shifted at all, other than the intensity, on the photographic plate when a change in distance is brought into play? Is the dark band between the two light bands increased when the distance is increased? Is the amount of light bands and dark bands increased or decreased? Or does nothing happen other than the intensity change as arunma already stated?
  5. Jan 2, 2010 #4
    Increasing B should affect the pattern by spreading the interference pattern out, ie increasing the distance between the fringes.

    Increasing or decreasing A shouldn't affect the pattern, unless the gun is brought so close to the slits that the waves hitting the slits can no longer be approximated as plane waves. That would only happen when A is of the same order as the distance between the slits, or several micrometers.
  6. Jan 2, 2010 #5
    The distance the laser/E-gun is from the slit makes no difference, provided at least one photon (or one electron) makes it to it's detector screen.
    Keep in mind that the diffraction pattern is observed when sending thru one particle at a time.

    One problem I have with quantum theory is the idea of superposition, which I think is absurd to apply to the double slit. I mean, when we send huge chunks of matter through the double slit (like Buckyballs), we still observe diffraction. Does certain of the contemporary Physicists actually expect us to believe that the massive chunk of matter is superpositioniong over itself as it goes through the slits?
  7. Jan 2, 2010 #6


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    The double-slit experiment is basically a measurement of the spatial coherence of the light field at the two slits. If you have spatially coherent light, you will see an interference pattern with full visibility. If you use incoherent light, you will not see an interference pattern. You can increase the spatial coherence of the light field by decreasing the spread in the emission angle you have at each of the slits. As you naturally decrease the spread of the emission angle at each of the slits if you increase the distance between light source and the slits, you can get better visibility (better contrast) of the interference pattern for long distances and the pattern will gradually disappear if you put the light source closer to the slits. This effect is not very important if you use lasers - laser light usually has high spatial coherence - but it is important for partially coherent light or incoherent light. It is for example possible to determine the angular extent of a star by examining the starlight in a double slit experiment.
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