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Young's double slit expt - purpose of the first slit

  1. Sep 12, 2010 #1
    Hi there,

    When performing the Young's double slit expt, we always pass the light source through a single slit first before the actual double slits. The typical textbook answer for the use of the first slit is to "produce 2 coherent light sources at the 2 slits", or "to ensure a constant phase difference between waves from the 2 double slits".

    If the light we are using is white light, i can understand the use of the first slit. However, I do not understand its significance if we are using single color lasers. For example, if we are using laser light which is monochromatic, arent all the waveforms already in phase? In other words, straight from the laser source to the double slits, the waveforms will still be in phase.

    Can fellow forummers shed some light pls?
  2. jcsd
  3. Sep 12, 2010 #2


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    The purpose of the single slit is to ensure that the light that reaches the double slit is effectively from a point source. This first slit would be unnecessary if the laser emitting device is itself a point source.

    Ironically, the business part inside a laser emitting device is in fact a point source. The laser light exits the cavity via a tiny aperture. Because the aperture is so small the light diverges. With a lense this divergent light is refracted to a parallel beam.

    What is exiting the laser emitting device is a parallel beam with significant spatial width. If the laser emitting device is close to the double slits then the width of the parallel beam prevents visible interference effects. One solution would be to move the laser emitting device back. The bigger the distance, the more it acts as a point source with respect to the double slit.

    A more compact solution is to use a single slit first to ensure that the light that reaches the double slit is effectively from a point source.
  4. Sep 12, 2010 #3


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    Let me elaborate some on my first answer.

    Interference effects to not require the light source to be 'coherent', or 'in phase'. Example: the colors of oil films on water that have a thickness in the order of the wavelength of visible light.

    The only factor that determines whether a Young double slit setup will produce visible interference fringes is whether you achieve a point source. Let's say your source is a Sodium lamp. Pretty close to monocromatic, but since the light comes from randomly emitting atoms it's not coherent. Remarkably, this makes no difference, you get interference effects

    In the case of laser light it doesn't matter either whether it's coherent or not. With laser light all of the luminosity is in a single quantum state. For laser light qualifications such as 'coherent' or 'in phase' are really not applicable. Two or more waves can be seen to be in phase when compared to each other, but laser light isn't a collection of a lot of waves; the luminosity is in a single quantum state.

    In physics light that is emitted from a point source is referred to as 'spatially coherent'. Similarly, if the light shines through a sufficiently narrow aperture then it acts as a point source, and the light is referred to as 'spatially coherent'.

    But this 'spatially coherent', doesn't say anything about being in phase or not. (It doesn't have to; being in phase or not makes no difference for interference effects.)
  5. Sep 12, 2010 #4

    Andy Resnick

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    If I understand you correctly, the source is first passed through a slit (or a pinhole) in order to create a source with a high spatial coherence.

    Spatial coherence is different than temporal coherence- temporal coherence (the so-called coherence length) relates to the polychromaticity of the source. Spatial coherence (transverse coherence) relates to the size of the source.

    Young's double slit experiment measures spatial coherence, while a mach-zender interferometer measures temporal coherence.
  6. Sep 12, 2010 #5

    Andy Resnick

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    I removed the parts of your post that are true- these parts are not. A 'raw' laser beam has fairly low spatial coherence- this is seen in the 'speckle' exhibited by a raw beam. For critical instrumentation, laser light is first passed through a spatial filter (which can be a single-mode fiber) in order to 'clean up' the beam.

    Interference *always* requires coherent signals. Otherwise, there is no interference.

    Laser light is not in a single quantum state- unless you are working with a very special types of lasers. 'Single mode' lasers are monochromatic, however, they too generally have multiple transverse modes.
  7. Sep 12, 2010 #6


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    I suppose I need to use the expressions 'coherent' and 'in phase' more precisely.

    The core is that the light source doesn't have to be in phase, interference effects can be obtained anyway. Being emitted from a point source is key.

    I won't make statements anymore about 'laser light as a single quantum state'. You have pointed out that generally that is not the case, and it was an unnecessary digression.
  8. Sep 12, 2010 #7


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    In the interest of offering further corrections, this is not usually true. Most lasers (and all of the ones I have ever worked with) use a partially reflecting, partially transmitting mirror instead of an aperture.

    I'll defer to Andy's expertise on this, and just add that there are lasers with sufficient spatial coherence that the first slit is not needed.
  9. Feb 20, 2011 #8
    Assume all vertical slits. The reason for the first slit is the same for non point-source laser light as it is for white light, namely to ensure each second slit receives photons with unique direction (ignore vertical component). This produces interference fringes. Photons with other directions produce offset fringes, masking the effect, so must be excluded.

    Point source laser photons will go through each second slit with unique direction (ignore vertical component) so no first slit is needed.

    Whether the photons are in phase with each other or not makes no difference. Photons do not arrive at the double slit simultaneously so they don't likely interfere with each other anyway. Also, experiments show that a sequence of single photons produces the same interference fringes as a constant light source. This strongly suggests that constant light source fringes occur only by single photon effect.

    The single photon may go through both second slits and use a probability process to get its path angle to the screen. The process could occur at the slits and involve various factors like photon entry points and direction, wavelength, and slit layout but photon phase should not be a factor. Successive screen strikes add up to the interference fringes.

    Since the fringes are made by single independent photons, and do not involve the phase of the photon, then phase differences between photons can have no effect on the fringes either.

    Coherent light can be defined as light which is in phase and of the same wavelength. Other places say coherent light is required for interference fringes so you might think the light has to be in phase. However they should really say "spatially coherent" which means point source. The word coherent is somewhat ambiguous.
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