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Monochromatic light vs Laser - Young's double slit

  1. Mar 13, 2014 #1

    I have a quick question about the distinguishing features between a 'monochromatic light source' and a laser.

    In my notes on wave optics, I have a diagram of monochromatic light being diffracted through a single slit, to produce a coherent wave formation that then hits a double slit. Since the waves 'created' by the double slit originated from the same wave, we can say they are in phase with one another.

    In my lab script, it appears that the single slit is unnecessary when using a laser. It says that the nature of the light produced by the laser means that the two beams leaving both slits will be in phase.

    What is the difference between these light sources that make the experimental set up different?

  2. jcsd
  3. Mar 13, 2014 #2


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    The difference is spatial coherence (which is also what a double slit measures). The more point-like a light source is, the higher its spatial coherence will be.

    Imagine comparing a nice laser and floodlight sent through a narrow spectral filter to make it more or less monochromatic. The floodlight has a huge surface area and light is coming from all of these points. This also means that the path difference between all of these points and the double slit differs strongly, which gives you a large spread in the relative phase of the light fields at the two slits of the double slit. In that case you will not be able to see a good interference pattern. The solution in that case is a narrow pinhole or slit that gives you an effective point-like source.
  4. Mar 13, 2014 #3

    Thanks for your answer

    With regards to the monochromatic light source, I have written "The single slit produces a coherent wave, whose pattern does not shift in time or space, relative to a regular periodic motion" Is this a description of 'high spatial coherence'? (i'm not familiar with that term).
  5. Mar 14, 2014 #4


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    The "in space" is the spatial coherence part. The "in time" would give you temporal coherence. A slit gives you spatial coherence.

    In a nutshell you can interpret spatial coherence as a measure of "If I know the phase of my light field at this point, how well can I predict the phase of the field at some other point at a distance".

    Temporal coherence then does the same for two times.
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