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Optical Path difference

  1. Oct 18, 2013 #1
    SOme one help me with the OPD.
    How is here the OPD=n2(AB + BC) -n1(AD)?
    I am not confused with the introduction of 'n'..Rather the path length is confusing me..Why is there a negative sign?

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  2. jcsd
  3. Oct 18, 2013 #2

    Simon Bridge

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    They are computing a path difference ...
  4. Oct 19, 2013 #3

    Claude Bile

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    Just to elaborate a little on Simon's response.

    Notice that the line connecting C and D is perpendicular to the rays. This indicates the C and D lie on the same wavefront.

    Ergo, the OPD is calculated as the difference in optical path lengths from A-C and from A-D.

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