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Homework Help: Thin film interference and external / internal reflection

  1. Nov 27, 2016 #1
    1. The problem statement, all variables and given/known data
    I am working on a thin film interference problem where i have to find the phase difference. What is the relationship between external/internal reflection and the thin film interference

    2. Relevant equations
    for external:

    for internal:

    3. The attempt at a solution
    So far i have that the DOPL=2dncosΘt
    d being the thickness of the film
  2. jcsd
  3. Nov 27, 2016 #2
    A picture might be helpful, in particular are we looking at reflection from a single layer or transmission?

    Internal means going from high index to low index: i.e. Leaving the film going into air. External is the opposite. The equations you posted show the magnitude of how much is reflected vs transmitted. The parallel and perpendicular symbols refer to the polarization of the light: parallel or perpendicular to the plane of incidence. None of that has anything to do with finding the relative phase.

    Let's suppose we are talking about reflection from a single layer. Draw a picture with the two possible paths: reflection from the top of the film and reflection from the bottom. We won't worry about multiple reflections for now. in drawing the picture remember that the light refracts going into the film. In setting the end points of the paths to compare, remember you have to get to a point where they are both back in air, parallel and equal distance from the assumed eventual destination. That is to say don't just stop the top path at the point of reflection. It has to be continued forward to a point that makes sense for comparison to the second path. Finally, when determining the relative phase don't forget that the path in the film accumulates more phase per distance travelled by the index of refraction.

    Draw a picture and post it.
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