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Thin film interference reflection

  1. Dec 3, 2014 #1
    1. The problem statement, all variables and given/known data
    A coating of n1 = 1.27 material is to be added to n2 = 1.50 glass, in order to make it "irridescent."
    a) What is the thinnest layer of material which will have a reflection maximum for normal-incidence light at BOTH 400 and 600 nm (vacuum) wavelength?
    b) This thickness will have a reflection minimum at some wavelength between 400 and 600 nm. What is the wavelength of the reflection minimum?

    2. Relevant equations

    2*d=lambda(coating), 2*d=lambda(coating)/2

    3. The attempt at a solution

    I got the first part correct and the answer was 472 nm but I can't get b right. For b I did 2*d=lambda(coating)/2 since it says that the reflection will be minimum.
     
  2. jcsd
  3. Dec 3, 2014 #2

    ehild

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    2. Relevant equations

    2*d=lambda(coating), 2*d=lambda(coating)/2

    3. The attempt at a solution

    I got the first part correct and the answer was 472 nm but I can't get b right. For b I did 2*d=lambda(coating)/2 since it says that the reflection will be minimum.[/QUOTE]

    In order to get minimum reflectance, the path difference between the directly reflected wave and that, which reflects from the glass-layer interface has to be odd number times half of lambda(coating) .
     
  4. Dec 3, 2014 #3

    BvU

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    Destructive interference occurs at ##2d =\lambda/2##, but also at other path differences....
     
  5. Dec 3, 2014 #4
    In your problem light moves from low u to high u.reflection minimum means the part of incident rays reflected will suffer destructive interference .hence 2ud=n(lambda).
     
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