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Homework Help: Thin Film Reflection

  1. Jan 14, 2007 #1
    1. The problem statement, all variables and given/known data
    A thin 1.00x10^-5 cm-thick film of MgF2 (n=1.38) is used to coat a camera lens. Are there any wavelengths in visible spectrum intensified in reflected light?

    2. Relevant equations
    thickness = wavelength/4n (n is refraction index)

    3. The attempt at a solution
    I am guessing that the incoming light in the material reflects inside the material to cause constructive interference at certain wavelengths (which will be multiples of the thickness of the MgF2).
    My main question is, how do I relate possible wavelengths to thickness of the material?
  2. jcsd
  3. Jan 14, 2007 #2

    Andrew Mason

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    Homework Helper

    Reflection cannot add to the incident light intensity. What happens is that some frequencies are not reflected as intensely due to destructive interference within the wave coating so the frequencies which experience constructive interference appear to be more intense.

    So the question is: at what frequency does the light reflected from the first surface (Air/MgF2) constructively interfere with light reflected from the second surface (MgF2/Glass).

    The reflections at both surfaces undergo a phase change of 180 so these phase shifts cancel out. In order to have constructive interference, what must the path from surface 1 to 2 and back to 1 represent in terms of wavelength of the light in the MgF2 medium?

    Work out the frequency of the light from the wavelength.

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