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Homework Help: Coating Eyeglass Lenses(destructive interference)

  1. Apr 23, 2017 #1
    1. The problem statement, all variables and given/known data

    Eyeglass lenses can be coated on the inner surfaces to reduce the reflection of stray light to the eye. If the lenses are medium flint glass of refractive index 1.62 and the coating is fluorite of refractive index 1.432, (a) what minimum thickness of film is needed on the lenses to cancel light of wavelength 550nm reflected towards the eye at normal incidence, and (b) will any other wavelengths of visible light be cancelled or enhanced in the reflected light?
    2. Relevant equations

    2nt = (m+.5)λ
    n = refractive index of film
    t = thickness of film
    m = 1, 2, 3, …
    λ = light wavelength in vacuum (air)

    3. The attempt at a solution
    I have already solved the first part to find that t=96.01nm but i don't know how to solve for part b. I was thinking that you would have to plug the thickness 96.01nm into 2nt = mλ to solve for any possible constructive interference wavelengths but i am not sure.
  2. jcsd
  3. Apr 23, 2017 #2


    User Avatar
    Homework Helper
    Gold Member
    2017 Award

    Yes, you are thinking correctly.
    From part (a), you now know the thickness t. So it is no longer an unknown.
    For part (b), λ is the unknown in the equations 2nt = (m+.5)λ and 2nt = mλ. (You want to investigate both constructive and destructive interference.)
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