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Homework Help: Thinnest film that will give constructive interference for red light

  1. Mar 6, 2013 #1
    1. The problem statement, all variables and given/known data

    When a thin soap film is very thin, we see it as black. How thick (in nm) is the film
    in the region where we see the first red band? Take the wavelength of red light to be 752
    nm and the index of refraction of the soap to be n = 1.33.

    2. Relevant equations

    [2nt/λ] - 1/2 = m

    3. The attempt at a solution

    Solving for t:
    First with m = 1 I moved the 1/2 over to get 3/2, Then I multiplied 3/2 by the wavelength 752e-9 and divided this result by 2n, n being 1.33.
    The answer I attained after solving for t was: 424 nm.
    The correct answer is 141 nm.

    I still don't know what I'm doing wrong here. I have noticed that if I divide my answer by 3 I get the correct answer, though this may just be coincidence.
    Also if anyone is using Walker edition 4, on page 988 the practice problem seems to be this exact same scenario and I am having the exact same problem.
  2. jcsd
  3. Mar 6, 2013 #2


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  4. Mar 6, 2013 #3
    So then the desired equation is 2ndcosθ=(m-.5)λ?
    How would I figure out the angle? I thought it was the right equation.

    Here is my instructors answer:
    If the thickness of the film is t, the difference in phase between a ray reflecting off the
    top surface of the film and one reflecting off the bottom surface of the film is (2πn/λ)2t + π
    the factor π coming from the phase change on reflection of the ray in air reflecting off
    the film. Set this phase change to 2π for the first constructive interference to get that
    t = λ/4n = 752/(4 × 1.33) = 141nm.

    I don't really understand how he got his answer either even though it is spelled out quite succinctly.
  5. Mar 6, 2013 #4


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    It's the angle of incidence, so for the OP it's 0.
    That's the same equation as I referenced. For the mth constructive interference phase, (2πn/λ)2t + π = 2πm. Rearranging: (2nt/λ) + 1/2 = m. You just had the sign wrong on the 1/2.
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