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Two waves of light in air travel through a layer

  1. Dec 18, 2016 #1
    1. The problem statement, all variables and given/known data

    Two waves of light in air, of wavelength 550 nm, are initially in phase. They then both travel through a layer of plastic as shown in the figure, with L1 = 4.00 μm, n1 = 1.40, L2 = 3.50 μm , and n2 = 1.60. What is their phase difference after they both have emerged from the layers, when they arrive at the same point?
    halliday_9e_fig_35_35-177-1408377159428.jpg


    2. Relevant equations
    N is the numbers of lemdas
    N1=L1*n1/λ
    N2=L2*n2/λ
    N in air = (L1-L2)/λ n(air)=1

    3. The attempt at a solution
    N2+Nair-N2=0.909 but the correct answer is 5.71?????
     
  2. jcsd
  3. Dec 18, 2016 #2

    TSny

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    "Phase difference" is an angle (in radians), not the number of λ's.
     
  4. Dec 18, 2016 #3
    What is the name of the difference in wavelengths?
     
  5. Dec 18, 2016 #4

    TSny

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    You could also call the difference in wavelengths the "phase difference". So, the answer of .909 wavelengths is a correct answer. However, does the answer of 5.71 include any units?
     
  6. Dec 18, 2016 #5
    no only number
     
  7. Dec 18, 2016 #6

    TSny

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    "Bad form" on the part of the answer sheet.
     
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