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Clarification on what the questions asking

  1. Apr 4, 2005 #1
    A glass plate (n=1.61) is covered with a thin uniform layer of oil (n=1.2). A light beam in air of variable wavelength is normally incident on the oil surface. Observation of the reflected beam shows destructive interference at 500 nm and constructive interference at 750 nm. Calculate the thickness of the oil film from this information.

    Is the question saying that when the wavelength is 500 its destructive and when wavelength equals 750 its constructive or after 500 nm it interferes destructively and at 750 nm it interferes constructively
  2. jcsd
  3. Apr 5, 2005 #2


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    I would interpret the question the way you did the first time around i.e. since the question says the incident beam's wavelength can vary (this is the key bit of info), I would intepret it to mean the following:

    1. When the incident beam has a wavelength of 500 nm, the reflected beam interferes destructively with it.

    2. When the incident beam has a wavelength of 750 nm, the reflected beam interferes constructively with it.

    To be honest, I find the wording of your second interpretation to be even more confusing than the original question :tongue2: , and I am really not sure what you are driving at because I can't understand what is meant by "after 500 nm" and "after 750 nm."

    I'm pretty confident what I wrote above (1 and 2) is the right interpretation.
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