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Homework Help: Light and Index of Refraction.

  1. Jul 27, 2009 #1
    1. The problem statement, all variables and given/known data

    A helium-neon laser beam has a wavelength in air of 633 nm. It takes 1.38 ns for the light to travel through 30 cm of an unknown liquid. What is the wavelength of the laser beam in the liquid?

    2. Relevant equations


    3. The attempt at a solution

    So, I'm having difficulty understanding how to calculate what the wavelength of the laser beam in the liquid is. In my attempt I tried using the index of refraction equation to determine the speed of light in the material (unknown liquid). Since, the question provided a time and distance I thought using the distance formula to find the speed would help too.

    First, I converted the 30 cm to m, which gave me 0.3m.
    Second, I plugged the given values into the distance formula. 0.3m= r(1.38ns) => 0.21739..
    Third, I plugged the values into the index of refraction equation... n=3.0x10^8/0.21739
    Now, I'm stuck. I'm not sure how to figure out the second wavelength. Do I incorporate the 633nm somehow?

    (Any help would be immensely appreciated.)
  2. jcsd
  3. Jul 27, 2009 #2


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    If you're going to rely on units, then you need to use units everywhere, not just somewhere. What is 0.21739? What are the units?

    Otherwise, you're on the right track. You need to answer this question: how much does the frequency of the light in the liquid differ from the freqency of the light in air. Then, use the wave relationship: fλ=v.
  4. Jul 27, 2009 #3
    Wait a minute you're right, I made a unit error.

    If I convert the ns to s, and then proceed with the process it should make a difference in my answer.

    So, if 0.3m/1.38s = 0.21739m/s <= that would be my speed of light in the material.
    Then, I could figure out the frequency using the wavelength relationship. f=633 x 10^-9/0.21739m/s
    Lastly, I could figure out the wavelength.
    I think I may get this afterall.

    (Thank you for your help!!)
    Last edited: Jul 27, 2009
  5. Jul 28, 2009 #4


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    You still have a unit error. Check again how much time the light takes to travel through the liquid.
  6. Jul 28, 2009 #5


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    Good idea. Try doing that conversion again.
  7. Jul 28, 2009 #6


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    Remember that the frequency of light doesn't change when the light enters a different medium. Since v=fλ and v=c/n, what must λ be?
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