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Homework Help: What is the wavelength of the helium-neon laser beam in the liquid?

  1. Feb 3, 2010 #1
    1. The problem statement, all variables and given/known data

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

    2. Relevant equations
    v= d/t

    3. The attempt at a solution
    First i found the frequency (since its the same in each medium) by v=wavelength*f
    I used the speed of light as v and the 633nm so 3.0e10^8=wavelength(633).. i get 819672.1311
    I have the distance and the time of the unknown liquid.. so i figure i can find v of the material by v=d/t
    I convert 30cm to nm (cause it wants the answer in nm) so its 300000000nm/1.38ns=217390000
    v=wavelength*frequency using my new v and the constant f --> 217390000/819672.1311=265nm
    The answer was wrong. I'm wondering if I made a mistake with converting or in my calculations.
  2. jcsd
  3. Feb 3, 2010 #2


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    Staff Emeritus
    Science Advisor
    Homework Helper

    Welcome to Physics Forums.
    There are a couple of problems in this last step.
    First, it should be
    3.0e8 m/s = 633nm x frequency​
    Also the frequency you got is wrong (wrong number, plus the units are missing)

    Looks like you have the right idea, just need to get that frequency calculation to start things off right. It's a little difficult to follow when you don't include units with the values.
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