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Index of refraction, calculate frequency. ~ stuck at 2 unknown variables

  1. Mar 3, 2008 #1
    1. The problem statement, all variables and given/known data

    An laser is used of wavelength 544 x 10^-7 m (in air) for eye surgery. The index of refraction of the fluid in an eye is n = 1.43. Calculate the frequency of the laser waves in the eye.

    n = 1.43
    L1=544 x 10^-7
    f =

    2. Relevant equations


    L1 being the wavelength in "air"
    L2 being the wavelength in the eye



    3. The attempt at a solution

    so, given n and L1 its simple to find L2, so I now know the wavelength in the eye. Yet I can't seem to derive the frequency.



    1.43= (5.44 x 10^-7) / (v/f)

    the frequency remains constant for both wavelengths.

    so simply I would have to find the velocity of L1 or L2 that would be the frequency "within the eye"


    the velocity is not the same so I now have 2 unknown variables.

    I KNOW this isn't hard, my minds just shot right now
    Last edited: Mar 3, 2008
  2. jcsd
  3. Mar 3, 2008 #2


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    You already said the correct answer. The frequency is the same. The wavelengths and velocities are different, but they both change by the same factor.
  4. Mar 3, 2008 #3
    Thanks Dick,

    I had though that, but then that same factor that they change by is simply equal to n. Using n for the frequency wouldn't create the correct answer. I'm just not thinking right.

    If you or anyone else could elaborate it would be greatly appreciated.

    I know this isn't complicated and I'm embarrassed to continue asking when I'm sure this is so simple..
    Last edited: Mar 3, 2008
  5. Mar 3, 2008 #4


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    Air has basically n=1. f=v/lambda. In a different medium v->v/n, lambda->lambda/n. The frequency is unchanged. This is also sort of obvious. A thing that measures frequency is counting waves as they pass. If it's located outside of the eye it counts f waves per second. If it's inside the eye it must also count f waves per second. Otherwise, some 'waves' are getting lost somewhere. Where could they go??
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