Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: A question relating to frequency i think a 10 year could do this but i cant lol

  1. Jan 5, 2010 #1
    1. The problem statement, all variables and given/known data

    I want to convert wavenumbers into wavelength. The wavenumber is 400cm-1 and i used the expression v=vbar*c to get the frequency in Hz to be 1.2*10^13, I also remembered to convert c into cm/s so that frequency is in its SI units (seconds). I then simply thought that I could use the expression lambda=c/v to get the answer i require and then multiply that answer by 1*10^9 to get my answer in nm instead of ms. I know the answer should be
    2500nm but i get 25000nm. Why do i have this factor of 10? where did it come from?

    2. Relevant equations

    i began with vbar=400 cm-1

    so then i said that

    v=vbar*c

    v=400*3*10^10

    v=1.2*10^13 Hz

    I then said that

    lambda nm=(c/v)*1*10^9

    lambda=(3*10^8/1.2*10^13)1*10^9

    lambda = 25000 nm when it should be 2500nm or 2.5um

    3. The attempt at a solution

    Please see above

    I know this forum is used mainly by graduates who set eachother calculus based challenges but any help for a simple folk like me would be much appreciated.
     
  2. jcsd
  3. Jan 5, 2010 #2
    You would need to convert to m/s for SI units; not cm/s.
     
  4. Jan 5, 2010 #3
    i thought that because vbar is equal to 400cm-1 that I would need to convert the speed of light to 3*10^10 so that its units become cm/s so that the cm cancel and leave the SI unit for frequency which is seconds? Is that where i went wrong?
     
  5. Jan 5, 2010 #4
    i got it now like a fool the nm is not 1*10^-9 but 1*10^-8, please post tho if u think im still wrong
     
  6. Jan 5, 2010 #5
    1 nm = 1*10^-9 m

    I get 25000nm as well (assuming wavenumber means 1/wavelength)
     
  7. Jan 5, 2010 #6
    I only read as far as the "convert to cm" and assumed that was the problem.
    I don't understand why you are using the speed of light and the wave equation to do this.
    If the wavenumber in 400 cm-1, that is all the information you need.
    This equals 40000m-1
    1/400000 = wavelength in meters
    =0.000025m
    =25000nm
     
  8. Jan 5, 2010 #7
    http://imgwe.com/accessbee/27/1.png [Broken]I'm[/URL] 30 and I still don't know this. It depends where your learn. You could be learning it from asia or the Caribbean. imo.
     
    Last edited by a moderator: May 4, 2017
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook