1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
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: Cosmic microwave background radiation

  1. Dec 23, 2012 #1
    1. The problem statement, all variables and given/known data

    The cosmic microwave background radiation has a blackbody type spectrum. Determine its max frequency and the correspondent wavelenght. Verify if found frequency is a microwave frequency and compare with the following curve:


    2. Relevant equations

    Using the Wien Law displacemente, and knowing that CMBR has a thermal black body spectrum at a temperature of 2.725 K. We can easily calculate the wavelength peak and its frequency.

    λmax = 1,06mm

    frequency = λmax/c = 2,83E11HZ

    We can verify that the frequency that we obtained corresponds to the microwave frequency.

    But when i am comparing with this curve Untitled(2).jpg , i did not find any relation...

    I just do not know what is the relation of the graphic with the obtained results.

    Anybody to help? Help is always appreciated.!!!
    Last edited: Dec 23, 2012
  2. jcsd
  3. Dec 23, 2012 #2


    User Avatar
    2017 Award

    Staff: Mentor

    That is a tricky problem.

    The curve shows the frequency-dependence of the intensity (waves/cm is proportional to frequency) - you can calculate the wavelength which corresponds to its peak, and it will not agree with your wavelength.

    This is not an error on your side, it is a surprising mathematical result: If you switch between wavelength and frequency dependence, you get different positions of the peaks, as "per wavelength" on the y-axis is different from "per frequency".
  4. Dec 26, 2012 #3
    So, the difference is the switching from wavelength to frequency dependence. The results are "mathematically" equal?
  5. Dec 26, 2012 #4


    User Avatar
    2017 Award

    Staff: Mentor

    The intensity maximum of the wavelength is different from intensity maximum of the frequency - even if the curves correspond to the same spectrum.
  6. Dec 26, 2012 #5

    Thanks a lot for the help!
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook