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!

Need help with blackbody radiation

  1. Jan 24, 2005 #1
    Can someone help me with this please?

    B(f) = [(2hf^3)/c^2]{1/[exp (hf/kT) - 1]} - Planck law in terms of frequency.

    "Derive expressions for B(f) for the cases i. hf << kT, ii. hf >> kT".

    I've done the first bit, that's just using a Taylor expansion. The second bit is where I'm stuck. In my lecture notes, I have that for hf >> kT, B(f) = [(2hf^3)/c^2]exp(-hf/kT). What I don't understand is why it's like that. If hf >> kT, shouldn't that exponential tend to infinity?

  2. jcsd
  3. Jan 24, 2005 #2


    User Avatar
    Science Advisor
    Homework Helper

    Yes,the exponential should much larger than 1 and therefore the 1 in the denominator would be neglected...The exp.would come on top with a "-" in its argument and from the Planck distribution,u'd be getting the Wien distribution...

  4. Jan 24, 2005 #3
    OH! Thanks :smile:.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Similar Discussions: Need help with blackbody radiation
  1. Blackbody radiator (Replies: 3)

  2. Blackbody radiation (Replies: 3)

  3. Blackbody Radiation (Replies: 0)