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Homework Help: Black Body Radiation (Awkward integral)

  1. Jun 2, 2010 #1
    1. The problem statement, all variables and given/known data
    What percentage of the Sun’s blackbody radiation spectrum falls into the visible light spectrum (400-700 nm). Where T=5000K
    Hint: Integrate over frequencies

    2. Relevant equations
    B=2h[tex]\nu[/tex]3c-2 (eh[tex]\nu[/tex]/kT-1)-1

    Where [tex]\nu[/tex] is the frequency of the light.
    3. The attempt at a solution

    Ok so the problem is very straightforward, i'm just having trouble evaluating the integral. I need to integrate the equation for the brightness (B) over the frequency [tex]\nu[/tex] where the limits are given by the span of wavelengths in the visible part of the spectrum. The integral then just looks something like this:
    [tex]\int B[/tex]=[tex]\int \nu[/tex]3(e[tex]\nu[/tex]-1)-1 , where i excluded the constants.

    I tried integration by parts but it didn't work. I also plugged it into mathematica and got a very weird, long answer that didn't make sense. When I asked my TA about it he told me I had to solve it numerically. What does it mean to solve an integral numerically? Am I just supposed to plug in the lower limit and then subtract that from what I get when I plug in the upper limit? I am thinking of the fundamental theorem of calculus here.

  2. jcsd
  3. Jun 2, 2010 #2


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    Homework Helper

    This integral doesn't have a primitive in terms of elementary functions. To evaluate this integral you can use the Raleigh-Jeans approximation by looking it up in your book or by using the Taylor series of the exponent up to first order.
  4. Jun 2, 2010 #3


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    Science Advisor

    When you integrate a function numerically, you basically divide the area up into a bunch of narrow rectangles or trapezoids and add them all up. There are a lot of ways to do this, try reading this:


    To accomplish this, you could write a program to do it, or use a canned program. Mathematica has a function NIntegrate which will do numerical integration and, given the function and the endpoints, will just return a number.
  5. Jun 3, 2010 #4
    Thanks guys
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