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Something that should be extremely simple - Blackbody Curve

  1. Jan 29, 2014 #1

    I am having an issue with something that should be extremely simple. Essentially, all I am trying to do is plot the blackbody curve at 2000K in terms of the wavelength. The formula I am using can be seen here.. (Don't feel like typing it)


    In the "Planck Radiation Formula" section, second box. Every time I try to plot it, it ends up looking like a normal exponential decay function, except symmetrical about the y axis, NOT the blackbody curve. I got the TA in my grad lab to help and he is also stumped. After calculating the numbers, I got the formula to be..

    [tex]\frac{5 \times 10^{-24}}{\lambda^5}\frac{1}{e^{\frac{0.144}{2000 \lambda}}-1}[/tex]

    Can anyone shed light on this?
  2. jcsd
  3. Jan 29, 2014 #2


    Staff: Mentor

    so you used the c = lambda * v equation right, solved for v = c / lambda and plugged it in to get your equation in lambda

    first how did you get lambda^5 in the first factor it seems it should be lambda^3

    and second did you plot using lambda or v for the x-axis?
  4. Jan 30, 2014 #3
    I used the equation labeled [itex]S_{\lambda}[/itex], it has a [itex]\lambda^5[/itex] term in it. I am using [itex]\lambda[/itex]for the x axis.
  5. Jan 30, 2014 #4
    Okay this seriously is not making any sense. I keep trying to plot this thing, thinking I made some sort of error with units or something and I keep getting the same thing. What is going on?!
  6. Jan 30, 2014 #5
    The equation is correct. You are probably mishandling the software. My guess is that the peak is squished too close to the Y-axis for you to see given the choice of coordinate scale. Play around with the axis scales and see what happens is my advice.
  7. Jan 30, 2014 #6
    I keep scaling the axis and am not seeing anything I should be. Could someone else try plotting it and let me know how it works out?
  8. Jan 30, 2014 #7


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    Science Advisor

    Looks OK when I plot it. See attachment. You must be doing the plotting wrong.

    Attached Files:

    • BB.png
      File size:
      16.2 KB
  9. Feb 4, 2014 #8


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    Science Advisor

    You are probably messing up typing the expression into the computer or calculator. Check your parentheses and your range of values for lambda. If you are using something like Matlab, Mathematica, or IDL, it should be pretty simple. If you still can't figure it out, post the code.
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