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Homework Help: Max Distance from which a python detects infrared radiation

  1. Feb 13, 2015 #1
    1. The problem statement, all variables and given/known data

    A python can detect thermal radiation with intensity greater than .60 W/m2. A typical human body has a surface area of 1.8 m2, a surface temperature of 30°C, and an emissivity e=0.97 at infrared wavelengths. What is the maximum distance from which a python can detect your presence? You can model the human body as a point source of radiation.

    I = .60 W/m2
    e = 0.97
    T = 303 K
    A = 1.8 m2
    d = ? (m)

    2. Relevant equations

    λ = (2.9*106 nm⋅K)/ Temp (in K)
    I = P/(4πr2)
    Q/Δt = eσAT4
    σ = 5.67 * 10 -8 W/(m2⋅K4)

    3. The attempt at a solution

    λ = (2.9*106 nm⋅K)/ 303 K = 9571 nm

    And if P = Watts, A = m2, and I = W/m2
    Then, P = I*A → .60*1.8 = 1.08 W

    .60 W/m2 = (0.97 * 1.08 W)/(4πr2)
    r = .37 m

    Honestly, I'm not sure what else to do. I don't think "P" is correct.
    Last edited: Feb 13, 2015
  2. jcsd
  3. Feb 13, 2015 #2


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    Hello R, welcome to PF :)

    I agree P probably isn't correct. Is it clear what the relevant equations you are listing stand for ?
    I like the third one: Q/time (aka P !), according to the Stefan-Boltzmann law
    You have all the ingredients to evaluate the human body emission ##P_{emit}##. It's rather a lot.
    Your first equation tells you the peak wavelength of the intensity distribution spectrum is at over 9000 nm, so most of this P is in the infrared.
    I suppose (but don't know for sure -- perhaps someone else can confirm or correct) that fraction is precisely the emissivity factor.

    And that is the P you want to distribute over a sphere using your second equation. By the time r is so big that I < 0.6 W/m2 you've found the range.
  4. Feb 13, 2015 #3
    Oh! So, using P = eσAT4, I found P = 834 W.
    Then, I = P/(4πr2), I found that r = 10.5 m!

    That's seems realistic! Thank you for all your help! :D
  5. Feb 14, 2015 #4


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    I hope it's the right answer...
    Tried to check by working it out for a mouse (after all, a much more likely item on the snake menu) of, say, 25 gram, so 1/3500 times the weight and some (1/3500)2/3 the area. About 66 cm, so life isn't all that easy for a snake...

    I also think I remember hearing that snakes only 'see' movement.

    Oh, well, with physics you can't know everything (although some physicists seem to think otherwise ;) )
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