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Power and rate of photon emission with a given temperature

  1. Oct 6, 2015 #1
    Assuming that your surface temperature is 99.1 F and that you are an ideal blackbody radiator (you are close), find (a) the wavelength at which your spectral radiancy is maximum,(b) the power at which you emit thermal radiation in a wavelength range of 1.0 nm at that wavelength, from a surface area of 4.10 cm2, and (c) the corresponding rate at which you emit photons from that area. Using a wavelength of 500 nm (in the visible range), (d) recalculate the power and (e) the rate of photon emission. (As you have noticed, you do not visibly glow in the dark.)

    i got a) by using wien's law, for the rest i don't know where to start
  2. jcsd
  3. Oct 6, 2015 #2

    Simon Bridge

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    You can start fro the same source where you got Wein's displacement law.
  4. Oct 7, 2015 #3


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    Dear Wang,
    We forgot to welcome you to PF :smile: ! Sorry about that. Welcome !

    Your two threads received similar responses and that started me wondering how we can help you effectively. Perhaps there is a culture issue on hand: here at PF we do all we can to help everyone in their efforts to learn while doing exercises, pondering questions and what not. But the principle is that the poster makes an effort (And I don't mean the effort of erasing the template -- if that's what happened in this thread).

    That's why threads of the type "I don't know where to start" without further information are in fact very frustrating for potential helpers. It doesn't really help you if we show you how to do the exercise. So what we really need to help you is an idea of what you can do, how far you do get, what is really preventing you from taking the next sensible step, etcetera.

    In short: we really want to help. So help us help you by giving us a clue what you need.
    Browse a few related or unrelated threads and check the guidelines to get an idea of how things work in PF.
  5. Nov 14, 2017 #4
    I am having issues will sections d) and e) of the problem. For d) we are given a new wavelength of 500nm. Should this wavelength be used to calculate a new spectral radiancy, and if so, what would the unit wavelength be?
  6. Nov 14, 2017 #5

    Simon Bridge

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    The clue is "you have noticed that you do not visibly glow in the dark."
    Initially (in a,b,c) you were to calculate how much "you" emit around the maximum spectral radiancy. That is why the temperature is "body temperature".
    Next you are asked to calculate how much you emit in the visible spectrum (ie, around 500nm).

    When you are working through problems like this you need to consider the broader context.
    Why are you asked to calculate specific figures: what do they tell you about life etc?
    Oftentimes a problem will be purely abstract, and the numbers are picked at random as an exercise.
    As you progress, the exercizes will more and more be relevant to observations you can make or have already made.
    It's the difference between practising climbing on a climbing wall in a gym vs practise by climbing progressively more difficult slopes and cliffs in real life.
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