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Rate of Heat Absorption from Visible vs Infrared Light

  1. Nov 19, 2006 #1
    I recently did an experiment with the Crookes's Radiometer.

    The radiometer was first placed under a 100W 1600 lm bulb and then a 100 W 2360 lm bulb. Though they both emit the same energy, the radiometer spun about 30% faster under the 2360 lm.

    Why is this? I was thinking maybe this was because visible light is absorbed faster than the infrared, but don't know for sure.

    Thanks,

    sphoenixee
     
  2. jcsd
  3. Nov 20, 2006 #2

    mathman

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    You're right. In general, the amount of energy absorbed by a surface depends on both the intensity and spectrum of the radiation hitting it.
     
  4. Nov 21, 2006 #3
    Just to clarify, it's the higher the spectrum, the higher the absorption rate?
     
  5. Nov 21, 2006 #4

    mathman

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    What do you mean by "higher the spectrum" - more intensity or higher frequency?
     
  6. Nov 21, 2006 #5

    mathman

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    What do you mean by "higher the spectrum" - higher frequency or higher intensity?

    In the case of frequency, there is no simple relationship - it depends specifically on the composition of the material.
     
  7. Nov 22, 2006 #6

    mathman

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    Sorry about all the repeated messages. I was experiencing internet problems so I couldn't tell what went through.
     
  8. Nov 23, 2006 #7
    My bad, but yes, I meant the higher the frequency. So the relationship between absorption rate and frequency is empirical? Or is there just a complicated relationship? Does anyone know of any good articles about this?

    Thanks,

    sphoenixee
     
  9. Nov 24, 2006 #8

    mathman

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    It is not quite empirical. There is a lot of theory there, but it is quite complicated, depending on the composition of the surface as well as the frequency of the radiation. the simplest example would be the fact that a red surface reflects red and absorbs other colors, while a blue surface reflects blue and absorbs others (this is overly simplified). Different parts of the spectrum (ranging from radio waves to gamma rays) alll act differently. Low frequency such as microwaves interact with molecules, while higher frequencies such as gammma rays interact with electrons and nuclei.
     
    Last edited: Nov 24, 2006
  10. Nov 25, 2006 #9
    Thanks for clearing that up mathman!
     
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