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Homework Help: Emissivity as a function of wavelength

  1. Feb 12, 2010 #1
    1. The problem statement, all variables and given/known data

    Hello, I'm a little confused. I did an experiment today with a Leslie Cube. One of the faces was a black paint, and another a white paint. I know that white reflects well and has a low emissivity, at least in the visual wavelengths. In the Leslie's cube, was hot water, so I was measuring the Irradiance of thermal radiation. From the results I concluded that the white and black faces had similar emissivities (in fact the white face was slightly higher). I understand that emissivity depends on several factors, temperature, angle and also wavelength. How come in the infrared spectrum, both faces have similar emissivities? I have a feeling the answer lies in a blackbody diagram. Can someone point me in the right direction?

    Thanks in advance :)
     
  2. jcsd
  3. Feb 12, 2010 #2
    I think I just worked it out. Colour has nothing to do with a materials ability to absorb and hence emit thermal radiation. Colour is important in reflecting and absorbing wavelengths in the visible spectrum only. A white material will reflect all visible wavelengths, but a black surface would absorb all, and re-emit as thermal radiation.

    I think oberlooked the way that visible light is then turned into thermal radiation. Thank you!
     
  4. Feb 12, 2010 #3

    turin

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    You may be correct. I thought of another problem. Maybe you are measuring reflected radiation that the cube reflects from external sources? How have you isolated this?
     
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