Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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


    User Avatar
    Homework Helper

    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?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook