Re: heat value in each color in sunlight - science demo doesn't work well
- From: Chris L Peterson <clp@xxxxxxxxxxxxxxxxxx>
- Date: Sun, 11 Dec 2005 16:33:00 GMT
On Sun, 11 Dec 2005 16:02:21 GMT, mack <jmack@xxxxxxxx> wrote:
>For a high school science demonstration, I've made
>up a panel of shallow boxes, each with a thermometer
>and different color internal back (white, black, red,
>yellow, green, blue) and put them in the window
>facing the sun...
>
>I had hoped that the white box would rise little in
>temperature and the black box the most with the other
>colors having temperature increases according to the
>number of photons at each color. While the order of
>the differences are approximately what I expect
>(over about a 10deg temp range), the temperature
>rise in each box is swamped by a 30deg increase for
>all boxes, which doesn't fit in well with what I
>expected (I had hoped the white and yellow boxes
>wouldn't rise much in temperature at all).
>
>The only explanation is that all the colors are dark
>in a photon sense (ie are absorbing most of the photons)
>and only reflecting back a relatively small number of
>photons of the color I see. The green color is relatively
>dark for instance, but the yellow looks (to me) relatively
>light. However maybe I don't know what bright yellow
>looks like.
I think you are seeing a couple of effects. First, by definition a
colored surface will absorb most of the photons striking it, since it
reflects only a relatively narrow band. Second, the pigments involved in
producing the colored papers are probably not very reflective at long
wavelengths, and about half of the photons reaching your boxes are
infrared. That is probably where most of your anomalous heating is
coming from- all of the boxes are pretty good absorbers of IR.
>Does anyone have an idea on how to do this better?
>ie how do I make a (Stephan-Boltzmann) white box?
>It seems like all the boxes I've made are black
>(or nearly so).
It is probably impractical to do this experiment, except qualitatively
with a black and white box. You just don't have enough control over the
spectral characteristics of the absorbing materials. There are IR
absorbing window tints- you could try using some to prefilter the
sunlight.
A better experimental approach is probably to use a prism to disperse
the light, and measure the temperature at different wavelengths. I did
this once, long ago, with three prisms and three medical thermometers.
Whatever you do, don't forget to allow for the fact that the temperature
rise isn't simply proportional to the number of photons absorbed, but
also to their energies, which are higher at shorter wavelengths.
_________________________________________________
Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com
.
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