Re: heat value in each color in sunlight - science demo doesn't work well

mack wrote:
Chris L Peterson wrote: 
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.


someone wrote to me off-list saying the same thing - that all my
boxes were black at IR.

Fundamentally that is your main problem. And the other is that very few coloured pigments are even remotely pure wavelength selectors.

This you can demonstrate easily in class using shovelware CDs as cheap high resoluion handheld spectroscopes.

And your coloured paint that is green is absorbing all the reds and blues. You really want coloured filters in front of identical boxes.
(or some othe means of splitting light by wavelength)

Few common materials that are hugely bad IR emitters. Bare metallic surfaces are the most common - aluminium foil and space blanket are amongst the most readily available.

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.

You might be able to do it more effectively with a prism or grating and a soot blackened thermocouple to explore the wavelength spectrum.

There are IR
absorbing window tints- you could try using some to prefilter the
sunlight.

a solution of Cu2+ ions is supposed to do it (from google). Other filters are expensive. I was hoping for a cheap and simple demonstration
of a simple concept :-)

The cheapest way is ring up a theatrical lighting supplier and explain you are a science teacher wanting to buy some gel filters. If you ask nicely they will send you a Lee Filters swatch of all the gel filters. Probably at 8x3cm maybe even big enough for a school experiment.

Of the ones I have to hand 027 "medium red", 781 "terry red", 021 "gold amber", 101 "yellow" are all pretty good low pass filters.

Pure greens are thin on the ground 735 "velvet green" isn't bad. IR leak at >750nm. Same with 115 "Pea*** blue, 195 "zenith blue". These would all need combining with a high pass IR blocking filter.

Note that the in band transmission of these filters is nothing like 100%

Regards,
Martin Brown
.

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