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What is the difference between emissivity and reflectivity?

by tom421421
Tags: difference, emissivity, reflectivity
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Sep27-09, 02:05 PM
P: 2
Hi first time posting so apologies if this is the wrong section to post this in!

I'm having a bit of trouble defining the difference between reflection and emission on the atomic level. As far as I can see both just essentially involve excitation of an electron by a photon, followed by de-excitation and re-emission of a photon at a different wavelength. The macroscopic situation I'm trying to apply it to is a painted metal roof in the sunlight. The roof is obviously going to get hot if it has poor emissivity and will be cooler if it has better reflectivity. If the two were the same then a material that was a good emitter would also be a good reflector. This is not the case with a metal, for example, as it is good reflector but gets very hot in the sunlight and must therefore be a poor emitter.

Anyway, I've obviously made a mistake in one of these assumptions so if anyone can let me know where it would be much appreciated!

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Nov26-11, 01:52 PM
P: 1
I'd like to bump this post because I have been struggling with the same concept. If reflection and emission could be considered the same phenomenon, then it would follow that both metal roofs are reflecting the same amount of energy, but the cooler roof is reflecting ( or emitting ) more of that energy in the visible light spectrum.

I am very unknowledgeable, all feedback appreciated

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