# Dark body vs light body and heat emission

1. Nov 13, 2014

### lomez

So, I've had several people insist that a dark colored object will radiate heat faster than an identical light colored body. I do not believe this is the case. We know that dark bodies ABSORB more short wave than light colors, and thus become hotter as a result.....but that's not the issue. Let's say I take two identical in every way objects, except one is black and one is white. I heat them both to exactly 200 c. and place them both in different rooms which are 0 degrees c. Which will cool down (radiate IR) faster? I say neither, and that both will cool down at exactly the same rate. I don't see how the color of an object would affect its RADIATION of IR.?

If anyone disagrees.....please explain the mechanism by which more energy will radiate from either one. Now remember, this is assuming a perfectly dark room. If it were not totally dark, I would expect the WHITE object to cool off faster, because the black object would be also simultaneously absorbing more em than the white colored one.

Either way, I see zero chance that the black object cools faster.

Last edited: Nov 13, 2014
2. Nov 13, 2014

### Low-Q

I assume both rooms are equally insolated, and that shape and volume are the same, also that the objects are located at similar spots in those rooms.

I would agree with you that both bodies might cool down equally. However, if these bodies had the same color, but covered with equal blankets with different colors, black and white, I would assume that the body with black blanket cools faster.

However, electric motors that is painted black runs cooler than an equal motor with white color.
The black motor feels hotter on the outside after a race, but the coils inside seems to be cooler.

3. Nov 13, 2014

### Andy Resnick

Also, you do realize that the appearance (absorbance/emissivity) of an object in the visible waveband is not generally correlated with the object's emissivity in other wavebands? For example, snow is black in the IR band (say, 2-20 microns:

http://www.icess.ucsb.edu/modis/EMIS/images/snowmam01.gif.

4. Nov 13, 2014

### Drakkith

Staff Emeritus
There's a bit of a complication here. The visible color of an object is related to how well it absorbs or emits visible light, but not the rest of the EM spectrum. A dark colored object will emit/absorb more visible light than a light colored one, but it may emit/absorb less radiation in other wavelengths. For example, a black trash bag is transparent to part of the IR band, so if much of the radiation falling on the trash bag is in that part of the spectrum, then a white object can absorb more total energy than the trash bag does.