Re: Blacker than Black

textfilter: chose text/plain from a multipart/alternative

On Fri, Nov 11, 2011 at 1:03 AM, K.H.Ranitzsch
<kh.ranitzsch@t-online.de>wrote:

> Tom B schrieb:
> >
> > TB: Admittedly, I'm confused now. It is 98% absorptive in infra-red,
> > but it also radiates away more heat... I can't quite reconcile this
in
> > my head.
>
> Indeed, this seems like a paradox, but actually, it's true.
>
> It may help to think about it as two different processes. One is how
the
> surface handles incoming radiation, the other is how it sends
radiation
> generated by the object.
>
> When we call an object black or white, we refer to the way it handles
> incoming radiation. A white object reflects radiation, a black one
> absorbs it. Note: of course this also depends on the incoming light.
At
> night everything looks black, because there is no light around.
>
> Any object above absolute zero temperature also sends out heat
> radiation. It is in the infrared at normal temperatures, but if you
heat
> it up, it starts to glow in visible light.
>
> By a paradoxic-seeming twist of the laws of physic, a black object is
> both the best absorber of incoming radiation and the most efficiet
> emitter of outgoing heat radiation.
>

An experiment for the student: take two similar objects, one black, one
white, and place them out in the sun for an hour or three (really, just
for
a long while). Note which one is warmer. Now take them out of the sun.
Let
sit for a few minutes. Note which one is warmer (but is cooling off
quickly
as it radiates away).

:-)

Mk