On 08/11/2011 10:16, Tim Lamb wrote:

Reflecting cold? That's an interesting concept.

On the other hand, being sat next to a cold surface is uncomfortable.

Apparently.

My wife has what must politely be called a *substantial* nose. Seated in
front of our lounge TV, I often find her face shrouded in a scarf.

We have large window openings to either side and the loss of radiated
heat causes her some discomfort.

Somebody once tried to explain *black body* radiation to me. AFAIR
everything absorbs incident radiation at infra red wavelengths and then
re-radiates it. The walls bounce back all they receive but the windows
do not absorb much and consequently feel cold.

One of our physicists may explain it better:-)

IANAP

I am more than a little rusty on this so may have a few of the niceties
wrong but AIUI:

Black bodies are so called because they absorb and radiate all
frequencies. In the real world most objects reflect some frequencies
rather than absorbing them (they must do one or the other) which is why
we have coloured objects.

All objects above absolute zero radiate to some extent, even cold walls
and substantial noses. Such radiation varies with the forth power of
absolute temperature so you get quite a noticeable difference even with
relatively minor differences in temperature.

Some surfaces radiate much better than others as anyone with a
thermometer that uses infra red should have learned by now. I can get
good reading from painted radiators but have to stick on a bit of black
tape to get much response from copper pipes (a matter of emissivity I
believe).

Glass reflects so it will radiate comparatively little so windows should
appear colder. They should usually also feel colder to the touch as even
double glazing tends to be a worse insulator than all but thinnest of
solid walls.

All this off the top of my head so take it with a pinch of salt until a
real scientist comes along and pulls it apart.

--
Roger Chapman