Matter at finite temperatures emits electromagnetic
radiation with a wave length distribution which peaks approximately for
Where 
Js is Plancks constant. Putting in numbers
we find for a hot bonfire where 
K
Such radiation is called thermal radiation. We cannot see it because
its wave length is too long for our eyes and we cannot pick it up
on our radio dial because the frequency is too high. The radiation is
generally called infra red radiation because its wave length is
longer than that corresponding to red light. The total
radiation emitted per area unit is given by Stefan Boltzmans law
where 
W/(m
K
) and 
is a thermal emissivity factor which is 1 for a perfectly absorbing so
called black body and very small for a perfectly reflecting
surface.
Bodies are as efficient at absorbing thermal radiation as they
are at emitting it. If it were not so then it would be impossible
to establish thermal equilibrium through the thermal radiation
phenomenon. Thus the net absorbed radiation from an environment
at temperature 
to a body at temperature 
is
This explains why you feel cold on a cold night even when there is no wind:
Your body is radiating more thermal energy to the surroundings than it is
picking up in return. We have a demonstration experiment which illustrates the
different thermal emissivities of various materials. We find that
the more shiny the less emissivity. This is perhaps part of the
reason why industrial grade
refrigerators are made from polished stainless steel.