Black Body Radiation
Consider a hollow cuboid with a small hole on one of it's sides(interior surface is mat black).and let this cuboid be in a vacuum.if we allow a light ray to pass trough this small small hole.
from which surface would the re radiation occur?
Does this happen?
the amount of radiations hitting the interior surface per unit time is re radiated per unit time from the surface.since all the re radiated waves cannot exit trough the hole the temperature inside the cuboid increases until the the amount of radiations leaving trough the hole is equal to the amount of radiations entering trough the hole.
[and also due to the temperature difference between the external surface and the surrounding can the object simply radiate according to newton's law of cooling.]