The process of heat transfer which occurs through empty space and can also occur in matter, in
the form of electro-magnetic waves is called radiation or radiant heat.
When Q amount of radiation occurs on any object, the object partly reflects
(Qr), partly transmits Qt) and absorbs the remaining part
(Qa) of heat.
It is obvious that
The quantities are called respectively coefficients of absorption,
reflection and transmission, of radiant heat.
Perfectly Black body
An object whose a = 1, and therefore r = t = 0, i.e. Q
a= Q or which absorbs all the incident radiant heat on it
is called a Perfectly black body. Lamp black is closest to a perfectly
black body (a = 0.98), but no object in our environment is otherwise
found to be a perfectly black body. An artificially constructed
perfectly black body is an important source of radiation. The analysis
of the spectrum of such an artificial black body leads to Planck's
Kirchoff's law of heat radiation
This law states that an object is as good an emitter of heat radiation as an absorber of it. In
other words, the coefficient of absorption and emissivity of any object are equal to each other; i.e. a = e.
This law can be verified by using Rritchie's apparatus.
The water in the arms of the glass tube is brought to an equal level before the experiment begins.
Then some hot water is poured into the cylinder C. After a sufficiently long interval of time, it
is observed that the water levels remain at their original level. The following observation is made :