Thermal radiation is the energy radiated from hot surfaces as electromagnetic waves. It does not require
medium for its propagation. Heat transfer by radiation occur between solid
surfaces, although radiation from gases is also possible. Solids radiate over a wide range of wavelengths,
while some gases emit and absorb radiation on certain wavelengths only.
When thermal radiation strikes a body, it can be absorbed by the body, reflected from the body, or transmitted through the body. The fraction of the incident radiation which is absorbed by the body is called absorptivity (symbol ). Other fractions of incident radiation which are reflected and transmitted are called reflectivity (symbol ) and transmissivity (symbol ), respectively. The sum of these fractions should be unity i.e.