Question

What are the two laws of thermal radiation?

Answer 1

The two laws of radiation are:

1. Stefan-Boltzmann law: The total energy emitted/radiated per unit surface area of a blackbody across all wavelengths per unit time is directly proportional to the fourth power of the black body’s thermodynamic temperature.
   $\mathrm{P}={\mathrm{e\sigma AT}}^4$
   where, $\mathrm{P}$ = radiated power
   $\mathrm{e}$ = emissivity
   $\mathrm{\sigma }$ = surface area
   $\mathrm{T}$ = absolute temperature
2. Kirchoff's Law: The ratio of the emissive power to the absorptive power for the radiation of a given wavelength is same for all substances at the same temperature and is equal to the emissive power of a perfectly black body for the same wavelength and temperature.
   $\mathrm{Emissive}\mathrm{power}\mathrm{of}\mathrm{a}\mathrm{black}\mathrm{body}=\frac{\mathrm{Emissive}\mathrm{power}\mathrm{of}\mathrm{a}\mathrm{body}}{\mathrm{Absorptive}\mathrm{power}\mathrm{of}\mathrm{a}\mathrm{body}}$