1. The four physical laws or equations that are most commonly used to explain the emission of radiation: 1) Kirchhoff's law; 2) Planck's law; 3) Stefan–Boltzmann law; and 4) Wien's displacement law. Of these, 1) and 2) are fundamental. The remaining two can be derived from 2). 2. All of the more inclusive assemblage of empirical and theoretical laws describing all manifestations of radiative phenomena, for example, Bouguer–Lambert law, Lambert's law, Stefan– Boltzmann law, etc.