In 1916, Albert Einstein applied this principle on an atomic level to the case of an atom radiating and absorbing radiation due to transitions between two particular energy levels, giving a deeper insight into the equation of radiative transfer and Kirchhoff's law for this type of radiation. If level 1 is the lower energy level with energy E 1, and level 2 is the upper energy level with energy E 2, then the frequency ν of the radiation radiated or absorbed will be determined by Bohr's frequency condition: E 2 − − E 1 = h ν ν .