The absorption coefficient determines how far into a material light of a particular wavelength can penetrate before it is absorbed. The absorption coefficient, α, in a variety of semiconductor materials at 300K as a function of the vacuum wavelength of light. We define the emission coefficient j as the amount of energy emitted per unit volume, per unit time, per unit frequency, into a solid angle (with units of ergs per second per cm³ per steradian per Hz). As the speed of light governs the amount of time that it takes for energy to pass along the cylinder, dl = c × dt. Einstein coefficients are mathematical quantities that are a measure of the probability of absorption or emission of light by an atom or molecule.