This is the seventh (and perhaps last) installment of a series of columns on Maxwell’s equations of electrodynamics. In previous columns (available at Spectroscopy’s website, www.spectroscopyonline.com/The+Baseline+Column), we have covered history, the background of the first three equations, and the mathematics underlying them. Here we will present the fourth equation, and after reaching it we’ll see how light is described in terms of these four mathematical expressions.