We examine the basic physics of type Ia supernova (SNe Ia) light curves with a view toward interpreting the relations between peak luminosity, peak width, and late-time slope in terms of the properties of the underlying explosion models. We develop an analytic solution of the radiation transport problem in the dynamic diffusion regime and show that it reproduces bolometric light curves produced by more detailed calculations under the assumption of a constant extinction coefficient. This model is used to derive the thermal conditions in the interior of SNe Ia, to address the issue of time dependence in the modeling of SNe Ia light curves and spectra and to show that these are intrinsically time-dependent phenomena. The analytic model is then used to study the sensitivity of light curves to various properties of supernova explosions. We show that the dominant opacity arises from line transitions and discuss the nature of line opacities in expanding media and the appropriateness of various mean opacities used in light curve calculations.