The energy density in a semiconductor laser cavity plays an important role in determining the above-threshold properties of the laser. There is, therefore, a need for accurate physical models for the distribution of light within laser cavities. This paper applies the probability-amplitude method for calculating distributed feedback laser spectra to the problem of calculating the distribution of light within a laser cavity. Results of the calculations are shown to be in agreement with results obtained by other methods, and physical explanations are given for some of the interesting aspects of the distributions of light in semiconductor lasers. The probability-amplitude model for calculation of the distribution of light has advantages over many other models in that it includes both the standing-wave effect and the quantum mechanical nature of the spontaneous emission within the cavity.