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A simple optical model has been used to describe the influence of light trapping on the performance of photocathodes. The model is based on the assumption that light trapping occurs as a result of total internal reflection in the faceplate-photocathode structure. The analysis based on the model predicts that successive light photocathode interactions are related through a geometric progression, and the ratios of light intensities and photoelectric yields associated with successive light photocathode interactions are equal to the common ratio of the progression. Experiments on S-1, S-11, and S-20 type photocathodes tend to confirm three predictions based on the analysis: that the logarithm of the photocurrent for a specific interaction is a linear function of the number denoting the interaction; that successive photocurrents and light intensities have the same ratio denoted K, and that the ratio of the total photocurrent to that of the first interaction approaches 1/1-K.