The rate at which one atom in a photodetector is ionized by a beam of classical light is considered. A detailed examination of the probability of transition is presented, using first- order time-dependent perturbation theory and the rotating wave approximation. This method usually leads to the well-known rule called 'golden' by Fermi. The authors have made a more accurate analysis based on numerical quadrature techniques. They discover a curious oscillation in the probability for photoionization for short times and a temporal shift in the curve as compared to the results given by the golden rule. The shift is approximated analytically and found to correspond with the numerical result to a high precision. Results for the probability of photoionization are shown without using the rotating wave approximation.