The paper by Bloembergen and Pershnan explores the behavior of light waves at the boundary between a linear and a nonlinear medium, focusing on the solutions to Maxwell's equations that satisfy the boundary conditions. The authors derive generalizations of the laws of reflection and refraction for harmonic waves, and describe the intensity and polarization conditions of these waves. They introduce the concept of an equivalent Brewster angle for harmonic waves and discuss the conditions for total reflection and transmission. The solution for harmonic waves in a nonlinear plane-parallel slab is presented, highlighting the asymmetry between the linear and nonlinear cases. The paper also sketches an integral equation formulation for wave propagation in nonlinear media and discusses the implications for experimental systems and devices. The theoretical results are shown to have direct bearing on experimental observations, particularly in the context of harmonic generation in nonlinear crystals.The paper by Bloembergen and Pershnan explores the behavior of light waves at the boundary between a linear and a nonlinear medium, focusing on the solutions to Maxwell's equations that satisfy the boundary conditions. The authors derive generalizations of the laws of reflection and refraction for harmonic waves, and describe the intensity and polarization conditions of these waves. They introduce the concept of an equivalent Brewster angle for harmonic waves and discuss the conditions for total reflection and transmission. The solution for harmonic waves in a nonlinear plane-parallel slab is presented, highlighting the asymmetry between the linear and nonlinear cases. The paper also sketches an integral equation formulation for wave propagation in nonlinear media and discusses the implications for experimental systems and devices. The theoretical results are shown to have direct bearing on experimental observations, particularly in the context of harmonic generation in nonlinear crystals.