"These lectures are part of the introductory course ""Photonics"", which is read to physics students at the University of Bath in the UK in the first semester of the fourth year. Studying this material allows students to get further through basic textbooks and monographs on nonlinear optics, and, in the future, to begin studying the scientific literature. To complete the course successfully, you need to know the general course of physics, common for the first two years of physical or engineering science education. This includes optical part, the Maxwell equations and their basic properties, as well as basic knowledge on methods for solving differential equations in ordinary and partial derivatives. Parts of the course requiring knowledge of quantum mechanics are not included in this abbreviated version."