This lecture covers the electromagnetic energy in a medium, interface conditions, and the behavior of electromagnetic waves in different mediums. The instructor explains the microscopic origin of dielectric permittivity, magnetic dipoles, spatial averages, and macroscopic Maxwell equations. The lecture also delves into the interface conditions between different materials and the phase velocity of electromagnetic waves in a medium, including the wave equation and refractive index. Snell's law of refraction and total internal reflection are discussed, along with Fresnel coefficients.