This lecture explores the matrix approach to interference in layers, discussing the links between different matrices, the formation of 2D cavities, and the concept of anti-reflective layers. It delves into the properties of systems with multiple dielectric layers and the transformation between diffusion and transfer matrices. The lecture also covers the Fabry-Perot interferometer, propagation in homogeneous media, and the design of semi-transparent mirrors. Additionally, it examines the interface between materials, reciprocal systems, and the reduction of reflection through anti-reflective coatings.