This chapter introduces geometrical optics, an approximation of light propagation valid in the short wavelength limit, where light is described as an ensemble of independent optical rays. It establishes that these rays propagate in straight lines in homogeneous media and follow curved trajectories in inhomogeneous media, always perpendicular to the wavefronts. The fundamental optical ray equation is derived, showing that rays bend towards regions of higher refractive index, a generalization that includes Snell-Descartes law for interfaces. A cornerstone of geometrical optics, Fermat’s principle of least time, is presented, stating that light travels along paths that make the travel time stationary. This principle is used to derive the laws of reflection and refraction and to explain the fundamental behavior of optical components. The chapter then establishes the properties of optical systems and image formation. Key definitions are introduced, including real and virtual objects and images, stigmatism (the ability of a system to form point-like images), and the concept of centered optical systems with an optical axis. The paraxial approximation is emphasized as a simplification that allows for perfect stigmatism and aplanatism (forming plane images of plane objects) for rays close to the optical axis. Crucial cardinal points of optical systems are defined: focal points, principal points, and nodal points. Their relationships are established through Newton’s relation and the lens equation, which relate object and image positions to focal lengths. The transverse magnification and longitudinal magnification are introduced to quantify image size and orientation. The chapter applies these principles to spherical lenses and spherical mirrors, deriving their imaging properties and demonstrating the concept of geometrical aberrations for non-paraxial rays. It also discusses aspheric lenses and Fresnel lenses as solutions for aberration correction and compact design, respectively. The human eye is analyzed as a complex optical system. Finally, ray tracing is presented as a practical graphical method for determining image formation, and the association of multiple optical systems is explored, culminating in Gullstrand’s formula for combined focal lengths.