When the wavelength of light is much smaller than the objects it meets, its wave nature can be set aside and light treated as straight-line rays: this is geometric optics, born with Euclid and refined by Snell, Descartes, Huygens and Newton. This section starts from the ray of light, moves through shadow, reflection and refraction, and arrives at mirrors and lenses, all governed by a single conjugate points equation. It closes with light intensity, the human eye and the magnifying glass.