Geometrical optics, or ray optics, is frequently used to study how images form in optical systems. It uses the concept of rays, which have direction and position but no phase information, to model the way Electromagnetic Radiation (EM Rad) travels through an optical system. Geometrical Optics is an approximation of how EM Rad behaves in an optical system – it is a very good model to use when the smallest dimension of the optical system is much larger than the wavelength of the incident EM Rad. But if the dimensions of the optical components are about the size of a wavelength or smaller, a different model (either the Electromagnetic Wave Optics Model or the Quantum Optics Model) must be used.
In geometrical optics, an object is viewed as a collection of many pin-point sources of EM Rad. Each source produces a bundle of rays that is traced through an optical system to determine what image will be formed. This technique is useful for designing lenses, developing systems of lenses for use in microscopes, telescopes, cameras and other devices, and for studying the aberrations of an optical system.