Most optical phenomena can be accounted for using the classical electromagnetic description of light. However, a complete electromagnetic description of light is often difficult to apply in practice. Therefore, simplified models are usually utilized in optics. One of the commonly used models is called geometrical optics, or ray optics. Geometrical optics treats light as a collection of light rays and describes light propagation in terms of light rays. A light ray is a straight or curved line that is perpendicular to the wavefront of light and is therefore collinear with the wave vector. A slightly more rigorous definition of light ray follows from Fermat’s principle, which states that the path taken between two points by a ray of light is the path that can be traversed in the least time. The light ray in geometrical optics is useful in approximating the paths along which light propagates in certain classes of media.