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Because of its usefulness, we start with the geometrical optics model. In this model a "point source" emits rays which are straight lines in a vacuum or in a homogeneous isotropic dielectric medium. Light travels at different speeds in different dielectrics. Its speed is given by c/n, where c is the speed in vacuum (299,792,458 m s^-1) and n, the refractive index, depends on the medium and on the frequency of the light.
A ray if refracted at an interface between two media. If r and r' are unit vectors along the incident and refracted directions, n and n' are the respective refractive indices, and n is the unit normal to the interface, then the ray directions are related by
nn x r = n'n x r',
which is the law of m Snell's law, in vector form. More conventionally, Snell's law can be written
n sin I = n' sin I' ,
where I and I' are the two angles formed where the normal meets the interface, the angles of incidence and refraction. The two rays and the normal must be coplanar.
>why_draw____by_guiltyorion-d3bnx68
We view light as particles of energy travelling through space. These particles follow trajectories that we call rays. We can describe an optical system comprising elements such as mirrors and lenses by tracing the rays through the system. In this example we term the speed of light in a transparent linear, homogeneous, isotropic material as v.