Light in a dielectric medium When light enters a dielectric medium (e.g. plastic or glass) the electric fields are damped by the dielectric material, which in turn affects the speed of light in the medium. We refer to the index of refraction n which gives the degree to which light is slowed in the medium. The index of refraction is always greater than unity, as nothing moves faster than c, unless one is in a vacuum.
The figure on the right illustrates this as the wave fronts are shown by the dashed lines. If light were to enter one dielectric from another one, n would be replace by the ratio of the indices of refraction. |