Light passes through many optical materials with no significant change in the properties of the wave other than an apparent reduction in the velocity (due to the refractive index). Often, in most uniform transparent materials, two light waves are able to pass through each other in the same volume of material without any interaction occurring between them. That this is so, leads naturally to the widespread success of the principle of linear superposition of the electric fields within transparent media, summarized as follows: under typical illumination conditions the electric fields of light waves are sufficient only to create minor polarizations of the material, so where two or more optical fields are present the effects of these tend to be independent of each other. There are, of course, materials in which this is not true such as where two or more optical fields can be made to interact by the effect of the material. One well-known example of this is the hologram.