Editing Refractive index (section)

===Nonlinearity===
{{Main|Nonlinear optics}}
The strong [[electric field]] of high intensity light (such as the output of a [[laser]]) may cause a medium's refractive index to vary as the light passes through it, giving rise to [[nonlinear optics]].<ref name=Hecht/>{{rp|502}} If the index varies quadratically with the field (linearly with the intensity), it is called the [[Kerr effect|optical Kerr effect]] and causes phenomena such as [[self-focusing]] and [[self-phase modulation]].<ref name=Hecht/>{{rp|264}} If the index varies linearly with the field (a nontrivial linear coefficient is only possible in materials that do not possess [[inversion symmetry]]), it is known as the [[Pockels effect]].<ref name=Hecht/>{{rp|265}}