<Ul> <Li> If the electrons emit a light wave which is 90 ° out of phase with the light wave shaking them, it will cause the total light wave to travel slower . This is the normal refraction of transparent materials like glass or water, and corresponds to a refractive index which is real and greater than 1 . </Li> <Li> If the electrons emit a light wave which is 270 ° out of phase with the light wave shaking them, it will cause the wave to travel faster . This is called "anomalous refraction", and is observed close to absorption lines (typically in infrared spectra), with X-rays in ordinary materials, and with radio waves in Earth's ionosphere . It corresponds to a permittivity less than 1, which causes the refractive index to be also less than unity and the phase velocity of light greater than the speed of light in vacuum c (note that the signal velocity is still less than c, as discussed above). If the response is sufficiently strong and out - of - phase, the result is a negative value of permittivity and imaginary index of refraction, as observed in metals or plasma . </Li> <Li> If the electrons emit a light wave which is 180 ° out of phase with the light wave shaking them, it will destructively interfere with the original light to reduce the total light intensity . This is light absorption in opaque materials and corresponds to an imaginary refractive index . </Li> <Li> If the electrons emit a light wave which is in phase with the light wave shaking them, it will amplify the light wave . This is rare, but occurs in lasers due to stimulated emission . It corresponds to an imaginary index of refraction, with the opposite sign to that of absorption . </Li> </Ul> <Li> If the electrons emit a light wave which is 90 ° out of phase with the light wave shaking them, it will cause the total light wave to travel slower . This is the normal refraction of transparent materials like glass or water, and corresponds to a refractive index which is real and greater than 1 . </Li> <Li> If the electrons emit a light wave which is 270 ° out of phase with the light wave shaking them, it will cause the wave to travel faster . This is called "anomalous refraction", and is observed close to absorption lines (typically in infrared spectra), with X-rays in ordinary materials, and with radio waves in Earth's ionosphere . It corresponds to a permittivity less than 1, which causes the refractive index to be also less than unity and the phase velocity of light greater than the speed of light in vacuum c (note that the signal velocity is still less than c, as discussed above). If the response is sufficiently strong and out - of - phase, the result is a negative value of permittivity and imaginary index of refraction, as observed in metals or plasma . </Li> <Li> If the electrons emit a light wave which is 180 ° out of phase with the light wave shaking them, it will destructively interfere with the original light to reduce the total light intensity . This is light absorption in opaque materials and corresponds to an imaginary refractive index . </Li>

As the density of a medium increases the speed of an electromagnetic wave