Transmission:

When radiation passes through a transparent medium, its velocit y is found to be reduced as compared to that in the vacuum. This is because of the interaction of radiation along with the constituents of matter. The extent of reduction within velocity depends on the nature and concentration of ions, atoms, or molecules present in the medium.  When the radiation interacts with the atoms and molecules present in the medium, it results in their periodic polarisation that is., the deformation of their electron clouds.  Within the absence of any absorption, the energy needed for polarisation is retained only momentarily (10 ^-14 - 10 ^-15 s) and reemitted without any alteration and the substance returns to its original state.  Since the re emission of the radiation is delayed for some time; i.e. the radiation takes a little longer to travel through a distance, its velocity decreases. Therefore, since there is no absorption of the radiation, the frequency of the radiation does not change. Therefore the transmission of radiation includes a series of polarisation processes wherein the atoms, ions and molecules act as intermediates.

The extent of interaction of the radiation with matter can be expressed in terms of an important parameter called the refractive index of the medium. It is defined by the following equation:

η_i  = c/V_i

where, ηi   is the refractive index for the radiation of a given frequency i, and c and ν i are the velocities of the radiation in vacuum and in the medium, respectively. The variation of refractive index of a substance with wavelength of the radiation is called as dispersion. If the refractive index rises gradually along with increasing frequency, it is called a normal dispersion, and if there is a sharp change in refractive index it is termed as an anomalous dispersion.