Optical Absorption in Dielectrics (Insulators)

OPTICAL ABSORPTION IN DIELECTRICS (INSULATORS)

In dielectrics the availability of free electrons (or) conduction electrons is very less and hence, when a dielectric is exposed to light it absorbs the light and the dielectric gets heated very slowly. The reason is as follows.

When light is allowed to fall on a dielectric for a longer time, the covalent bonds are broken and the electrons in the valence band absorbs the light energy and moves to conduction band thereby creating a hole in valence band. While it returns from conduction band to valence band it may radiate the energy in two process. viz.

(i) Radiative transition: It is the process in which the electrons in conduction band recombines with the holes in valence band and reradiate the energy in the form of light.

(ii) Non-radiative transition: In this process the electrons in conduction band will have collision with the lattice points in the dielectric material and thereby radiate the energy in the form of heat.

In both the process a part of energy is released in the form of heat and thus the material is heated.