In dielectrics the availability of free electron (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. To valence band it may radiate the energy in two processes.
- Radiate transition.
2. 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.
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