OPTICAL MATERIALS:- COLOUR CENTERS



 Pure ionic crystals like NaCl, KCl etc., are transparent when visible light is passed through them. However, these crystals may be made to appear coloured by any one of the following ways
  •     Adding impurities
  •     Adding metal ions
  •     By bombarding the crystal with X-ray,  -ray, neutron (or) electron.
  •     Electrodes.

In all these methods, vacancies will be created in the crystal. These vacancies, so called defect centres will capture the electrons (or) holes and capable of absorbing the visible light. Thus the crystal will be coloured.

Colour centres:

    Colour centres are the lattice defects (or) crystal imperfections which absorb the visible light, so that the crystal coloured.

Types of colour centres:

    There are different types of colour centres, F- centre,  centre,  -centre,  -centre, M-centre, V-centre etc.,
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