Define F–centre. Mention its one consequence.
One or more unpaired electrons occupy an anionic vacancy in a crystal in a crystallographic defect known as an F-center, FARBE centre, or colour centre (from the German Far Zentrum). Vacancies like these tend to absorb visible spectrum light, turning normally clear materials into colourful ones. As a result, it is used to detect a wide variety of chemicals, including zinc oxide.
