Name the non-stoichiometric point defect responsible for colour in alkaline metal halide.

Frenkel defects are not found in alkali metal halides.

What do you mean by non-stoichiometric defect? How this defect is classified? Give example of each class.

Frenkel defect is not found in halides of alkali metal why?

Name two solid non-metal .

Ionic solids, which have anionic vacancies due to metal excess defect, develop colour. Explain with the help of a suitable example.

A blackish brown coloured solid 'A' when fused with alkali metal hydroxides in presence of air, produces a dark green coloured compound 'B' which on electrolytic oxidation in alkaline medium gives a dark purple coloured compound 'C' Identify A, B and C and write the reactions involved.

Name the defect of vision in a person whose far point is less than infinity. What type of lens is used to correct it?