Glass is a

The band gaps of an insulator, semiconductor and conductor are respectively ${\mathrm{E}}_{{\mathrm{g}}_1,} {\mathrm{E}}_{{\mathrm{g}}_2} \mathrm{and} {\mathrm{E}}_{{\mathrm{g}}_3}$.The relationship between them can be given as:

The chromium ions doped in the ruby rod: