Co2+ ion was found to produce a variety of colour shades ranging from pinkish-blue to intense blue to greenish blue in a series of binary alkali silicate glasses. The optical absorption characteristics of divalent cobalt produce three strong bands in the visible and two weak bands in the near infrared region, respectively in lithium silicate, sodium silicate and potassium silicate glasses. In this study, the molar extinction coefficients of Co2+ ion were calculated using Bouger-Beer's equation and the intensities of the bands were reported quantitatively in glasses at their wavelength maxima. The absorption bands in the molar extinction curves for Co2+ ion were found to shift more towards longer wavelengths with an increase in ionic radius of alkali ion on replacing lithium by sodium and sodium by potassium in glasses.
