Question

Explain the following causes of color:

a. Why are rubies red? They are basically single crystals of Al2O3 with 1% of Cr impurities. Bulk Al2O3 has a band gap of 8.8 eV.

b. Contrast the different causes of color in chlorophyll, in Si and in the gemstone Al2O3.

Answer 1

A.) first of all we know that Ruby is a crystal of a Al2O3 and due to presence of very minute quantity or percentage of chromium it looks red. In Crystal to replace aluminium atom balancing the charge chromium has to lose three electron and become chromium +3 but as we know that chromium +3 are larger than aluminium ion and the electron orbitals of chromium +3 are in different direction then of aluminium and that’s why the octahedral arrangement of crystal get distorted  and this lead to change in energy gap of the crystal this energy gap corresponds to the absorption in Ultraviolet  and yellow green region so when a white light falls on Crystal It absorbs the yellow and the green light and so the crystals looks red.

B.) Chlorophyll is a pigment that absorb mostly the red and blue light of the specific wavelengths and reflects the green colour light and that’s why it looks green. This absorption of a specific wavelength of light is due to presence of a band gap of a particular energy and similar is the reason for the other gemstone of Al2O3 they have a particular colour because of the absorption of light of a particular color.