The optical properties of solids can be studied with the complex refractive index n:
n* = n + ik (19)
in which n is the real part of the refractive index and к is the imaginary part or absorption index. Values of n and к from 0.008731 pm to 600 pm (142.000-0.00207 eV) are given to high accuracy and many wavelengths for alumina in . In the wavelength range from 0.1454 pm to 4.000 pm (8.529-0.31 eV), the value of к for highly pure alumina is less than 10-6 [49, 50], so the alumina is effectively transparent. Values of n for this wavelength range are given to four significant figures in Table 21. Values of n in the fifth or sixth significant figure are different for different investigations, probably because of different purities of samples and different measuring techniques and errors. A two-term equation for n (ordinary ray) in this wavelength range is 
with l the wavelength in micrometers. This equation gives n accurately to about four significant figures (Table 21). A more accurate three-term Sellmeier equation is
with the constants A. and X. given in Table 22.