Question

16.2. (a) Refer to Table 16.1. Identify the materials with the highest and lowest values of n. Explain the differences in terms of what you know about polarizabilities of the constituent ions. (b) What differences in the indices of refraction and dispersion would you expect between LiF and PbS? Explain. (c) Which oxide would you expect to have the higher index of refrac- tion, MgO or BaO? Explain. Refractive Indices of Selected Ceramic Materials Table 16.1 Material Material Halides and Sulfides NaF 1.430 CaF2 1.770 Nal 1.480 1.560 1.510 1.390 BaF2 1.780 PbF2 PbS KCl 2.370 TIBr LiF NaCI 2.200 ZnS 1.550 Oxides 2.610 РЬО Al2O, (Sapphire) 1.760 3Al20,.2Sio2 1.640 BaTiO 2.710 1.810 2.490 SrO SrTiO, 2.400 1.980 BaO 1.920 1.720 BeO CaCO MgAl 04 Mgo 2.000 1.658, 1.486 ZnO ZrSiO4 1.950 1.720 ZrO2 2.190 1.740 Covalent Ceramics C (diamond) a-SiC 2.424 a-SiO, (quartz) 1.544, 1.553 2.680 Glasses Pb-silicate glasses Fused quartz 2.500 Soda-lime-silicate glass Na O-CaO-sio Vycor 1.510 2.126 1.458 Pyrex 1.470 1.458 Dense optical flint : Borosilicate 96% silica

Answer 1

a) Highest refractive index material = PbS

Lowest refractive index material = NaF

Given the polarizability, a, and the volume, V, of the cluster, the average refractive index, n, is readily given by n = (1+a/v)

therefore PbS has a higher refractive index and NaF the lowest

b)

LiF has a much lower density and molar mass than PbS and this is consistent with the values of refractive index. the Dispersion index is a measure of the amount of dispersion, since LiF is a smaller unit compared to PbS it will have a higher refractive index for lower wavelengths

c) Barium is larger than Magnesium so one would expect more 'resistance' per unit volume for Magnesium and hence a higher refractive index is expected for MgO than BaO