The complex index of refraction N=n-ik for amorphous SiO2 is derived in the energy range 0.03-1.0 eV by Kramers-Kronig analysis of reflectance data. The results are used to compute the transmission, reflectance, and absorptivity of thin layers of SiO2 on Si substrates in the vicinity of the 0.14-eV (9 μ) lattice absorption band of SiO2. The dependence of these quantities on the layer thickness and angle of incidence is discussed.