thickness of the oxide layer 52 using techniques known in the art in one implementation the optical monitoring system comprises an interferometer capable of generating a collimated light beam and an interference signal as described in u s pat no 5 964 643 the entire disclosure of which is incorporated herein by reference without being limited to any particular theory one possible source of attenuation is scattering of the light beam at the interface between the window 36 and the slurry 38 as shown in fig4 a if the window 36 has scratches or surface roughness both the outgoing light beam 34 and the incoming light beam 60 can be scattered at the window slurry interface 40 this scattering can increase the signal to noise ratio refraction is the bending of light as the light passes from one medium to another when there is a difference in the index of refraction between the two mediums when the two refractive indices of two mediums are equal the light passes from the first medium to the second medium without refraction referring to fig4 b the window 36 can be formed from a material having a refractive index equal to or nearly equal to the refractive index of the slurry 38 at the wavelengths of interest to the optical monitoring system e g if the light source 32 is a laser then at the wavelength of the beam 34 emitted by the laser thus the light beam.