The reflectivities have been calculated by the matrix method using a digital computer for light polarized parallel (P) and perpendicular (S) to the plane of incidence for dielectric materials with continuously varying index of refraction profiles. Assuming various index of refraction profiles up to the computed penetration depth for positive ions, it was found that the phase difference between theP‐ andS‐waves is sensitive to the magnitude and shape of the index perturbation. Results for 150 keV Ar+bombardments of transparent vitreous silica are presented which show that implanted argon increases the refractive index of the solid by 0.02. The implanted ions appear to out diffuse between 200° and 600°C.