Take a beam of monochromatic light pointed at a sheet of quartz 2" thick. The beam is striking the surface of the quartz at a fixed angle different than zero. Part of the beam will be reflected and another part of the beam will be refracted. Its exact path through the sheet will depend on the refraction index of the quartz and the refraction index of the first media. Question: What is the minimum thickness this sheet of quartz must have to conserve the same refraction index of the original 2" thick quartz sheet? I suppose there is a minimum of material below which it makes no sense talking about refraction index or about any other material property. Would this limit be 2 layers of atoms, or three or maybe 100 layers? What does it depend on?