While the large difference in refractive indices (𝑛 ≈ 2.4 and 𝑛 ≈ 3.4) would suggest a much smaller depth of penetration for Si, this is not observed in the flat Si sample carriers in the AFM-IR illumination geometry. The reason being that refraction at the grooves on the bottom of the substrate leads to an angle of incidence of 30° at the top surface for Si while the angle of incidence for ZnSe is 45°. (The refractive index data used for the calculation for was taken from literature. ) Thus, similar depths of penetration are reached for both substrates (see fig. ). The change of refractive index with wavelength is considered, but absorption in the sample layer is ignored. For ZnSe prisms, an angle of incidence of 45° is used for flat Si substrates the angle of incidence is set to 30°, considering the refraction of the free space beam at grooved side of the carrier.