English: Representation of the penetration depth ${\displaystyle d_{p}={\frac {\lambda }{2\pi {\sqrt {n_{1}^{2}\sin ^{2}(\Theta )-n_{2}^{2}}}}}}$ of evanescent field as a function of incidence angle (from the lot) in the case of total reflection at the interface between a theoretical wavelength-refractive index material of the 2,4 (eg diamond) and 1.4 (eg water). The critical angle for these two materials is 36.275 °. The figure illustrates, that the penetration depth near the critical angle is greater than half the wavelength of the incident light. With increasing angle of incidence the penetration depth rapidly decreases and for the grazing incidence the penetration depth is approximately one tenth of the wavelength.