The spectral features of the optical functions of the Drude model are investigated over wide ranges of parameters — the plasma frequency ωp of the current carriers, their transport relaxation frequency g, and the dielectric constant ε∞ due to high-energy electronic transitions in the system. The conditions are determined for: a) the square-root frequency dependence of the modulus and phase of the reflection; b) linearity of the phase θ=2ω/ωpε∞+θ0; c) the existence of a plasma reflection edge. Approximate relations are obtained which permit simplifying the analysis of optical reflection and electron characteristic energy loss data both in “good” metals and in materials with a strong temperature dependence of the electrical conductivity (semiconductors, doped and nonstoichiometric oxides, granular metal films, and amorphous alloys). The parameters of the systems of conduction electrons in Al and U2Zn17 are determined.