English: Monostatic radar cross section (RCS) of a perfectly conducting metal sphere as a function of frequency (computed by Mie theory). The x-axis is the relative frequency, defined as the number of wavelengths in the circumference, and equal to the wavenumber times the radius i.e. f_rel = 2πR/λ = kR. The y-axis is the RCS relative to the projected area of the sphere i.e. σ/(πR²). The asymptotes are σ/(πR²) = 9 (f_rel)⁴ in the low frequency (Rayleigh) region (λ > 2πR) , and σ/(πR²) = 1 in the high frequency (optical) limit (λ << R).