The Leontovich-Levin equation for optical scattering by an achiral carbon nanotube (CNT) of finite length is formulated, based on a quantum-mechanical microscopic model of the conductivity. Application over a wide frequency range from the terahertz to the ultraviolet is possible. The CNT polarizability scalar in the low-frequency regime and the scattering pattern in the regime of optical interband transitions as well as in the vicinity of plasmon resonance are calculated. Geometric resonances of strongly retarded surface waves emerge, and can be used for the qualitative interpretation of experimentally observed features in the optical response characteristics of CNT-based composite mediums. The potential of isolated CNTs as optical nanoantennas of both the receiving and the transmitting types is established.