The optical conductivity measurements give a powerful tool to investigate the
nature of the superconducting gap for conventional and unconventional
superconductors. In this article, first, general analyses of the optical
conductivity are given stemmed from the Mattis-Bardeen formula for conventional
BCS superconductors to unconventional anisotropic superconductors. Second, we
discuss the reflectance-transmittance (R-T) method which has been proposed to
measure far-infrared spectroscopy. The R-T method provides us precise
measurements of the frequency-dependent conductivity. Third, the optical
conductivity spectra of the electron-doped cuprate superconductor
Nd_{2-x}Ce_xCuO_4 are investigated based on the anisotropic pairing model. It
is shown that the behavior of optical conductivity is consistent with an
anisotropic gap and is well explained by the formula for d-wave pairing in the
far-infrared region. The optical properties of the multiband superconductor
MgB_2, i n which the existence of superconductivity with relatively high-T_c
(39K) was recently announced, is also examined to determine the symmetry of
superconducting gaps.Comment: 44 pages, 15 figure