Optical properties of cubic boron nitride were studied in the spectral range from 400 to 50 000 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$. Raman-scattering, absorption, and reflection spectra were measured. The regions of the one-phonon and multiphonon absorption were studied in detail and the energies of the phonons were determined. The dispersion of the refractive index in the range 400--28 000 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ was obtained from measurements of interference spectra on plateletlike samples. A model including one-phonon and Sellmeier electron oscillators was used to fit the dispersion of the refractive index. Both the static and high-frequency dielectric constants ${\mathrm{\ensuremath{\varepsilon}}}_{0}$ and ${\mathrm{\ensuremath{\varepsilon}}}_{\mathrm{\ensuremath{\infty}}}$ were obtained and found to be 6.80 and 4.46, respectively. The parameters of the phonon oscillators were used for simulation of the interference spectra in transmission and reflection. Techniques for preparing free-standing samples of dimensions of the order of 150\ifmmode\times\else\texttimes\fi{}150 \ensuremath{\mu}m and of thickness down to 0.2 \ensuremath{\mu}m are described as well as methods to measure their thickness.