Monochromatic light

Monochromatic light is light concentrated at a single temporal frequency in the ideal model. Its vacuum wavelength is ${\lambda }_0=c/f$, while in a transparent medium the wavelength is set by the medium's dispersion relation. The frequency stays the same across a stationary interface even when the wavelength changes.

A real source has finite spectral width, and a finite pulse cannot be exactly monochromatic. A narrow spectral line or a stable narrowband laser can nevertheless approximate monochromatic light for a stated experiment. A spectrometer tests this approximation by comparing the line width with the frequency scale being resolved. A single frequency does not by itself specify polarization of waves or spatial optical coherence: two fields may share a spectral line while having different propagation directions or fluctuating relative wave phases.