How does monochromatic light produce a saturated hue?

Monochromatic light produces a saturated hue because it consists of radiation at a single frequency within the visible spectrum.1

In the study of colour, saturation—also referred to as purity or chroma—describes the vividness of a colour. When light is composed of a single wavelength, it lacks the mixture of white light or other wavelengths that would otherwise dilute its intensity and create a paler, less saturated appearance.2

On a chromaticity diagram, these pure spectral colours are located along the outer horseshoe-shaped circumference, which represents the maximum level of saturation. Conversely, as colours become less pure—such as when a vivid shade is mixed with white light—they move toward the central achromatic point, resulting in lower saturation. Therefore, because monochromatic light is restricted to a specific point on the spectrum without additional spectral components, it is perceived by the eye as a fully saturated hue.321