Britannica Sources used in this answer

How does a monochromator select a narrow range of wavelengths?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

Learn about the use of gamma-ray spectroscopy to identify the quarry that was the source of granite found in ancient Roman ruins — spectroscopy See how gamma-ray spectroscopy is used to identify the quarry that was the source of granite found in ancient Roman ruins.

A monochromator selects a narrow range of wavelengths by dispersing a beam of light into its constituent wavelengths and then isolating a specific portion of that spectrum.

The process typically involves the following steps:

Dispersion: The instrument uses a diffraction grating or a prism to separate incoming light into its component wavelengths. A diffraction grating achieves this through constructive interference, where different wavelengths are diffracted at distinct angles based on the formula $m λ = d (\sin i - \sin r)$ .
Selection: Because different wavelengths emerge at different angles, the instrument can isolate a specific range by physically blocking unwanted wavelengths or by rotating the dispersive element (such as the grating) to align a desired wavelength with an exit slit.
Filtering: In some configurations, additional filters are used to remove unwanted wavelengths or stray light, ensuring the output consists of a single color or a very narrow spectral band.12