an interferometer consisting of two glass plates that reflect approximately half of each ray of light incident upon them and that are separated by a small, fixed distance: used to compare wavelengths and to study atomic spectra.
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British Dictionary definitions for etalon
physics a device used in spectroscopy to measure wavelengths by interference effects produced by multiple reflections between parallel half-silvered glass or quartz plates
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012