An optical device consisting of two parallel, partially reflective surfaces used to measure wavelengths of light or create interference patterns in physics.
From French 'étalon' meaning 'standard' or 'gauge,' derived from 'étaler' meaning 'to spread out.' The term was adopted in optics because the device serves as a standard for measuring light wavelengths.
The Fabry-Pérot etalon is absolutely fundamental to modern lasers and spectrometers—it's like an optical tuning fork that lets scientists measure the exact color of light!
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