Fabry–Pérot Interferometer
Definition
Two ultra-parallel mirrors bouncing light hundreds of times — the sharpest color filter in optics.
Technical definition
Multiple-beam interference in an etalon; transmission peaks occur at 2nd = mλ, with finesse set by mirror reflectivity and resolving power near 10⁶.
Example
Laser cavities are Fabry–Pérot interferometers — only the exact resonant wavelengths build up, which is why laser light is so pure in color.
Common confusion
It is not just 'two mirrors' — the magic is the hundreds of round trips. Each pass sharpens the resonance; low-reflectivity mirrors give a blurry filter.