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glossary

Refracting surface: surface interfacing two transparent media with different refraction indexes
Refraction index: for a transparent medium, ratio between the light speed in the air and the light speed in the medium ; for a reflective surface in the air or vacuum, the refraction index is « - 1 »
Effective focal length: algebraic distance from the primary/secondary principal plane of an optical system to its front/back focal plane
Front focal length: for an optical system, algebraic distance from the vertex of the first surface to the front focal point
Back focal length: for an optical system, algebraic distance from the vertex of the last surface to the back focal point