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glossary

Lens: optical component with two refractive surfaces axially symmetrical for redirecting the light in a controlled manner
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