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glossary

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 »
Wavelength ( of a light wave ): distance between two consecutive identical states of the electric ( or magnetic ) field
Vertex: top of a surface
Object: element (point, line, plane, or whatever,…) of the space linked to another element called image in a way that rays coming from the object and transmitted by an optical system focus on the image ; object and image are said to be conjugate by the optical system
Image: conjugate of an object by an optical system (see object)
Chromatism: aberration occurring in optical systems with refractive surfaces and caused by the dependence of the refractive index on the wavelength
Axial chromatism: wavelength dependance of the position of the paraxial image by an optical system caused by the chromatic aberration for an on-axis object
Height chromatism: for an off-axis object, wavelength dependance of the height of the paraxial image by an optical system caused by the chromatic aberration