An Etymological Dictionary of Astronomy and Astrophysics
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René Descartes (1596-1650), French mathematician, physicist, and philosopher, who made valuable contributions to mathematics, physics, and philosophy. Descartes was a pioneer and major figure in 17th century rationalism. He represents a major break with the Aristotelianism and Scholasticism of the Medieval period. René Descartes is regarded as the father of modern philosophy. → Descartes ray, → Cartesian coordinates, → Cartesian vortex theory.

Same as → rainbow ray.

→ Descartes; → ray.

A formula that gives the position of an image formed by highly → paraxial rays from a → spherical mirror. It is quite accurately given by: 1/x_o + 1/x_i = 2/x_C, where x_o is the distance along the → principal axis from the mirror to the object, x_i is the distance from mirror to image, and x_C is the distance from the mirror to its center of curvature. Any distance measured on the same side of the mirror as the reflecting surface is positive; on the other side, negative. Thus for a → concave mirror  x_C is positive; for a → convex mirror, negative.

→ Descartes; → formula.