An Etymological Dictionary of Astronomy and Astrophysics
English-French-Persian

The angle formed between a ray of light striking a surface and the normal to that surface at the point of incidence. Also called → incidence angle.

→ angle; → incidence.

1) Fact, event, or condition of coinciding.
2) The occurrence of events simultaneously in a striking manner but without any causal connexion between them.

→ coincide.

Light striking a surface at an angle almost perpendicular to the normal. → grazing-incidence telescope.

→ grazing; → incidence.

A telescope design used for focusing → extreme ultraviolet, → X-rays, and → gamma rays by means of → grazing incidence. Such short wavelengths do not reflect in the same manner as at the large incidence angles employed in optical and radio telescopes. Instead, they are mostly absorbed. To bring X-rays to a → focus, one has to use a different approach from → Cassegrain or other typical → reflecting telescopes. In a grazing-incidence telescope, incoming light is almost → parallel to the → mirror surface and strikes the mirror → surface at a very → shallow angle. Much like skipping a stone on the water by throwing it at a low angle to the surface, X-rays may be → deflected by mirrors arranged at low incidence angles to the incoming energy. Several designs of grazing-incidence mirrors have been used in various → X-ray telescopes, including → plane mirrors or combinations of → parabolic and → hyperbolic surfaces. To increase the collecting area a number of mirror elements are often nested inside one another. For example, the → Chandra X-ray Observatory uses two sets of four nested grazing-incidence mirrors to bring X-ray photons to focus onto two → detector instruments. → Bragg's law; → X-ray astronomy.

→ grazing incidence; → telescope.

1) The degree, extent, or frequency of occurrence, especially of something unwanted.
2) The act of striking a → light ray or → beam of particles on a surface. Also the direction of striking.

M.E., from M.Fr. incidence, from L.L. incidentia, from incidere "to happen, befall," from in- "on" + -cidere, combining form of cadere "to fall," → case.

Fotâd,literally "fall, the act of falling," from fotâdan "to fall," variant of oftâdan "to fall; to happen," → coincide.

Same as → angle of incidence.

→ incidence; → angle.