Etymology
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ray (n.1)

"beam of light, light emitted in a given direction from a luminous body," early 14c., rai, from Old French rai (nominative rais) "ray (of the sun), spoke (of a wheel); gush, spurt," from Latin radius "ray, spoke, staff, rod" (see radius). Not common before 17c. [OED]; of the sun, usually in reference to heat (beam being preferred for light).

Ray is usually distinguished from beam, as indicating a smaller amount of light; in scientific use a beam is a collection of parallel rays. In ordinary language ray is the word usually employed when the reference is to the heat rather than the light of the sun .... [OED]

Science fiction's ray-gun is recorded by 1931 (in Amazing Stories; electric ray gun as an imaginary weapon is from 1924; death-ray gun from 1926 as a prop in a vaudeville act), but the Martians had a Heat-Ray weapon in "War of the Worlds" (1898).

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roentgen 

in physics, 1896, in Roentgen rays "X-rays," in recognition of German physicist Wilhem Conrad Röntgen (1845-1923), who discovered X-rays in 1895. As a unit of exposure to radiation, it is attested from 1922, proposed in French in 1921.

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ray (n.2)

"a skate, type of fish related to sharks and noted for its broad, flat body," early 14c., raie, from Old French raie (13c.) and directly from Latin raia. De Vaan describes this as a word of unknown origin but with apparent cognates in Germanic (Middle Dutch rogghe, Old English reohhe), perhaps a loan-word from a substrate language. The old etymology (Century Dictionary, etc.) was that the fish was so called from its resemblance to the rays of a fan and from the source of ray (n.1).

All skates are rays, but all rays are not called skates, this name being applied chiefly to certain small rays of the restricted genus Raia, of both Europe and America. [Century Dictionary]
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X-ray (n.)

1896, X-rays, translation of German X-strahlen, from X, algebraic symbol for an unknown quantity, + Strahl (plural Strahlen) "beam, ray." Coined 1895 by German scientist Wilhelm Conrad Röntgen (1845-1923), who discovered them, to suggest that the exact nature of the rays was unknown. As a verb by 1899. Meaning "image made using X-rays" is from 1934, earlier in this sense was X-radiograph (1899).

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sting-ray (n.)

also sting ray, 1620s, from sting + ray (n.2). First in Capt. John Smith's writings: "Stingraies, whose tailes are very dangerous ...."

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REM (n.1)

also R.E.M., rem, unit for measuring ionizing radiation, 1947, acronym of roentgen equivalent man.

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tesseract (n.)

"four-dimensional 'cube,'" 1888, from tessera + Greek aktis "ray" (see actino-).

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moonbeam (n.)

"ray of light from the moon," 1580s, from moon (n.) + beam (n.).

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mammogram (n.)

"X-ray image of the breast," by 1937, from mammo- "breast" + -gram.

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angiogram (n.)

"X-ray image of the arteries, veins, and/or heart chambers," 1933, from angio- + -gram.

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