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| radiation in the form of elementary particles emitted by an atomic nucleus produced by decay of radioactive substances or by nuclear fission |
| radiation consisting of electromagnetic waves, including radio waves, infrared, visible light, ultraviolet, x-rays, and gamma rays |
| reference frame (rěf'ər-əns) Pronunciation Key
A basis of a four-dimensional coordinate system in which the first coordinate is understood as a time coordinate, while the other three coordinates represent spatial dimensions. Inertial frames and non-inertial frames are both examples of reference frames. Also called frame of reference. See also General Relativity, space-time, Special Relativity. |