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The transmission of energy through space by electromagnetic waves or particles. Of electromagnetic waves, the most familiar is the visible light spectrum. Longer wavelengths than visible light have infrared, microwave, or radio radiation; shorter wavelengths have ultraviolet, X-rays, and gamma radiation. From particle radiation, we distinguish alpha and beta radiation, neutrons, and protons. Radiation is often categorised as ionizing or non-ionizing according to the energy of the particles, because particles with high energies have the ability to strip electrons from the electron shell of atoms (ionize).