Radiation are electromagnetic waves, that is, synchronized oscillations of electric and magnetic fields that propagate at the speed of light through a vacuum, each one perpendicular to each other and to the direction of wave propagation. Each electromagnetic wave can be instantiated by the frequency, i.e. the number of occurrences of a repeating event per unit time, the wavelength, i.e the distance between consecutive corresponding points of same phase and radiant energy, i.e. the energy associated with the wave. The more radiant energy a wave has, the higher its frequency is. On the contrary, the larger the wavelength of a wave, the lower its energy. The characteristic oscillations and properties of electromagnetic waves originate the electromagnetic spectrum, from low energetic radiation (e.g. infrared radiation), composed of large wavelengths, to highly energetic radiation (e.g. gamma rays and X-rays), short wavelengths. Visible light stands for a tiny fraction of the electromagnetic spectrum, more specifically waves of 400-700 nanometers wide.
Radiation and Radiant Energy
A common misconception, frequently encountered even on scientific books, is that radiation is “energy”. Radiation is not energy, as energy is a property of entities, not an entity itself. Radiation “has” energy, but “is not” energy. The same thing happens between house and color: a house “has” color, but “is not” color. The difference should remain clear. Radiation, alongside matter and fields, composes the Universe content. That is why it is wrong to say that the Universe is composed of “matter and energy”. In quantum mechanics, the photons, said to be the “particles of light”, constitutes the quanta of the electromagnetic fields, that is, tiny bundles of wave packets or ripples in the electromagnetic field. Each photon carries energy, as well as other physical properties like linear momentum and spin (angular momentum), but they are not “pure energy” as some say.
Radiation and Radioactivity
Although related, they are disparate terms. Radiation refers to oscillating electric and magnetic fields. Radioactivity is a property of unstable atoms to spontaneously decay into other elements, emitting radiation through the process. Irradiation, in turn, refers to the process of emission of radiation by a radiant source. Eradiation, oppositely, to the imission of radiation.