A debate arose over whether light had a wave nature or a particle nature with René Descartes, Robert Hooke and Christiaan Huygens favouring a wave description and Newton favouring a particle description. Starting in 1666, Newton showed that these colours were intrinsic to light and could be recombined into white light. Isaac Newton was the first to use the term spectrum for the range of colours that white light could be split into with a prism. Light was intensively studied from the beginning of the 17th century leading to the invention of important instruments like the telescope and microscope. The ancient Greeks recognized that light traveled in straight lines and studied some of its properties, including reflection and refraction. Humans have always been aware of visible light and radiant heat but for most of history it was not known that these phenomena were connected or were representatives of a more extensive principle. See also: History of electromagnetic theory, History of radio, History of electrical engineering, and History of optics Spectroscopy is used to study the interactions of electromagnetic waves with matter. Throughout most of the electromagnetic spectrum, spectroscopy can be used to separate waves of different frequencies, so that the intensity of the radiation can be measured as a function of frequency or wavelength. Visible light and radiation of longer wavelengths are nonionizing their photons do not have sufficient energy to cause these effects. Gamma rays, X-rays, and extreme ultraviolet rays are called ionizing radiation because their high photon energy is able to ionize atoms, causing chemical reactions. Gamma rays, at the high-frequency end of the spectrum, have the highest photon energies and the shortest wavelengths-much smaller than an atomic nucleus. They can be emitted and received by antennas, and pass through the atmosphere, foliage, and most building materials. Radio waves, at the low-frequency end of the spectrum, have the lowest photon energy and the longest wavelengths-thousands of kilometers, or more. The electromagnetic waves in each of these bands have different characteristics, such as how they are produced, how they interact with matter, and their practical applications. From low to high frequency these are: radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, and gamma rays. The spectrum is divided into separate bands, with different names for the electromagnetic waves within each band. The electromagnetic spectrum is the full range of electromagnetic radiation, organized by frequency or wavelength. Range of frequencies or wavelengths of electromagnetic radiation A diagram of the electromagnetic spectrum, showing various properties across the range of frequencies and wavelengths, as well as the equivalent blackbody temperature
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