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The electromagnetic spectrum is the full range of electromagnetic radiation , organized by frequency or wavelength. The spectrum is divided into separate bands, with different names for the electromagnetic waves within each band.
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From low to high frequency these are: radio waves , microwaves , infrared , visible light , ultraviolet , X-rays , and gamma rays. 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. Radio waves, at the low-frequency end of the spectrum, have the lowest photon energy and the longest wavelengths—thousands of kilometers , or more.
They can be emitted and received by antennas , and pass through the atmosphere, foliage, and most building materials. 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. 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.
Longer-wavelength radiation such as visible light is nonionizing; the photons do not have sufficient energy to ionize atoms. 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. Spectroscopy is used to study the interactions of electromagnetic waves with matter.
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. The ancient Greeks recognized that light traveled in straight lines and studied some of its properties, including reflection and refraction.
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Light was intensively studied from the beginning of the 17th century leading to the invention of important instruments like the telescope and microscope. 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. Starting in , Newton showed that these colours were intrinsic to light and could be recombined into white light.
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. Huygens in particular had a well developed theory from which he was able to derive the laws of reflection and refraction. Around , Thomas Young measured the wavelength of a light beam with his two-slit experiment thus conclusively demonstrating that light was a wave.
In , William Herschel discovered infrared radiation. He noticed that the highest temperature was beyond red. He theorized that this temperature change was due to "calorific rays", a type of light ray that could not be seen. The next year, Johann Ritter , working at the other end of the spectrum, noticed what he called "chemical rays" invisible light rays that induced certain chemical reactions.
These behaved similarly to visible violet light rays, but were beyond them in the spectrum. The study of electromagnetism began in when Hans Christian Ørsted discovered that electric currents produce magnetic fields Oersted's law.
Light was first linked to electromagnetism in , when Michael Faraday noticed that the polarization of light traveling through a transparent material responded to a magnetic field see Faraday effect. During the s, James Clerk Maxwell developed four partial differential equations Maxwell's equations for the electromagnetic field. Two of these equations predicted the possibility and behavior of waves in the field.
Analyzing the speed of these theoretical waves, Maxwell realized that they must travel at a speed that was about the known speed of light. This startling coincidence in value led Maxwell to make the inference that light itself is a type of electromagnetic wave.
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Maxwell's equations predicted an infinite range of frequencies of electromagnetic waves , all traveling at the speed of light. This was the first indication of the existence of the entire electromagnetic spectrum. Maxwell's predicted waves included waves at very low frequencies compared to infrared, which in theory might be created by oscillating charges in an ordinary electrical circuit of a certain type.
Attempting to prove Maxwell's equations and detect such low frequency electromagnetic radiation, in , the physicist Heinrich Hertz built an apparatus to generate and detect what are now called radio waves. Hertz found the waves and was able to infer by measuring their wavelength and multiplying it by their frequency that they traveled at the speed of light.