The electromagnetic spectrum is a continuous range of frequencies or wavelengths (each determines the other) of electromagnetic radiation. The spectrum ranges from long-wavelength, low frequency radio waves to short-wavelength, high frequency gamma rays. The electromagnetic spectrum is traditionally divided into radio waves, microwaves, infrared radiation, visible light, ultraviolet rays, x rays, and gamma rays. The divisions between these types of rays are invented, not physical.
Scottish physicist James Clerk Maxwell's (1831–1879) development of a set of equations that accurately described electromagnetic phenomena allowed the mathematical and theoretical unification of electrical and magnetic phenomena. When Maxwell's calculated speed of light fit well with experimental determinations of the speed of light, Maxwell and other physicists realized that visible light should be a part of a broader electromagnetic spectrum containing forms of electromagnetic radiation that varied from visible light only in terms of wavelength and wave frequency. Frequency is defined as the number of wave cycles that pass a particular point per unit time, and is commonly measured in Hertz (cycles per second). Wavelength defines the distance between adjacent points of the electromagnetic wave that are in equal phase (e.g., wavecrests).
Exploration of the electromagnetic spectrum quickly resulted practical advances. German physicist Henrich Rudolph Hertz regarded Maxwell's equations as a path to a “kingdom” or “great domain” of electromagnetic waves. Based on this insight, in 1888, Hertz demonstrated the existence of radio waves. A decade later, Wilhelm Röentgen's discovery of high-energy electromagnetic radiation in the form of x rays quickly found practical medical use.
At the beginning of the twentieth century, German physicist, Maxwell Planck, proposed that atoms absorb or emit electromagnetic radiation only in certain bundles termed quanta. In his work on the photoelectric effect, German-born American physicist Albert Einstein used the term photon to describe these electromagnetic quanta. Planck determined that energy of light was proportional to its frequency (i.e., as the frequency of light increases, so does the energy of the light). Planck's constant, h = 6.626 × 10−34 joule-second in the meter-kilogram-second system (4.136 × 10−15 eV-sec), relates the energy of a photon to the frequency of the electromagnetic wave and allows a precise calculation of the energy of electromagnetic radiation in all portions of the electromagnetic spectrum.
Although electromagnetic radiation is now understood as having both photon (particle) and wavelike properties, descriptions of the electromagnetic spectrum generally utilize traditional wave-related terminology (i.e., frequency and wavelength).
Electromagnetic fields and photons exert forces that can excite electrons. As electrons transition between allowed orbitals, energy must be conserved. This conservation is achieved by the emission of photons when an electron moves from a higher potential orbital energy to a lower potential orbital energy. Accordingly, light is emitted only at certain frequencies characteristic of every atom and molecule. Correspondingly, atoms and molecules absorb only a limited range of frequencies and wavelengths of the electromagnetic spectrum, and reflect all the other frequencies and wavelengths of light. These reflected frequencies and wavelengths are often the actual observed light or colors associated with an object.
The region of the electromagnetic spectrum that contains light at frequencies and wavelengths that stimulate the rod and cones in the human eye is termed the visible region of the electromagnetic spectrum. Color is the association the eye and brain make with various frequencies in the visible region; that is, particular colors are associated with specific wavelengths of visible light Mixed wavelengths produce more complex color sensations. A nanometer (10−9 m) is the most common unit used for characterizing the wavelength of visible light. Using this unit, the visible portion of the electromagnetic spectrum is located between 380 nm–750 nm and the component color regions of the visible spectrum are red (670–770 nm), orange (592–620 nm), yellow (578–592 nm), green (500–578 nm), blue (464–500 nm), indigo (444–464 nm), and violet (400–446 nm). Because the energy of electromagnetic radiation (i.e., the photon) is inversely proportional to the wavelength, red light (longest in wavelength) is the lowest in energy. As wavelengths contract toward the blue end of the visible region of the electromagnetic spectrum, the frequencies and energies of colors steadily increase.
Like colors in the visible spectrum, other regions in the electromagnetic spectrum have distinct and important components. Radio waves, with wavelengths that range from hundreds of meters to less than a centimeter, transmit radio and television signals. Within the radio band, FM radio waves have a shorter wavelength and higher frequency than AM radio waves. Still higher frequency radio waves with wavelengths of a few centimeters can be utilized for radar imaging.
Microwaves range from approximately 1 ft (30 cm) in length to the thickness of a piece of paper. The atoms in food placed in a microwave oven become agitated (heated) by exposure to microwave radiation. Infrared radiation comprises the region of the electromagnetic spectrum where the wavelength of light is measured region from one millimeter (in wavelength) down to 400 nm. Infrared waves are discernible to humans as thermal radiation (heat). Just above the visible spectrum in terms of higher energy, higher frequency and shorter wavelengths is the ultraviolet region of the spectrum with light ranging in wavelength from 400 to 10 billionths of a meter. Ultraviolet radiation is a common cause of sunburn even when visible light is obscured or blocked by clouds. X rays are a highly energetic region of electromagnetic radiation with wavelengths ranging from about ten billionths of a meter to 10 trillionths of a meter. The ability of x rays to penetrate skin and other substances renders them useful in both medical and industrial radiography. Gamma rays, the most energetic form of electromagnetic radiation, are light with wavelengths of less than about ten trillionths of a meter and include waves with wavelengths smaller than the radius of an atomic nucleus (1015 m). Gamma rays are generated by nuclear reactions (e.g., radioactive decay and nuclear explosions).
Cosmic rays are not a part of the electromagnetic spectrum because they are not a form of electromagnetic radiation. Rather, they are high-energy charged particles with energies similar to, or higher than, observed gamma electromagnetic radiation energies.
Wavelength, frequency, and energy
The wavelength of radiation is sometimes given in units with which we are familiar, such as inches or centimeters, but for very small wavelengths, they are often given in angstroms (abbreviated Å). There are 10,000,000,000 angstroms in 3.3 ft (1 m).
An alternative way of describing a wave is by its frequency, or the number of peaks which pass a particular point in one second. Frequencies are normally given in cycles per second, or hertz (abbreviation Hz), after Hertz. Other common units are kilohertz (kHz, or thousands of cycles per second), megahertz (MHz, millions of cycles per second), and gigahertz (GHz, billions of cycles per second). The frequency and wavelength, when multiplied together, give the speed of the wave. For electromagnetic waves in empty space, that speed is the speed of light, which is approximately 186,000 miles per second (300,000 km per sec).
In addition to the wavelike properties of electromagnetic radiation, it also can behave as a particle. The energy of a particle of light, or photon, can be calculated from its frequency by multiplying by Planck's constant. Thus, higher frequencies (and lower wavelengths) have higher energy. A common unit used to describe the energy of a photon is the electron volt (eV). Multiples of this unit, such as keV (1000 electron volts) and MeV (1,000,000 eV), are also used.
Properties of waves in different regions of the spectrum are commonly described by different notation. Visible radiation is usually described by its wavelength, for example, while x rays are described by their energy. All of these schemes are equivalent, however; they are just different ways of describing the same properties.
The electromagnetic spectrum is typically divided into wavelength or energy regions, based on the characteristics of the waves in each region. Because the properties vary on a continuum, the boundaries are not sharp, but rather loosely defined (Table 1).
Radio waves are familiar to us due to their use in communications. The standard AM radio band is at 540–1650 kHz, and the FM band is 88–108 MHz. This region also includes shortwave radio transmissions and television broadcasts.
We are most familiar with microwaves because of microwave ovens, which heat food by causing water molecules to rotate at a frequency of 2.45 GHz. In astronomy, radiation emitted at a wavelength of 8.2 inches (21 cm) has been used to map neutral hydrogen throughout the galaxy. Radar is also included in this region.
The infrared region of the spectrum lies just beyond the visible wavelengths. It was discovered by William Herschel in 1800 by measuring the dispersing sunlight with a prism, and measuring the temperature increase just beyond the red end of the spectrum.
The visible wavelength range is the range of frequencies with which we are most familiar. These are the wavelengths to which the human eye is sensitive, and which most easily pass through Earth's atmosphere. This region is further broken down into the familiar colors of the rainbow, which fall into the wavelength intervals listed in Table 2.
A common way to remember the order of colors is through the name of the fictitious person ROY G. BIV (the I stands for indigo).
The ultraviolet range lies at wavelengths just short of the visible. Although humans do not use UV to see, it has many other important effects on Earth. The ozone layer high in Earth's atmosphere absorbs much of the UV radiation from the sun, but that which reaches the surface can cause suntans and sunburns.
We are most familiar with x rays due to their uses in medicine. X radiation can pass through the body, allowing doctors to examine bones and teeth. Surprisingly, x rays do not penetrate Earth's atmosphere, so astronomers must place x-ray telescopes in space.
Gamma rays are the most energetic of all electromagnetic radiation, and we have little experience with them in everyday life. They are produced by nuclear processes, for example, during radioactive decay or in nuclear reactions in stars or in space.