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Multiwavelength Milky Way: Electromagnetic Spectrum

Visible and invisible light

mosaic image of portions of separate Milky Way images

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I. The visible rainbow
II. Long-wavelength light - infrared and radio waves
III.Short-wavelength light - ultraviolet, x-rays, and gamma rays

light - a form of energy having wave-like and particle-like properties.

I. We see by visible light. We characterize visible light by colors ranging from red to green to purple. The range of visible colors is often called a "rainbow." People throughout the ages have been familiar with the rainbow. It was realized that a rainbow could be produced by sunlight (white light) passing through a prism. In 1672, Isaac Newton studied this and described how white light is actually a mixture of colored light.

prism and rainbow

As mentioned on the preceding page, every form of light is associated with a wavelength and an energy. The color red has the longest wavelength of the visible spectrum. Its wavelength is around 700 nanometers (that's the same as 0.00007 centimeters). Every shade of red has its own unique wavelength. It is kind of like a fingerprint for light! The energy associated with a 700 nm wavelength is 1.77 eV (see Planck's Law on how to calculate this), which is the lowest energy for visible light (again, see Planck's Law for more on this). "eV" stands for electron volt, and is a unit of energy. 1 eV = 1.602 x 10-19 joules. The color purple (or violet) has the shortest wavelength of the visible spectrum, at around 400 nm.

rainbow and wavelengths

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II. Past the red end of the visible spectrum is light of longer wavelengths, that our eyes cannot see. This area of the electromagnetic spectrum includes infrared light, microwaves, and radio waves (yes! radio waves are a form of light!).
Infrared light has wavelengths that extend from the visible red to about 1 mm (there are 1,000,000 microns in a meter). Infrared waves include thermal radiation, or heat. Sir Frederick William Herschel, a German astronomer and muscian, is credited for having discovered infrared light. He discovered infrared light in 1800, while conducting experiments on the temperatures of visible light colors. See the list of Related Online Resources below to learn more about Herschel's discovery.
Microwaves have a longer wavelength than infrared light, between 1 mm and 30 cm. The term "microwave," is attributed to Nello Carrara, an Italian physicist who used it is his publication The Detection of Microwaves (1932). The term became widely used around 1940.
Radio waves have the longest wavelengths of the electromagnetic spectrum, and the lowest energy (or frequency). Radio waves pick up where microwaves leave off. This light has wavelength a few centimeters and longer. The discovery and understanding that radio waves are a form of electromagnetic radiation, has its beginnings in the mid-1800s. In the 1870s, James Clerk Maxwell, a physicist, developed a theory of electromagnetic waves. About fifteen years later, another physicist, Heinrich Hertz, proved Maxwell's theory by producing these waves electrically. Upon the foundation built by Maxwell and Hertz, Sir Oliver Joseph Lodge perfected a device called a "coherer," which could detect radio waves. Guglielmo Marconi carried the "invention" of radio farther by developing an apparatus that allowed him to produce and detect radio waves over long distances.

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III. Beyond the violet end of the visible spectrum is light of shorter wavelengths and higher energies. Our eyes cannot detect this light either. This area of the electromagnetic spectrum includes ultraviolet light, x-rays, and gamma rays.
Ultraviolet light extends from the violet region of the visible spectrum, down to about 10 nm. Following Herschel's discovery of infrared light, Johann Ritter discovered ultraviolet radiation in 1801. By observing the effects of radiation from the sun on silver chloride, Ritter determined that radiation existed beyond the visible violet.
X-rays have wavelengths shorter than about 10 nm, but longer than about 0.01 nm. X-rays were discovered in 1895 by Wilhelm Roentgen, a physicist, while looking for cathode rays from glass vacuum tubes.
Gamma rays have the shortest wavelengths of the electromagnetic spectrum, and the highest energies. Gamma rays have such short wavelengths, that astronomers usually discuss them in terms of energy. Gamma rays have energies greater than about 100 keV (keV stands for kilo-electron volt. There are 1000 eV in 1 keV). The discovery of gamma rays is primarily credited to Paul Villard in 1900, who recognized that they could penetrate material more deeply than x-rays, and Ernest Rutherford, who determined in 1914 that gamma rays were a form a light with wavelengths shorter than x-rays.

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Related Online Resources:
Color Science - spectra, chromaticity, blackbody, color links
Discovery of Infrared - an account of Herschel's discovery of infrared light
Electromagnetic Spectrum - an introduction by Imagine the Universe

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