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The electro-magnetic spectrum includes far more than just visible light. There are no limits to how short or long the wavelength of light can be. Wavelength and frequency are inversely related...the shorter the wavelength, the higher the frequency and energy. EMR with high frequencies, like X-ray and gamma rays, was created by high electron activity in their sources (wild explosions!). At shorter wavelengths, light behaves more like a particle; at longer wavelengths, light behaves more like a wave. Electro-magnetic radiation includes: radio has wavelengths longer than about 1 mm. The atmosphere is transparent to radio waves at wavelengths longer than a few tenths of a mm and shorter than a few hundred meters. Astrophysical radio waves are generated mostly by electrons oscillating in a magnetic field, processes similar to those we employ to generate AM and FM radio signals. FM radio and television operate at wavelengths of a few meters (about 100 megahertz), while AM radio operates at wavelengths of a few tenths of a kilometer (about 1000 kilohertz). AM radio penetrates better than FM across canyons, or into mountains, because of a wave-like property: radiowaves can bend (or diffract) around objects the size of the wavelength or smaller. Radio frequencies include radar (like on ships), microwave radiation (like ovens and cell phones), and the familiar AM, FM, and TV bands. infra-red (IR) has wavelengths longer than red light, from about 1000nm (1 micron) to about 1 millimeter. Some of this penetrates the atmosphere, mostly at wavelengths less than 10 microns. Infrared radiation is electromagnetic energy that, on contact with an object, is absorbed by the object, which in turn gives off heat. IR itself is not heat. IR radiation is generated by objects with temperatures less than about 1000K. You have body temperatures of about 300K (this is in degrees Kelvin, and is the same as 98.6 Fahrenheit and 37.0 Celsius), and radiate strongly at wavelengths of 10 microns. (This is the secret behind night vision goggles.). Light with wavelengths greater than about 100 microns is often called sub-millimeter (sub-mm) or millimeter (mm) radiation visible (optical) penetrates the atmosphere, and can be seen with the naked eye. Astronomically, the optical runs from 300-1000nm. ultraviolet (UV) is light with wavelengths between the optical and X-rays. It is often divided into extreme-UV (EUV: 10-91nm), the far-UV (FUV: 91-200nm), and near-UV (NUV, 200-300nm). This light is generated by gas with temperatures above about 1000K. Most UV light is absorbed by atmospheric ozone, but some near-UV radiation penetrates the Earth's atmosphere, and can cause sunburn. X-rays are also high energy, penetrating radiation. Wavelengths range from about 0.01 to 10nm. When you get X-rayed at your dentist's office, you get illuminated by short wavelength X-rays (about 0.012nm). These X-rays penetrate soft tissue, but are stopped by the minerals in bone. X-rays are produced by gas with temperatures of 1-100 million degrees, as in stellar coronae, or matter falling onto neutron stars or black holes. Although there are a lot of gamma rays and X-rays in the universe, Earth's atmosphere prevents it from penetrating and therefore protects us. Gamma rays. These are the highest energy photons. Gamma rays are produced by extremely energetic events, including solar flares and terrestrial lightning, and are produced by gas at temperatures near a billion degrees. Gamma rays carry much energy, and hence are dangerous to living things because they can penetrate tissue. Wavelengths are less than about 0.01nm. An important point to realize, is that the shorter the wavelength, the greater the amount of energy that is needed to create the radiation. A gamma ray burst is the result of extremely high energy levels. A flux of radiowaves is the result of a much smaller energy level. By looking at one cosmic body in a wide range of wavelengths, you can see how the energy levels are distributed. On the next page, you will read about the 'black box distribution curve'. By mapping the amounts of energy found in the various wavelengths onto a graph, the distribution pattern of a cosmic body becomes apparent.

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This image of Mars was captured in visible light by NASA's Mars Global Surveyor. You can explore the Martian surface as these ways: - hold down your left mouse button, and drag it around the orb to view it from any angle. - hold down... more