Electro-magnetic Radiation


	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.

	All these spectral regions, including the visible, collectively make up the electromagnetic spectrum. Remember that, despite their greatly differing wavelengths and the very different roles they play in everyday life on Earth, all types of radiation are basically the same phenomenon, and all move at the same speed—the speed of light c, 300,000 kilometers per second, in a vacuum. This is an extremely high speed. In the time it takes to snap a finger, about a tenth of a second, light can travel three quarters of the way around our planet. Just like us, it slows down a little when the path it takes goes through a denser medium, like when you run through a mud puddle. The picture above is worth studying carefully, as it contains a great deal of information. Note that wave frequency (in hertz, which is number of waves per second) increases from left to right, and wavelength (in meters) increases from right to left. The slide bar midway down the picture lets you draw waves of varying frequencies. Check it out. These wave properties behave in opposite ways because, as noted earlier, they are inversely related. Notice that the wavelength and frequency scales do not increase by equal increments of 10. Instead, successive values marked on the horizontal axis differ by factors of 10—each successive value is 10 times greater than its neighbor. This type of scale, called a logarithmic scale, is often used in science in order to condense a very large range of some quantity into a manageable size. Had a linear scale been used for the wavelength range shown above, the figure would have been many light years long! Another important characteristic of EMR is that no single wave changes its wavelength or frequency. Once the wave starts at the source, that wavelength, and therefore frequency, are fixed for its duration. Only the amplitude changes. The picture shows wavelengths extending from the height of mountains for radio radiation to the diameter of an atomic nucleus for gamma-ray radiation. The box at the upper right emphasizes how small the visible portion of the electromagnetic spectrum is. Most objects in the universe emit large amounts of invisible radiation. Indeed, many objects emit only a tiny fraction of their total energy in the visible range. A wealth of extra knowledge can be gained by studying the invisible regions of the electromagnetic spectrum. Only a small fraction of the radiation arriving at our planet actually reaches Earth's surface because of the opacity of Earth's atmosphere. Opacity is the extent to which radiation is blocked by the material through which it is passing—in this case, air. The more opaque an object is, the less radiation gets through. Earth's atmospheric opacity is plotted along the wavelength and frequency scales at the bottom of the picture. The extent of shading is proportional to the opacity. Where the shading is greatest, no radiation can get in or out—the energy is completely absorbed by atmospheric gases. Where there is no shading at all, our atmosphere is almost totally transparent. Extraterrestrial radiation in these regions of the electromagnetic spectrum can reach Earth's surface and terrestrial radiation from human transmissions can pass virtually unhindered into space. There are only a few windows, at well-defined locations in the electromagnetic spectrum, where Earth's atmosphere is transparent. In much of the radio and in the visible portions of the spectrum, the opacity is low, so we can study the universe at those wavelengths from ground level. In parts of the infrared range, the atmosphere is partially transparent, so we can make certain infrared observations from the ground. In the rest of the spectrum, however, the atmosphere is opaque. As a result, ultraviolet, X-ray, and gamma-ray observations can be made only from above the atmosphere, from orbiting satellites.
Coming up next: Black Body Radiation Distribution Curve