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Narrative Description of Electromagnetic Spectrum Poster
Visible light is only one kind of electromagnetic radiation--the kind to which human eyes are sensitive. There is also invisible radiation, to which our eyes are completely insensitive.
This poster sketches the entire range of electromagnetic radiation. Starting from the left side, at the longer wavelengths (or smaller frequences), the whole spectrum includes:
- radio radiation, including radar, microwaves, and the familiar AM, FM, and TV communications bands.
- infrared radiation, which we sense as heat.
- visible light, displaying its familiar color band--red, orange, yellow, green, blue, violet--within a narrow domain near the center of the electromagnetic spectrum.
- ultraviolet radiation, lying just short of visible light, and largely responsible for suntans and sunburns.
- x-ray radiation, perhaps best known for its ability to penetrate human tissue and reveal the state of our bones.
- gamma rays, often associated with radioactivity and invariably damaging to living cells.
Technically, the length of a cycle of radiation--the wavelength--is plotted increasing toward the left (in units of centimeters); conversely, the wave frequency (in units of cycles per second, or Hertz) increases toward the right. Also note that the wavelength and frequency scales are non-linear--that is, they do not increase by equal increments of ten. Instead, the horizontal scale plots additional factors of ten for each interval shown--a so-called logarithmic scale. (In this way, we can condense the very large span of the whole electromagnetic spectrum into a manageable size; otherwise, this poster would have stretched for many light-years!)
The icons are meant to reinforce the notion that wavelengths extend from the size of mountains for radio radiation (at left) to the size of an atomic nucleus for gamma-ray radiation (at right). The superposed images of the Sun show the falsely colored views of our star as perceived at various wavelengths.
Only a small fraction of the radiation produced by cosmic objects actually reaches our eyes, in part because of the opacity of Earth's atmosphere. Opacity is the extent to which radiation is blocked by the material through which it passes--in this case, air. Atmospheric opacity--the opposite of transparency--is plotted on this poster as a variably intense silver ribbon atop the frequency scale. The extent of shading is proportional to the opacity: where shading is greatest (strong silvering or mirroring), no radiation can get in or out of our atmosphere; where there is no shading at all, the air is almost completely transparent, enabling extraterrestrial radiation to reach Earth's surface and terrestrial radiation from transmissions made by humans to pass virtually unhindered into space.
Accordingly, 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 observe the Universe at those wavelengths from ground level. Throughout the infrared range, the atmosphere is partially transparent, so we can also make limited infrared observations from the ground. Moving to the tops of mountains, above as much of the atmosphere as possible, improves such observations. In the rest of the spectrum, however, the atmosphere is opaque. Ultraviolet, x-ray, and gamma-ray observations can be made only from above the atmosphere, mostly from orbiting satellites.