The electromagnetic spectrum categorizes types of radiation according to wavelength, with the shortest wavelengths being cosmic rays, and in increasing wavelength, gamma rays, X-rays, ultraviolet rays, visible rays, infrared rays, microwave rays, radio waves, and television waves. The environment contains a low level of constant background radiation, mostly from the radioactive decay of minerals and radioactive gases such as radon and thoron. Some background radiation, known as cosmic radiation, comes from space. The sun emits solar radiation consisting of visible light, ultraviolet radiation, and infrared waves spanning the entire spectrum of electromagnetic wavelengths from radio waves to X-rays. The sun also emits high-energy particles such as electrons, especially from solar flares. X-rays, because of their extremely short wavelength, are able to penetrate soft tissue and some sands and soils and reflect off internal denser material such as bones or rocks. This property has made X-rays useful for diagnostic medicine and geologic mapping of subsurface materials. short-wavelength radiation is measured in nanometers (nm), where 1 nm equals 3.937 x 10-8 inches). Short-wavelength, high-frequency elec tromagnetic waves from 3.93 x 10-7 to 1.57 x 10-5 inch (10-400 nm) are known as ultraviolet radiation, which is powerful and useful to people for many applications but harmful in strong doses. Visible radiation includes all that humans see with their eyes, including the wide range of colors of the rainbow. Higher energy forms of electromagnetic radiation such as infrared, microwave, X-rays, and gamma rays can ionize materials and damage tissue.
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