Radiation
Radiation has two meanings in physics. The first refers to propagating energy, and sometimes to the energy itself. An example is a light wave--its photons each have energy. The second meaning is the process of a body emitting energy, as in a pipe filled with hot water that radiates heat.
Radiation (in the first sense above) comes in several forms: electromagnetic radiation, acoustic radiation, and particle radiation.
Electromagnetic radiation is the radiation composed of electromagnetic waves, or, from another point of view, photons. In the relatively small band of wavelengths in which our eyes can see, we call it light. At longer wavelengths, beyond our eyes' capabilities, we call it infrared radiation, and beyond that microwave radiation, then short radio waves, radio waves, and long radio waves. At shorter wavelengths we refer to light as ultraviolet radiation, x-rays, and gamma rays.
Acoustic radiation, or waves of sound, is also described by its wavelength. Infrasonic waves are those with the lowest frequencies (the longest wavelength), sonic waves are longer and overlap the range of human hearing, and ultrasonic waves have high frequencies (short wavelengths) and have been utilized to produce images of organs and other objects in the body such as a fetus.
Particle radiation are the alpha and beta particles, and gamma rays, of radioactivity, created from transitions in the nuclei of atoms. An alpha particle is a doubly-ionized helium atom, a beta particle is an energetic electron, and a gamma particle is an energetic photon in the x-ray region of the spectrum. Cosmic rays--radiation from space--consists largely of very high-energy particles such as protons, neutrons, heavier nuclei, beta particles and other, more exotic particles.
Radiation has a wide variety of impacts on the human body, depending on its wavelength. Visible light, with a wavelength of 300-700 nanometers (nm), is not harmful to the eye or skin. Shorter wavelength radiation like ultraviolet radiation, with a wavelength around 100-300 nm, can cause skin cancer and other problems with long exposures. (Most ultraviolet radiation from the sun is blocked by the Earth's ozone layer.) X-rays have wavelengths of about 10-0.01 nm, are highly penetrating, and are used in medicine and the study of crystal structure. Gamma rays have extremely small wavelengths, less than 0.01 nm, and are very energetic. They are used in medicine for cancer treatment, and for diagnostic, and in industry for the inspection of castings and welds.
In the other direction, infrared radiation has a wavelength of about 1 micrometer, and is thermal radiation, or heat. Radio waves have a wide variety of wavelengths, from about 1 mm to more than 1 km, and include radar waves. Biological effects from this extremely long wavelength, very low frequency radiation is not yet completely understood, and has often been termed electromagnetic pollution.
Particle radiation can be extremely dangerous, and is called ionizing radiation because it can cause ionization of the atoms in bodily tissue. (X-rays and gamma rays also fall in this category.) Cells lose their reproductive capacity when exposed to ionizing radiation. Effects may take years to appear, and include genetic mutations passed along to subsequent generations, tumor development, and shortening of life span. It is important to stay above a certain threshold exposure when exposed to ionizing radiation.
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