X Rays
X rays are a form of high-energy electromagnetic radiation. Electromagnetic radiation, or light energy, occurs as a continuous gradient, or spectrum, of forms. Exhibiting wave-like properties, the electromagnetic spectrum consists of light energy of many differing wavelengths and frequencies. Wavelength is the distance between the same points on adjacent light waves, and frequency is defined as the number of waves that pass a given point within a specified time period (wave fronts per second, or Hertz). Therefore, smaller wavelengths have greater frequencies, and vice versa. The entire electromagnetic spectrum spans wavelengths smaller than and greater than 100,000 meters. On one end of the spectrum, radio waves are very long wavelength light waves (around 100,000 m or more) with low frequency. On the other end, x rays are very short wavelength (10-13 meters or less), high frequency waves. X rays specifically have wavelengths that range from 10-8 to 10-10 meters.
X rays were produced artificially for the first time in 1895 by Wilhelm Röntgen. They are produced when electrons, accelerated through high voltages, strike metal. The resulting collision releases x-ray radiation. Also, like other forms of light, x rays are produced naturally. The Sun, for example, radiates x rays in addition to the visible spectrum (the spectrum visible to humans). Because x rays are a form of light, their energy is inversely proportional to their wavelength. Thus, since x rays have very short wavelengths, x-ray photons have very high energy. This intense energy makes x rays a form of penetrating radiation. In other words, x rays penetrate, and sometimes pass entirely through, matter.
X rays can traverse several centimeters of solid material, depending upon its density. When x rays pass through living tissue, their energy can damage cells. Damage to cellular DNA is of particular concern. X rays can cause chromosomal aberrations or mutation that can lead to cancer upon prolonged, or chronic, exposure. Therefore, shielding is used when x rays are employed. The penetrability of x rays is limited, however, and dense materials such as lead effectively block x-ray radiation.
Because of their penetrating energy, x rays have many practical applications. In medicine and industry, x rays are used to visualize interior structures in a manner not possible using visible light wavelengths. X-ray imaging exposes photographic film to x rays that pass through an object (living or inanimate). The darkening of the film is proportional to the degree of x-ray exposure. Thus, lighter areas are denser than darker areas. In this way, cracks in human bone or imperfections in metal castings are detected. A more complex medical application of x rays is Computerized Axial Tomography, or CAT scan, sometimes shortened to CT scan. CAT scans take multiple, rotating x-ray exposures through a subject that are recorded digitally within a computer. As the exposures progress longitudinally along the patient, digital transverse sections are formed that in composite can create a three-dimensional x-ray image of the patient's interior. Because CAT scans have greater sensitivity than x-ray techniques used to assess bone fractures or dental health, they are able to detect some forms of cancerous tumors that would be undetected by other forms of x-ray analysis. X rays are also important to the study of crystals. When x rays pass through crystals, they are scattered by electrons in the atoms that make up the crystal lattice. Because the atoms are close and spaced in a regular fashion, the scattered x-rays will interfere with each other in a process called diffraction. The diffraction pattern that is created can be recorded and then used to deduce the structure of the crystal.
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