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Ward's World+MGH Radiation - Teacher Key

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Radiation (continued) + ward ' s science Electromagnetic radiation Electromagnetism is one of the four fundamental interac- tions of the universe. The carrier of the electromagnetic interac- tion is the massless, elementary particle known as the photon. All forms of electromagnetic radiation consist of photons. The form that is most familiar in everyday life is visible light. All other forms of electromagnetic radiation are fundamen- tally the same as visible light, except that they happen to be invisible to humans. Each photon has a set of unchangeable properties, including energy, wavelength, and frequency. The energy of a photon is directly proportional to its frequency and inversely proportional to its wavelength. Therefore, all forms of electromagnetic radiation can be ordered on a spectrum from low photon energy (corresponding to low frequency and long wavelength) to high photon energy (corresponding to high fre- quency and short wavelength). Starting with the lowest photon energy, the electromagnetic spectrum consists of: radio waves, infrared waves, visible light, ultraviolet rays, x-rays, and gamma rays. Of the various forms, only extreme ultraviolet rays, x-rays, and gamma rays are ionizing radiation. Each band (interval of frequency) in the electromagnetic spectrum is further divided into subbands. For instance, radio waves include microwaves, terahertz waves, and the various broadcast bands. Electromagnetic radiation is emitted any time an electri- cally charged particle accelerates. The mechanisms that emit electromagnetic radiation include: infrared thermal radiation, incandescence, braking radiation, Cerenkov radiation, cyclotron radiation, synchrotron radiation, antenna radiation, particle- antiparticle annihilation, and the radiation emitted during quantum transitions. Quantum transitions can take the form of molecular transitions, atomic transitions, nuclear transitions, and energy-band transitions. The emission of electromagnetic radiation occurs in manmade devices such as light bulbs, lasers, computer displays, and radio antennas; as well as in natural sources such as fires, stars, lightning bolts, and fireflies. Because of the ease of generating, transmitting, and detecting the vari- ous forms of electromagnetic radiation, they are often used in wireless communication, fiber-optic communication, imaging, and heating applications. Among the various forms of electro- magnetic radiation, x-rays are particularly effective at penetrat- ing materials. For this reason, x-rays are used in the detection of concealed weapons and in medical imaging. Particle radiation Particle radiation consists of rapidly moving particles with mass. Although x-rays and gamma rays are electromagnetic in nature and do not have mass, they are also sometimes classified as particle radiation. Common examples of particle radiation include alpha radiation, beta radiation, proton radia- tion, neutron radiation, muon radiation, and neutrino radia- tion. Alpha radiation consists of high-speed helium nuclei. Beta radiation consists of high-speed electrons or positrons. As the names suggest, proton radiation consists of high-speed protons, neutron radiation consists of high-speed neutrons, and so forth. Particle radiation is often emitted during nuclear reactions. Nuclear fusion, nuclear fission, and radioactive decay can all lead to the emission of alpha radiation, beta radiation, gamma radiation, proton radiation, neutron radiation, and neutrino radiation. In everyday life, radioactive decay is the most com- mon source of particle radiation. In fact, radioactive decay occurs steadily in almost all objects. For this reason, radioactive decay can be used to determine the age of an object. Other mechanisms that produce significant amounts of particle radia- tion include solar flares, supernovas, lightning, cosmic ray air showers, and black hole jets. Most forms of particle radiation are ionizing. Because of their ability to penetrate materials, the various forms of particle radiation are used in medical imaging, cancer treatment, and industrial subsurface imaging. Acoustic radiation Acoustic radiation, or sound, consists of propagating pat- terns of vibration in a medium such as a gas, liquid, or solid. All acoustic radiation may be classified according to frequency as infrasonic, sonic, or ultrasonic. Infrasonic radiation includes all sound waves with frequencies below the range that humans can hear. Sonic radiation, which includes all audible sound waves, spans the frequency range from about 16 to 20,000 Hz. Ultrasonic radiation includes all frequencies above the range that humans can hear. Infrasonic radiation can result from explosions, earthquakes, or other sources of low-frequency vibration. When traveling through the ground, infrasonic radiation is known as a seismic wave. Sonic radiation is often produced by the collision, the rubbing, or the vibration of everyday objects. Common sources of sonic radiation include musical instruments, loudspeak- ers, vocal chords, footsteps, and machinery. Sonic radiation is used extensively by biological organisms for communication. Ultrasonic radiation can be produced by means of crystals which vibrate rapidly in response to alternating electric volt- ages. Because of its ability to safely penetrate tissue, ultrasonic radiation is widely used in medical imaging. Examples include obstetric ultrasonography, such as the imaging of a gestat- ing fetus (colloquially "fetal ultrasound"), and the imaging of a beating adult heart (an echocardiogram). All forms of acoustic radiation are non-ionizing.

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