Key Concepts
The emission and outward propagation of energy-carrying particles or waves; also, the emitted particles or waves themselves. Radiation is a common phenomenon that transfers energy, matter, momentum, and information from one place to another. The major types of radiation are electromagnetic radiation (Fig. 1), particle radiation, acoustic radiation, and gravitational radiation. Generally speaking, any particle or wave can become radiation if it is emitted from a source and then propagates directly away from its source. In science, the word “radiation” is used in this general sense and does not necessarily imply that the particle or wave is harmful. Radiation that is directly harmful is referred to as ionizing radiation. Only ionizing radiation can, depending on the exposure, directly cause radiation sickness, mutation, and cancer. Radiation tends to travel in straight lines through space as it propagates away from its source. Radiation continues to propagate in this way until it encounters and interacts with a physical object. Through this interaction, the radiation can transfer to the object some or all of its energy, matter, momentum, or information. In the situation where all of the radiation is traveling generally in the same direction, the radiation is referred to as a beam. For instance, the radiation from a laser is known as a laser beam. Similarly, the radiation from a proton accelerator is known as a proton beam. See also: Electromagnetic radiation; Energy; Laser; Matter; Momentum
Electromagnetic radiation
Electromagnetism is one of the four fundamental interactions of the universe. The carrier of the electromagnetic interaction 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 fundamentally 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 frequency 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. See also: Fundamental interactions; Gamma ray; Infrared radiation; Light; Microwave; Photon; Ultraviolet radiation
Electromagnetic radiation is emitted any time an electrically 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 various forms of electromagnetic radiation, they are often used in wireless communication, fiber-optic communication, imaging, and heating applications. Among the various forms of electromagnetic radiation, x-rays are particularly effective at penetrating materials. For this reason, x-rays are used in the detection of concealed weapons and in medical imaging. See also: Acceleration; Bioluminescence; Cerenkov radiation; Electric charge; Fire; Incandescence; Lightning; Medical imaging; Mobile communications; Star; Synchrotron radiation; X-ray
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 radiation, neutron radiation, muon radiation, and neutrino radiation. 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. See also: Alpha particles; Atomic nucleus; Beta particles; Cosmic ray; Electron; Mass; Neutron; Proton
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 common 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 radiation 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. See also: Antimatter; Cosmic ray; Elementary particle; Helium; Hydrogen; Nuclear fission; Nuclear fusion; Supernova; Sun
Acoustic radiation
Acoustic radiation, or sound, consists of propagating patterns 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. See also: Hearing (human); Infrasound; Sound
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, loudspeakers, 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 voltages. 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 gestating fetus (colloquially "fetal ultrasound"), and the imaging of a beating adult heart (an echocardiogram). All forms of acoustic radiation are non-ionizing. See also: Biomedical ultrasonics; Earthquake; Echocardiography; Heart (vertebrate)
Gravitational radiation
A gravitational wave, also known as gravitational radiation, consists of a self-propagating pattern of spacetime curvature. Gravitational waves travel at the speed of light and are generated by the acceleration of mass (Fig. 2). German-born U.S. theoretical physicist Albert Einstein first predicted the existence of gravitational waves in his general theory of relativity in 1916. Gravitational waves proved too weak to be directly detected until 2015, when an experiment called the Laser Interferometer Gravitational-Wave Observatory (LIGO) detected waves passing through the Earth. LIGO is sensitive to a strain of one part in 1022, which is on the order of one ten-thousandth of the diameter of a proton. The gravitational waves were generated by the merger of black holes more than a billion light-years away. LIGO has since made numerous detections of black hole and neutron star mergers. Gravitational radiation is extremely weak and is non-ionizing. See also: Black hole; Gravity; Gravitational radiation; LIGO (Laser Interferometer Gravitational-wave Observatory); Neutron star; Relativity; Spacetime
Ionizing versus non-ionizing radiation
An important classification of radiation is ionizing versus non-ionizing. Ionizing radiation has enough energy per particle to eject electrons from atoms and break chemical bonds. Because of this, ionizing radiation can trigger chemical reactions, wear down materials, induce radiation sickness, produce genetic mutation, and cause cancer. In contrast, non-ionizing radiation does not have enough energy per particle to directly cause permanent damage. However, large amounts of non-ionizing radiation can still indirectly cause damage through excessive heating or pressure effects. Extreme ultraviolet rays, x-rays, gamma rays, and most types of particle radiation are ionizing. Radio waves, infrared waves, visible light, low-frequency ultraviolet, acoustic radiation, and gravitational waves are non-ionizing. See also: Cancer; Ionization; Radiation injury to plants and animals
