- Related Topics:
- light
- luminescence
- X-ray
- photoelectric effect
- emission
- On the Web:
- National Center for Biotechnology Information - Natural Radioactivity and Radiation (Oct. 25, 2026)
radiation
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What is radiation in physics?
In physics, radiation refers to the flow of energy as atomic and subatomic particles and waves. These waves include heat rays, light rays, and X-rays. All matter is constantly bombarded by radiation from cosmic and terrestrial sources.
Electromagnetic radiation involves energy flow at light speed through space or a material medium via electric and magnetic fields. Examples include radio waves, visible light, and gamma rays. Quantum theory describes electromagnetic radiation as photons moving at light speed, with energy proportional to frequency.
Ionizing radiation includes electromagnetic waves and particle radiation that can create ions by interacting with atoms or molecules. Examples of electromagnetic waves are X-rays and gamma rays. Particle radiation involves subatomic particles like protons and alpha particles.
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What are the main types of radiation?
Radiation is the emission or transmission of energy in the form of waves or particles. It can be categorized into two primary types:
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Electromagnetic radiation Electromagnetic radiation includes gamma rays, X-rays, visible light and radio waves. These waves transfer energy through space and differ in wavelength and frequency.
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Particle radiation Particle radiation consists of subatomic particles, such as alpha particles, beta particles, neutrons, protons, and carbon ions. Alpha particles are identical to helium nuclei, while beta particles are electrons. Alpha decay is a type of radioactive disintegration in which unstable atomic nuclei dissipate excess energy by spontaneously ejecting an alpha particle. Beta decay is any of three processes of radioactive disintegration by which some unstable atomic nuclei spontaneously dissipate excess energy and undergo a change of one unit of positive charge without any change in mass number.
Ionizing radiation, including X-rays, gamma rays and particle beams, can be directly or indirectly ionizing. Directly ionizing radiation disrupts the structure of tissue, while indirectly ionizing radiation produces fast-moving particles that damage tissue.
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How is radiation different from conduction and convection?
Radiation, conduction, and convection are all mechanisms of heat transfer. Conduction transfers energy between adjacent molecules and is usually slow. It occurs through molecular or atomic agitation and contact between particles. Convection transfers heat through the movement of a heated fluid, such as air or water, and is fairly rapid. Radiation transmits energy as electromagnetic radiation, which is emitted by a heated surface and absorbed by another surface. Radiation doesn't need a medium to transfer energy.
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What is electromagnetic radiation?
Electromagnetic radiation is the flow of energy at the speed of light through space or a material medium. This energy takes the form of electric and magnetic fields, creating electromagnetic waves like radio waves, visible light, and gamma rays. These waves are characterized by their intensity and frequency, with electric and magnetic fields linked at right angles to each other and perpendicular to the direction of motion.
In quantum theory, electromagnetic radiation consists of photons, which are energy packets that always move at the speed of light. These photons interact with charged particles, and the resulting phenomena depend on the radiation's frequency. The electromagnetic spectrum ranges from low-frequency radio waves to high-frequency gamma rays.
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How does ionizing radiation affect living things?
Ionizing radiation's impact on living things stems from its ability to create ions when it interacts with atoms, leading to the generation of chemically reactive radicals that can harm DNA and proteins. This type of radiation includes electromagnetic waves (gamma and X-rays) and particle radiation (alpha and beta particles, neutrons, and positrons).
Exposure to high doses of ionizing radiation can cause various health issues, including a higher chance of cancer, cellular damage, and developmental abnormalities. In extreme cases, it can also lead to death within days or months. The severity of the damage depends on the dose and exposure conditions, with dividing cells being more vulnerable. The effects can be somatic (affecting the exposed individual) or genetic (affecting their offspring).
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How is radiation measured and detected in physics experiments?
Radiation measurement in physics experiments involves detecting and quantifying ionizing radiation, which includes charged particles and uncharged quanta. Since only charged radiation interacts continuously with matter, it is directly detectable, while uncharged quanta must undergo a major interaction to transform their energy into secondary charged radiations.
Detectors are either passive, indicating radiation presence post-exposure, or active, producing real-time signals. Common instruments include Geiger counters that detect radiation through ionization of gas, scintillation counters that use light flashes from phosphors, and solid-state detectors employing semiconductors. Other methods involve cloud chambers and thermoluminescent dosimeters (TLDs).
Units for measuring ionizing radiation include roentgen, gray, sievert, becquerel, and curie.
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radiation, flow of atomic and subatomic particles and of waves, such as those that characterize heat rays, light rays, and X rays. All matter is constantly bombarded with radiation of both types from cosmic and terrestrial sources. This article delineates the properties and behaviour of radiation and the matter with which it interacts and describes how energy is transferred from radiation to its surroundings. Considerable attention is devoted to the consequences of such an energy transfer to living matter, including the normal effects on many life processes (e.g., photosynthesis in plants and vision in animals) and the abnormal or injurious effects that result from the exposure of organisms to unusual types of radiation or to increased amounts of the radiations commonly encountered in nature. The applications of various forms of radiation in medicine and technological fields are touched upon as well.
General background
Types of radiation
Radiation may be thought of as energy in motion either at speeds equal to the speed of light in free space—approximately 3 × 1010 centimetres (186,000 miles) per second—or at speeds less than that of light but appreciably greater than thermal velocities (e.g., the velocities of molecules forming a sample of air). The first type constitutes the spectrum of electromagnetic radiation that includes radio waves, microwaves, infrared rays, visible light, ultraviolet rays, X rays, and gamma rays, as well as the neutrino (see below). These are all characterized by zero mass when (theoretically) at rest. The second type includes such particles as electrons, protons, and neutrons. In a state of rest, these particles have mass and are the constituents of atoms and atomic nuclei. When such forms of particulate matter travel at high velocities, they are regarded as radiation. In short, the two broad classes of radiation are unambiguously differentiated by their speed of propagation and corresponding presence or absence of rest mass. In the discussion that follows, those of the first category are referred to as “electromagnetic rays” (plus the neutrino) and those of the second as “matter rays.”
At one time, electromagnetic rays were thought to be inherently wavelike in character—namely, that they spread out in space and are able to exhibit interference when they come together from two or more sources. (Such behaviour is typified by water waves in the way they propagate and periodically reinforce and cancel one another.) Matter rays, on the other hand, were considered to be inherently particle-like in character—i.e., localized in space and incapable of interference. During the early 1900s, however, major experiments and attendant theories revealed that all forms of radiation, under appropriate conditions, can exhibit both particle-like and wavelike behaviour. This is referred to as the wave–particle duality and provides in large part the foundation for the modern quantum theory of matter and radiation. The wave behaviour of radiation is apparent in its propagation through space, while the particle behaviour is revealed by the nature of interactions with matter. Because of this, care must be exercised to use the terms waves and particles only when appropriate.
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Electromagnetic rays and neutrinos
Visible light and the other components of the electromagnetic spectrum
According to the theory of relativity, the velocity of light is a fixed quantity independent of the velocity of the emitter, the absorber, or a presumably independent observer, all three of which do affect the velocities of common wavelike disturbances such as sound. In an extended definition, the term light embraces the totality of electromagnetic radiation. It includes the following: the long electromagnetic waves predicted by the Scottish physicist James Clerk Maxwell in 1864 and discovered by the German physicist Heinrich Hertz in 1887 (now called radio waves); infrared and ultraviolet rays; the X rays discovered in 1895 by Wilhelm Conrad Röntgen of Germany; the gamma rays that accompany many radioactive-decay processes; and some even more energetic (with higher energy) X rays and gamma rays produced as the normal accompaniment of the operations of ultrahigh-energy machines (i.e., particle accelerators such as the Van de Graaff generator, the cyclotron and its variants, and the linear accelerator).
The behaviour of light seems to have interested ancient philosophers but without stimulating them to experiment, though all of them were impressed by vision. The first meaningful optical experiments on light were performed by the English physicist and mathematician Isaac Newton (beginning in 1666), who showed (1) that white light diffracted by a prism into its various colours can be reconstituted into white light by a prism oppositely arranged and (2) that light of a particular colour selected from the diffracted spectrum of a prism cannot be further diffracted into beams of other colour by an additional prism. Newton hypothesized that light is corpuscular in its nature, each colour represented by a different particle speed, an erroneous assumption. Furthermore, in order to account for the refraction of light, the corpuscular theory required, contrary to the wave theory of the Dutch scientist Christiaan Huygens (developed at about the same time), that light corpuscles travel with greater velocity in the denser medium. Support for the wave theory came in the electromagnetic theory of Maxwell (1864) and the subsequent discoveries of Hertz and of Röntgen of both the very long and the very short waves Maxwell had included in his theory. The German physicist Max Planck proposed a quantum theory of radiation to counter some of the difficulties associated with the wave theory of light, and in 1905 Einstein proposed that light is composed of quanta (later called photons). Thus, experiment and theory had led around from particles (of Newton) that behave like waves (Huygens) to waves (Maxwell) that behave like particles (Einstein), the apparent velocity of which is unaffected by the velocity of the source or the velocity of the receiver. Furthermore it was found, in 1922, that the shorter-wavelength electromagnetic radiations (e.g., X rays) have momentum such as may be expected of particles, part of which can be transferred to electrons with which they collide (i.e., the Compton effect).
Neutrinos and antineutrinos
Neutrinos and their antiparticles are forms of radiation similar to electromagnetic rays in that they travel at the speed of light and have little or no rest mass and zero charge. They too are produced by ultrahigh-energy particle accelerators and certain types of radioactive decay.