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Technologies to optimize the performance of optical systems wherein controllable optical elements maintain a sharply focused image in the presence of wavefront aberrations. The practical development of adaptive optics started in the late 1960s. Main…

An analytical technique for determining the composition of a sample in terms of the chemical elements present and their quantities or concentrations. Unlike other methods of elemental analysis, however, the sample is decomposed into its constituent…

The ongoing lower level of species extinction intensity occurring between episodes of mass extinctions. Extinction is a normal part of the history of life. Out of every 100 species that have ever lived on Earth, 99 are no longer in existence.…

A quantum-mechanical theory of the motion of electrons in solids. The band theory of solids' name comes from the fact that it predicts certain restricted ranges, or bands, for the energies of electrons in solids. In a solid, the energy level of an…

That portion of a fluid flow near a solid surface where shear stresses are significant and the inviscid-flow assumption, in which viscous-shear and normal stresses are negligible, may not be used. In physics broadly and fluid mechanics specifically,…

Cameras in mobile phones or smartphones that can capture images and videos.The cellphone, the world’s favorite must-have gadget, has been with us for nearly a half-century, while experiencing continuous and often phenomenal improvements. Much of the…

An apparent outward force on a particle rotating about an axis which, by Newton's third law, is equal and opposite to the centripetal force. In classical mechanics, a particle that is rotating about an axis experiences an apparent outward…

Light emitted by a high-speed charged particle when the particle passes through a transparent, nonconducting, solid material at a speed greater than the speed of light in the material. The emission of Cerenkov radiation—also spelled Cherenkov—is…

The attractive force that holds atoms or ions together. Chemical bonds are very strong. To break one bond in each molecule in a mole of material typically requires an energy of many tens of kilocalories.It is convenient to classify chemical bonding…

The basic quantitative law of electrostatics that quantifies the physical force that exists between two electrically charged points. For electrostatics, Coulomb's law states that the direct force F of point charge q1 on point charge q2, when the…

A general term referring to a system or part of a system of conducting parts and their interconnections through which electric currents are intended to flow. A circuit is made up of a voltage source along with active and passive elements, as well as…

The measure of the opposition that an electrical circuit presents to the passage of a current when a voltage is applied. In quantitative terms, electrical impedance is the complex ratio of the voltage to the current in an alternating-current (ac)…

The ability of a material to resist or allow the flow of electrical current. Material properties largely determine the resistance and conductance—quantified by electrical resistivity and conductivity—of a given object, such as a circuit or…

The measures of a material's opposition to the flow of electricity and a material's ability to conduct electricity, respectively. Resistivity quantifies resistance, which is a material's opposition to the flow of electrons (an electric current or…

A range of physical phenomena that involve electric charges, their motions, and their effects. In conventional usage, the term “electricity” often refers to the energy that powers lighting, appliances, and other electronic devices in homes,…

The process in which an electron, which has been excited from the valence band to the conduction band of a semiconductor, falls back into an empty state in the valence band, which is known as a hole. When atoms come together to form a crystal, the…

Typically, an aquatic vertebrate with scales, gills, and paired fins. Fishes (Fig. 1) are found throughout the world's waterways, from saltwater oceans to freshwater streams and ponds. About 35,000 species of fishes have been described to date. Many…

The branch of physics and engineering science concerned with the relation between the forces acting on fluids (liquids and gases), the motions of fluids, and the forces caused by fluids on their surroundings. Fluid mechanics is comprised of two…

In thermodynamics, the amount of useful work that can be extracted from a closed thermodynamic system. The Helmholtz free energy applies to a process that takes place at a constant temperature T (isothermal process). The Gibbs free energy applies to…

A vacant electron energy state near the top of an energy band in a solid. A hole, or more formally an electron hole, represents the absence of an electron where one could exist as part of an atom or within a crystal structure (an atomic lattice)…

An applied science and technology concerned with the mechanical properties and practical uses of liquids. The physical description of fluids in motion is based upon the conservation laws of mass, momentum, and energy. The mathematical statement of…

One of the three fundamental particles that transmit the weak force. The three intermediate vector bosons are known as the W+, W− , and Z0 particles or bosons. were discovered in 1983 in very high energy proton-antiproton collisions. It is through…

Any scientific law in which a physical quantity varies with distance from a source inversely as the square of that distance. When energy is being radiated by a point source (see illustration), an inverse-square law holds, provided the space between…

Fundamental laws dealing with electric currents at junctions and voltages around any loop within a network. Also known as Kirchhoff's circuit laws, the laws were first described by German physicist Gustav Kirchhoff in 1845. The laws are divided into…

A transparent object that uses refraction in order to alter the light that passes through the object. In general, when a ray of light crosses the interface between two different transparent materials, the ray experiences a bending of its path. This…

The spectrum of a beam of light when only a few isolated colors are present. A beam of light (technically, a beam of electromagnetic radiation) typically contains a mixture of pure colors. The particular pure colors that are present in the beam…

The relationship stating that the magnitude of the electrical current in a circuit is directly proportional to the applied voltage and inversely proportional to the circuit's resistance. One of the simplest and most intuitive electrical laws, Ohm's…

An electric circuit in which the elements, branches (elements in series), or components are connected between two points, with one of the two ends of each component connected to each point. The illustration shows a simple parallel circuit. In more…

Computation that encodes data in quantum objects to later process that data using operations based on the concepts and principles of quantum mechanics. Quantum objects used in quantum computation include the nuclei of atoms, ions in a crystal,…

The study of chemical changes resulting from the absorption of high-energy ionizing radiation. Such radiation includes alpha particles, electrons, gamma rays, fission fragments, protons, deuterons, helium nuclei, and heavier charged projectiles.…

A general theory of physics, primarily conceived by German-born U.S. theoretical physicist Albert Einstein, which involves a profound analysis of time and space, leading to a generalization of physical laws with far-reaching implications in…

A solid crystalline material whose electrical conductivity is intermediate between that of a metal and an insulator. Semiconductors exhibit conduction properties that may be temperature-dependent, permitting their use as thermistors…

The theory that explains the three fundamental interactions of elementary particle physics—the strong interaction responsible for nuclear forces, the weak interaction responsible for radioactive decay, and the electromagnetic interaction—in terms of…

The branch of mechanics that describes bodies which are acted upon by balanced forces and torques so that the bodies remain at rest or in uniform motion. Statics includes point particles, rigid bodies, fluids, and deformable solids in general.…

A subdiscipline of paleobiology that investigates the processes of preservation and their influence on information in the fossil record. Taphonomy is the study of fossilization and considers the processes at work in the incorporation of living…

The description of natural phenomena in mathematical form. As a branch of physics, theoretical physics involves the construction of models and abstractions (see illustration) to both explain and predict nature. Theoretical physics fundamentally…

The traveling pattern of motion exhibited by a wave, combining both oscillatory motion and forward motion. Examples of wave motion include the motion of ocean waves, light waves (Fig. 1), sound waves, and waves on a string. The oscillatory motion…

An extensively used electrical network of four resistances R1–4 (or impedances Z1–4) if alternating currents are involved. A Wheatstone bridge network (see Fig. 1), was first described by British physicist and mathematician Samuel Hunter Christie in…

The scattering of x-rays by matter with accompanying variation in intensity for specific directions (or wavelengths) due to interference effects. X-ray diffraction is a powerful and readily available method for determining atomic arrangements in…

The use of intense acoustic waves to hold a body that is immersed in a fluid medium against the force of gravity without obvious mechanical support. A common demonstration of the phenomenon of acoustic levitation is suspending droplets in a column…

Any one of a number of related integral quantities which serve as the basis for general formulations of the dynamics of both classical and quantum-mechanical systems. The term "action" has been associated with four quantities: the fundamental action…

The cross section of a body that is placed in an airstream to produce a useful aerodynamic force in the most efficient manner possible. The cross sections of wings, helicopter rotor blades (Fig. 1), propeller blades, windmill blades, compressor and…

Most commonly, a generic name for all fossil resins, although it has been restricted by some to refer only to succinite, the mineralogical species of fossil resin that includes most of the Baltic Coast deposits. The time necessary to transform resin…

A genus of fossil birds in the extinct order Archaeopterygiformes, characterized by flight feathers like those of modern birds. Archaeopteryx is generally accepted as the oldest known fossil bird. It represents a unique snapshot of evolution; most…

A device that uses an internal resonance frequency of atoms (or molecules) to measure the passage of time. Atomic clocks (Fig. 1) are relied on internationally as the most accurate time and frequency standards with myriad uses in commerce, industry,…

An algebraic relation between pressure, velocity, and elevation for the flow of an idealized, inviscid (non-viscous) fluid. In fluid dynamics, a subdiscipline of fluid mechanics, Bernoulli's theorem describes the principle whereby a fluid increases…

A region of spacetime exerting a gravitational field so strong that neither matter nor radiation can escape. Black holes are extreme cosmic objects predicted by German-born U.S. physicist Albert Einstein's theory of general relativity. Within a…

The irregular motion of a body arising from the thermal motion of the molecules of the medium in which the body is immersed. A body that is immersed experiences many collisions with the molecules in the medium, which impart energy and momentum to…

In its simplest form, a vector is a directed line segment. Physical quantities, such as velocity, acceleration, force, and displacement, are vector quantities, or simply vectors, because they can be represented by directed line segments. The algebra…

The measurement of the quantity of heat transferred during chemical reactions, changes of state, mixing of substances, as well as other chemical and physical processes. The unit of energy in the International System of Units is the joule, symbolized…

A class in the phylum Mollusca that is exclusively marine and includes squids, octopuses, and cuttlefishes. The molluscan class Cephalopoda is one of the major taxonomic groupings of marine animals and contains both extant members—squids (Fig. 1),…

An order of the class Reptilia, including the turtles, terrapins, and tortoises; also termed Testudines. The order Chelonia (often referred to as Testudines) comprises one of the oldest groups of reptiles on Earth. Members of the Chelonia…

Any living or fossil member of the plant division Pinophyta, consisting chiefly of woody trees or shrubs that produce cones and evergreen needles. Conifers are predominantly cone-bearing, evergreen trees and shrubs belonging to the order Pinales, as…

The computer manipulation of visual data to solve a problem. The problems under consideration are of two primary types: (1) those that require a computer to act on the resultant images, and (2) those that provide images for human interpretation. The…

A definable parameter of physical, temporal, or other spaces in fields including physics, mechanics, and mathematics. The concept of a dimension has various contextual meanings depending on the mathematical or scientific field where it is being…

A technique that involves the study of dimensions of physical quantities. Dimensional analysis is used primarily as a tool for obtaining information about physical systems too complicated for full mathematical solutions to be feasible. It enables…

The part of classical mechanics which deals with the relation between the motions of material bodies and the forces acting upon them. The English physicist and mathematician Isaac Newton. originally developed the physical laws governing dynamics. In…

A condition in space in the vicinity of an electrically charged body such that the forces due to the charge are detectable. An electric field (or electrostatic field) exists in a region if an electric charge at rest in the region experiences a force…

The physical field extending throughout space that delivers electric and magnetic effects. The electromagnetic field is a unified field that consists of two inseparable components, the electric field and the magnetic field (Fig. 1 and Fig. 2).…

The production of an electromotive force either by motion of a conductor through a magnetic field in such a manner as to cut across the magnetic flux or by a change in the magnetic flux that threads a conductor. The phenomenon of electromagnetic…

A disturbance of the electromagnetic field, typically produced by the acceleration of an electrically charged object, which propagates as a wave. Electromagnetic waves consist of oscillating electric field and magnetic field components that form…

A device for forming greatly magnified images of objects by means of electrons. Electron microscopes (Fig. 1) serve primarily two purposes: the visual examination of structures too fine to be resolved with ordinary, or light, microscopes, and the…

In a mechanical system the condition under which no acceleration takes place. In any given system, an object can be subject to various forces. When these forces acting upon the object are in an exact balance, resulting in no net force acting on the…

The death and disappearance of a species. The extinction of a species usually represents an end point in a long series of population extinctions. During the extinction process, unique evolutionary history is lost at every stage, but the death of the…

Lenses worn to improve vision. Eyeglasses and contact lenses are eyewear devices used to correct or improve defective eyesight (see illustration). The primary function of the eye is to focus light energy on the retina. In the optics of a normal eye,…

Any interaction between objects that causes motion. Fundamentally, objects interact through the four known physical forces: gravity, electromagnetism, and the strong and weak nuclear interactions. In practice, however, many of the forces worked with…

The application of physics for purposes of civil or criminal law. Indirectly, physics has contributed to forensic science via the invention of the microscope, the electron microscope, the mass spectrometer, and optical spectrometers. A direct…

A record of earlier life buried in rock. Originally meaning any distinctive object that has been dug up (from Latin fodio, dig), the term “fossil” soon came to refer particularly to things resembling animals and plants. These objects were known…

A massive body producing distorted, magnified, or multiple images of more distant objects when its gravitational fields bend the paths of light rays. Examples of foreground bodies that can act as gravitational lenses of background objects include…

Optical instruments that establish an optical line or axis for the purpose of aiming a weapon. The axis includes the observer's eye, a suitable mark in the instrument, and the target. For a pistol, the traditional iron sights employ a rear notch and…

The form of energy transfer that occurs due to a temperature difference between a higher temperature “source” and a lower temperature “sink.” Heat transfer always occurs between two objects when they exist at different temperatures (see…

The quantity of heat required to raise a unit mass of homogeneous material one unit in temperature along a specified path, provided that during the process no phase or chemical changes occur is known as the heat capacity of the material in question.…

A force which acts only during a short time interval but which is sufficiently large to cause an appreciable change in the momentum of the system on which it acts. An impact is often studied in the context of the collision between two bodies. In the…

The integral of a force over an interval of time. As force is applied over time to a body, the force produces an impulse, which is a vector change in the body's linear momentum. An impulse accordingly influences a body's motion and velocity (see…

A property of matter that manifests itself as a resistance to any change in the motion of a body. Inertia is a familiarly encountered phenomenon. For instance, a ball on a level surface will resist motion unless prodded or kicked (see illustration).…

The process whereby two or more waves overlap to form complex patterns and effects. When two or more waves exist at the same location at the same time, they overlap to form a composite wave which is the mathematical sum of the original waves (see…

The common name for a member of the agnathan fishes. The fish superclass Agnatha (subphylum Vertebrata, phylum Chordata) comprises the jawless vertebrates (agnathans). Members include cyclostomes (modern lampreys and hagfishes), various extinct…

That branch of mechanics which deals with the motion of a system of material particles without reference to the forces which act on the system. Kinematics differs from dynamics in that the latter takes these forces into account. Dynamics Mechanics…

A smooth, streamline type of viscous fluid motion characteristic of flow at low-to-moderate deformation rates. The name derives from the fluid's moving in orderly layers, or laminae, without the formation of small eddies or irregular fluctuations.…

A type of transportation system that uses magnetic forces to levitate vehicles above a dedicated guideway. Magnetic levitation is also known as maglev. Typically, maglev vehicles are magnetically guided and propelled. Maglev has been a niche form of…

A measure of the effectiveness of an optical system in enlarging or reducing an image. For an optical system such as a magnifying glass (see illustration) or a microscope that forms a real image, such an effectiveness measure is the lateral…

A lineage of true elephants in the extinct genus Mammuthus that occupied Eurasia and North America in the late Cenozoic Era. Mammoths (genus Mammuthus) are extinct elephants assigned to the mammalian order Proboscidea (Fig. 1). They inhabited…

The widespread, catastrophic loss of many species in a relatively short period of geological time. Earth has supported life for approximately 3.5 billion years, and it is normal for species to become extinct and for new species to arise through…

A term that traditionally refers to the substance of which all bodies consist. In everyday usage, matter refers to the ubiquitous materials made of atoms, which in turn are made up by subatomic particles. In modern physics and chemistry, matter…

Four equations that concisely convey the fundamentals of classical electromagnetism, classical optics, and electric circuits. Completed by Scottish physicist James Clerk Maxwell in 1864, these partial differential equations govern a wide range of…

An instrument used to obtain an enlarged image of a small object. Microscopes enable the visualization and gathering of information from samples and portions of samples that by definition are microscopic or cannot be seen with the naked eye. A broad…

The smallest unit of a compound, which consists of atoms bonded together in a unique arrangement. A diatomic molecule consists of two atoms linked together by chemical bonds. Examples include the oxygen and nitrogen of the air (O2 and N2,…

A major phylum of the animal kingdom comprising a diverse group of unsegmented, soft-bodied invertebrates. The phylum Mollusca is one of the most significant groupings of invertebrate animals. Members of the Mollusca, termed mollusks or molluscs,…

A mammalian order containing the egg-laying platypus and echidnas. Members of the order Monotremata (monotremes) are egg-laying mammals belonging to the subclass Prototheria (in some alternative classifications, the monotremes have been assigned to…

The change in the position of an object in a physical space over time. A familiarly encountered phenomenon, motion (see illustration) occurs on all levels of reality, from the constant movements of subatomic particles (conveyed by probabilities of…

The branch of acoustics that deals with the generation of sound by musical instruments, the transmission of sound to the listener, and the perception of musical sound. Musical acoustics has fascinated scientists at least since the time of the Greek…

A synthetic particle that ranges in diameter from 1 to 100 nanometers. Semiconductor nanoparticles around 1–20 nm in diameter are often called quantum dots, nanocrystals, or Q-particles. These particles possess short-range structures that are…

Techniques and products involving nanometer-scale structures, with dimensions ranging from 1 to 100 nanometers, especially those that transform matter, energy, and information using nanometer-scale components with precisely defined molecular…

The Nobel Prizes for 2021 included the following awards for scientific disciplines. Chemistry The chemistry prize was awarded jointly to Benjamin List of the Max-Planck-Institut für Kohlenforschung, Mülheim an der Ruhr, Germany, and David W.C.…

A fluid that departs from the classic linear Newtonian relation between stress and shear rate. In contrast to Newtonian fluids, non-Newtonian fluids can become either thicker or thinner when stress is applied. Many of the fluids encountered in…

The formation of nuclides present in the universe by various nuclear reactions. Theories of the origin of the chemical elements involve synthesis with charged and neutral particles (neutrons, protons, neutrinos, photons) and other nuclear building…

A small extinct order of articulated brachiopods that ranges in age from Early to Middle Cambrian and includes the earliest known calcitic brachiopods. Classification The order Obolellida is included within the class Obolellata, subphylum…

An order of coleoid cephalopods, characterized by eight appendages (which encircle the mouth), a saclike body, and an internal shell that is much modified or reduced from that of its ancestors. Members of the order Octopoda (subclass Coleoidea,…

A morphologically distinct type of trace fossil left by boring organisms in hard biogenic substrates. Oichnus Bromley 1981 is a formal taxonomic name with the rank of ichnogenus (a trace fossil genus). Oichnus is a Latin-derived term that…

A suborder of trilobites (a class of extinct arthropods) and a diverse clade consisting of roughly 100 distinct species. The trilobites constituting the suborder Olenellina belong to the order Redlichiida in the class Trilobita. These extinct…