Either the taking up of energy from radiation by the medium through which radiation is passing, or the taking up of matter in bulk by other matter. A simple example of the absorption of energy is how sunlight warms a tree's leaves (see illustration). Wavelengths of green light are reflected by molecules in the leaves, making them appear green, while other wavelengths of light are absorbed for use in photosynthesis, with some energy being converted into heat. A simple example of absorption by matter of other matter is the dissolving of carbon dioxide gas into water to create soda water. See also: Carbon dioxide; Chemistry; Energy; Photosynthesis; Physics; Tree; Water

Propagating oscillations in electrically conducting fluids or gases in which a magnetic field is present.

The maximum magnitude (value without regard to sign) of the disturbance of a wave. The term “disturbance” refers to that property of a wave which perturbs or alters its surroundings. It may mean, for example, the displacement of mechanical waves, the pressure variations of a sound wave, or the electric or magnetic field of light waves. Sometimes in older texts the word “amplitude” is used for the disturbance itself; in that case, amplitude as meant there is called peak amplitude. This is no longer common usage. See also: Light; Sound

The analog of linear momentum for rotational motion. A particle with mass m moving with velocity v has linear momentum p = mv (Note: This article follows the convention of using bold, non-italicized variables for vectors and using non-bold, italicized variables for scalars.) The angular momentum of the particle can be written as the vector cross-product of its position vector r, and its linear momentum, p:

(1)

The reduction in level of a transmitted quantity as a function of a parameter, usually distance. It is applied mainly to acoustic or electromagnetic waves and is expressed as the ratio of power densities. Various mechanisms can give rise to attenuation. Among the most important are geometrical attenuation, absorption, and scattering.

System behavior that depends so sensitively on the system's precise initial conditions that it is, in effect, unpredictable and cannot be distinguished from a random process, even though it is deterministic in a mathematical sense. This article begins with a discussion of the notions of order, chaos, and noise as they occur in deterministic dynamical systems, the relation of chaos and periodicity, and the concept of an attractor. Applications of chaos to atmospheric prediction, weather (Fig. 1), climate, electronic circuits, astronomy, acoustics, and atoms are then discussed.

The unusual physical properties displayed by substances near their critical points. The study of critical phenomena of different substances is directed toward a common theory.

The process of changing energy from one form to another. There are many conversion processes that appear as routine phenomena in nature, such as the evaporation of water by solar energy or the storage of solar energy in fossil fuels. In the world of technology, the term "energy conversion" is more generally applied to operations in which the energy is made more usable. Traditional examples include the burning of coal or natural gas in power plants to convert chemical energy into electricity and the burning of gasoline in automobile engines to convert chemical energy into propulsive energy for a moving vehicle. The conversion of energy in sunlight via solar cells into electricity, as well as the conversion of kinetic energy in wind into electricity, are further examples (Fig. 1). See also: Automobile; Coal; Electricity; Energy; Energy sources; Evaporation; Fossil fuel; Gasoline; Natural gas; Power plant; Propellant; Propulsion; Solar cell; Solar energy; Thermoelectric power generator; Thermoelectricity; Water

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 when solving everyday problems are macroscopic manifestations of these four fundamental interactions that take place at the level of subatomic particles. For example, the macroscopic contact force between two objects—typically called the normal force—is fundamentally an electrostatic interaction between the electron clouds of molecules at the interface of the two objects. At the introductory physics level, though, it is typically not useful to describe the interaction in fundamental terms, and normal force considerations are sufficient (Fig. 1). In this way, Newton's laws—first postulated by English physicist and mathematician Isaac Newton—provide an essential framework for understanding how forces determine the motion of objects. While widely applicable in most situations, Newton's laws fail to describe motion in certain, less-frequently-encountered circumstances, for instance involving objects moving close to the speed of light or in non-inertial reference frames. These circumstances require application of the special and general theories of relativity, respectively. See also: Electromagnetism; Fundamental interactions; Gravity; Newton's laws of motion; Relativity; Strong nuclear interaction; Weak nuclear interaction

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 contribution is the use of the photoluminescence phenomenon for physical evidence examination, with latent fingerprint detection (Fig. 1) the most notable application in criminalistics (and the focus of this article). Forensic physics techniques are complementary to forensic chemistry techniques, such as analytical chemistry, and forensic biology techniques, including serology and DNA profiling. See also: Deoxyribonucleic acid (DNA); Forensic biology; Forensic chemistry; Forensic microscopy; Forensic science; Forensic science evidence; Mass spectrometry; Microscope; Serology; Spectroscopy