Electromagnetic radiation

Electromagnetic radiation (EMR) is a self-propagating wave of the electromagnetic field that transports both momentum and radiant energy across space. Its spectrum is broad and organized by frequency, which is inversely related to wavelength, and includes radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, and gamma rays. All forms of EMR travel at the speed of light in a vacuum and display wave–particle duality, meaning they behave as both waves and discrete particles called photons.

This radiation is generated by accelerating charged particles, whether from natural sources like the Sun and other celestial bodies or from artificial creation for various uses. How EMR interacts with matter depends on its wavelength, which determines its applications in communication, medicine, industry, and scientific research. Radio waves support broadcasting and wireless communication, infrared is used in thermal imaging, visible light enables vision, and higher-energy forms like X-rays and gamma rays are employed in medical imaging, cancer treatment, and industrial inspection. Higher-energy EMR, particularly ultraviolet and above, is linked to negative health effects.

In quantum mechanics, EMR can be understood as consisting of photons—uncharged elementary particles with zero rest mass that serve as the quanta of the electromagnetic field and are responsible for all electromagnetic interactions. Quantum electrodynamics describes how EMR interacts with matter at the atomic level. Quantum effects also produce additional sources of EMR, such as when electrons transition to lower energy levels in an atom and through black-body radiation.

The electromagnetic nature of radiation is described by Maxwell's equations, which show that the electric and magnetic field components have amplitudes that decrease inversely with distance from the source (1/r), allowing detection at great distances. These components are transverse to the direction of propagation, with the electric field perpendicular to the magnetic field and both perpendicular to the direction of travel. The fields are related by the equation B = (n × E) / c, and they propagate through space at 299,792,458 meters per second.

EMR is produced by accelerating charged particles and can be emitted naturally or artificially. The energy in electromagnetic waves is sometimes called radiant energy, and these waves do not require a medium to travel through space; they move through a vacuum at the speed of light. Electric and magnetic fields obey superposition, so fields from any particle or time-varying field contribute to the total field in the same space. As vector fields, all magnetic and electric field vectors add through vector addition. For example, in optics, two or more coherent light waves can interact through constructive or destructive interference, yielding a resultant irradiance different from the sum of the individual irradiances. The electromagnetic fields of light are unaffected by static electric or magnetic fields in a linear medium like a vacuum, but in nonlinear media such as certain crystals, interactions like the Faraday effect and Kerr effect can occur.

In refraction, a wave changes speed and direction when crossing from one medium to another of different density. The degree of refraction is determined by the ratio of the media's refractive indices, as summarized by Snell's law. Light of composite wavelengths, such as natural sunlight, disperses into a visible spectrum when passing through a prism due to the wavelength-dependent refractive index of the prism material (dispersion), bending each component wave by a different amount.

EM radiation simultaneously exhibits both wave and particle properties, a phenomenon confirmed by many experiments. Wave characteristics are more apparent when measuring over relatively large timescales and distances, while particle characteristics are more evident at small timescales and distances. For instance, when EMR is absorbed by matter, particle-like properties become more obvious when the average number of photons in a cube of the relevant wavelength is much less than 1. Experimentally observing non-uniform energy deposition during light absorption is not alone evidence of particulate behavior; rather, it reflects the quantum nature of matter. The quantum theory of interaction between EMR and matter, such as electrons, is described by quantum electrodynamics.

Electromagnetic waves can be polarized, reflected, refracted, or diffracted, and can interfere with each other. Some experiments, like the self-interference of a single photon, display both the wave and particle natures of electromagnetic waves. When low-intensity light is sent through certain setups, these dual characteristics become evident.