Electromagnetic radiation is energy that propagates through space in coupled electric and magnetic fields. It includes radio waves, microwaves, infrared radiation, visible light, ultraviolet radiation, X-rays, and gamma rays. Unlike sound, it requires no material medium and can travel through a vacuum. Classical electromagnetism describes its wave behavior, while its emission and absorption can be described in terms of discrete quanta called photons. These descriptions concern the same physical phenomenon at different levels of analysis. (science.nasa.gov)
Classical description and propagation
Maxwell’s equations relate electric and magnetic fields to charges, currents, and changes in the fields themselves. In empty space, they admit traveling-wave solutions. For a plane electromagnetic wave, the electric and magnetic fields are perpendicular to one another and to the direction of propagation: the wave is transverse. The fields oscillate together, with their magnitudes related by (E=cB). (openstax.org)
All electromagnetic radiation travels in vacuum at the speed of light, (c), regardless of frequency. Its value is exactly 299,792,458 meters per second in the International System of Units. Frequency (f), the number of oscillations per second, and wavelength (\lambda), the distance between successive points of equal phase, satisfy
[ c=f\lambda. ]
Thus, higher-frequency radiation has a shorter vacuum wavelength. In matter, propagation depends on the material’s electromagnetic response; different frequencies may travel at different phase velocities. (nist.gov)
Polarization describes the orientation and evolution of the electric field. Linear polarization has a fixed oscillation direction, whereas circular or elliptical polarization involves a rotating field direction. These properties distinguish electromagnetic waves beyond their frequency and intensity. (science.nasa.gov)
The electromagnetic spectrum
The electromagnetic spectrum is the continuous range of electromagnetic frequencies. Its conventional regions, ordered from lower to higher frequency, are radio, microwave, infrared, visible, ultraviolet, X-ray, and gamma-ray radiation. These are not fundamentally different kinds of wave: they share the same underlying physical description but interact differently with matter because their wavelengths and photon energies differ. Visible radiation occupies only a small part of this range. (science.nasa.gov)
Different regions are commonly specified using different quantities. Radio signals are often identified by frequency, infrared and visible radiation by wavelength, and high-energy radiation by photon energy. Frequency is measured in hertz, wavelength in units of length, and photon energy often in electron volts. These conventions express equivalent information when the propagation conditions are specified. (science.nasa.gov)
Quantum description
In quantum mechanics, electromagnetic radiation exchanges energy in discrete amounts. A photon’s energy is
[ E=hf=\frac{hc}{\lambda}, ]
where (h) is the Planck constant and (\lambda) is the vacuum wavelength. Consequently, an individual ultraviolet photon carries more energy than an individual infrared photon. This does not by itself determine a beam’s total power, which also depends on how many photons it delivers per unit time. (openstax.org)
Wave behavior remains essential: radiation exhibits interference and diffraction, even though detection involves localized energy exchanges. The photoelectric effect, in which radiation ejects electrons from a material, illustrates the importance of photon energy rather than a purely classical account of energy delivery. Quantum and wave descriptions are therefore complementary, not classifications into separate kinds of radiation. (openstax.org)
Production and interaction with matter
Classically, accelerating electric charges generate electromagnetic radiation. In a transmitting antenna, an alternating current drives charges back and forth, producing outgoing waves. A receiving antenna operates in reverse: the incoming field drives oscillating currents that can be selected and amplified by electrical circuits. A steady current’s static magnetic field is not itself an outgoing radiation wave. (openstax.org)
Thermal emission provides another important source. Blackbody radiation is the idealized continuous spectrum emitted by a perfect absorber in thermal equilibrium. Its spectral distribution depends on temperature; hotter blackbodies have their wavelength-distribution peaks at shorter wavelengths. Stellar spectra often approximate this continuous background. (science.nasa.gov)
Radiation encountering matter may undergo reflection, transmission, absorption, refraction, or scattering. Their relative importance depends on wavelength and material composition. Absorption transfers energy to matter, while refraction changes propagation direction when waves cross an interface under appropriate conditions. These wavelength-dependent interactions underpin spectroscopy, which uses radiation to investigate physical and chemical properties. (science.nasa.gov)
Sufficiently energetic photons can cause ionization by removing bound electrons. Whether this occurs depends on both photon energy and the target material’s binding energies. The distinction between ionizing and non-ionizing radiation therefore concerns an interaction mechanism, rather than simply whether radiation is visible or invisible. (openstax.org)
Energy transfer, momentum, and applications
Radiation also transports momentum. Its absorption or reflection exerts radiation pressure on matter. For a normally incident beam of intensity (I), the pressure is (I/c) on a perfect absorber and (2I/c) on a perfect reflector. Although often small, this pressure produces measurable forces and contributes to astronomical phenomena such as comet dust tails. (openstax.org)
Electromagnetic waves support communication, imaging, remote sensing, and astronomical observation. Different spectral regions reveal different properties of objects, while instruments convert received radiation into electrical signals or images. Radiation is also central to Earth’s energy budget: the planet absorbs incoming solar energy and emits thermal infrared radiation. The greenhouse effect involves atmospheric absorption and emission of part of this outgoing infrared energy, affecting energy exchange between the surface, atmosphere, and space. (science.nasa.gov)