Light is electromagnetic radiation, especially the portion detectable by the human eye. In broader scientific usage, the term can include radiation outside the visible range, such as infrared and ultraviolet. Light transports energy through space and interacts with matter through emission, absorption, and scattering. It can be described as an electromagnetic wave or, in quantum mechanics, in terms of photons. Its production, propagation, and detection are central subjects of optics. (science.nasa.gov)
Spectrum and physical properties
Visible light occupies a narrow interval within the electromagnetic spectrum, commonly described as approximately 380–700 nanometers in wavelength. These boundaries are approximate rather than abrupt divisions. Violet lies toward the shorter-wavelength end and red toward the longer-wavelength end. Beyond visible light, the spectrum extends through ultraviolet, X-rays, and gamma rays at shorter wavelengths, and infrared, microwaves, and radio waves at longer wavelengths. These categories describe the same fundamental type of radiation. (science.nasa.gov)
Wavelength, denoted by (\lambda), is the spatial separation between successive wave crests; frequency, (\nu), is the number of cycles per second. In a vacuum they satisfy
[ c=\lambda\nu. ]
The speed of light in vacuum, (c), is exactly 299,792,458 meters per second. This fixed value forms part of the definition of the meter in the International System of Units. Thus, shorter vacuum wavelengths correspond to higher frequencies, not to different vacuum propagation speeds. (science.nasa.gov)
Wave behavior and propagation
In classical electromagnetism, light consists of changing electric and magnetic fields. Unlike sound, it requires no material medium and can propagate through empty space. Polarization describes the orientation and evolution of the electric field: linearly polarized light has a fixed oscillation direction, whereas unpolarized light has no single persistent direction. Polarizing filters selectively transmit particular field orientations and can reduce reflected glare. (science.nasa.gov)
A ray model approximates propagation when wavelengths are small compared with the relevant objects or openings. It is useful for describing mirrors and lenses, but wave effects become essential at smaller scales. In diffraction, light spreads around obstacles or through apertures; the effect becomes prominent when their dimensions approach the wavelength. Interference occurs when overlapping waves reinforce or cancel one another according to their relative phases, producing patterns such as bright and dark fringes or the colors of thin films. (openstax.org)
Reflection, refraction, and scattering
Reflection redirects light at a surface. For a smooth reflecting interface, the angle of reflection equals the angle of incidence, with both measured from the surface normal. Refraction is a change in propagation direction associated with differing optical properties between media. The refractive index is defined by (n=c/v), where (v) is the phase speed in the material. For ordinary interfaces, Snell’s law relates the incident and refracted angles:
[ n_1\sin\theta_1=n_2\sin\theta_2. ]
These relationships underlie image formation by lenses and many familiar apparent displacements of submerged objects. (openstax.org)
Materials may transmit some wavelengths while absorbing or scattering others. Scattering redirects light through interactions with particles or variations in a medium. Atmospheric scattering helps explain the blue appearance of the daytime sky. An object’s appearance depends partly on which wavelengths reach the observer after reflection or transmission; absorption removes energy from the incident radiation and transfers it to the material. (science.nasa.gov)
Photons and quantum interactions
A photon carries energy according to
[ E=h\nu, ]
where (h) is the Planck constant. Higher-frequency light therefore has greater energy per photon. Photons also carry momentum and have zero rest mass. Wave and particle descriptions concern complementary aspects of light’s behavior: propagation can produce interference and diffraction, while detection involves discrete energy exchanges. (science.nasa.gov)
The photoelectric effect illustrates this quantization. Light incident on a metal can eject electrons only when its frequency exceeds a material-dependent threshold. In the basic single-photon description, the maximum electron kinetic energy is (h\nu-\Phi), where (\Phi) is the metal’s work function. Increasing intensity at a fixed frequency above threshold increases the supply of photons, rather than the energy of each photon. Albert Einstein extended quantum ideas to explain this behavior, which classical wave theory alone could not account for. (openstax.org)
Emission and spectral information
Matter emits light when it releases energy. In atoms, transitions between allowed energy levels can produce photons at characteristic frequencies. Conversely, atoms can absorb photons corresponding to suitable transitions. These processes generate emission and absorption lines, allowing light to carry information about the material with which it has interacted. (science.nasa.gov)
Spectroscopy separates and analyzes radiation by wavelength or frequency. In astronomy, spectra reveal chemical constituents and physical conditions in distant objects; shifts in spectral features also provide information about motion. Lasers generate amplified light through stimulated emission, producing radiation with properties suitable for controlled optical applications. Laser light is used in communication through optical fibers and in barcode readers. (science.nasa.gov)
Biological significance
Light supplies the energy that drives photosynthesis. Pigments such as chlorophyll absorb selected wavelengths and transfer excitation energy into reactions that produce chemical energy carriers. Different pigments broaden the usable range of radiation, while their selective absorption also contributes to the colors of plants and algae. (openstax.org)
Human vision likewise begins with the absorption of visible light by retinal pigments. The eye’s sensitivity covers only a small portion of the electromagnetic spectrum, so radiation detectable by scientific instruments need not be visible to humans. Light therefore serves both as a biological energy source and as a carrier of sensory information. (science.nasa.gov)