light

Light is electromagnetic radiation that is visible to the human eye, as well as portions of the spectrum adjacent to it, including infrared and ultraviolet. In vacuum, light travels at the speed of light, about 299,792 kilometers per second. Light can be described as both a wave and as particles called photons, a duality described in quantum electrodynamics.

In materials, light can be reflected, transmitted, or absorbed. Its propagation is governed by the medium's refractive index, which reduces its speed to v = c/n and shortens its wavelength. Light may be refracted at interfaces, diffracted around obstacles, or interfered with itself and other waves. Dispersion splits white light into component wavelengths, producing spectra.

Sources range from natural to artificial. The Sun emits a broad spectrum of radiation, while artificial sources such as incandescent lamps, LEDs, and lasers produce light with more restricted spectra. Photons are emitted when atoms transition to lower energy states, and their energy is inversely proportional to wavelength, E = hc/λ.

Perception and measurement: the eye responds to wavelengths roughly 380 to 740 nanometers. Color is determined by the spectral distribution of received light. Photometric quantities such as lumen, candela, and lux quantify perceived brightness; color temperature characterizes the spectral composition of a light source.

Applications and significance: light underpins vision, photography, and communication through fiber optics. It enables solar energy collection, medical imaging and procedures, and a wide range of industrial and everyday technologies. Safety considerations include potential eye and skin damage from intense or ultraviolet light, leading to guidelines and protective measures.