Light

Light is a form of electromagnetic radiation produced by changing electric charges and described by oscillating electric and magnetic fields. In a vacuum it travels at exactly 299,792,458 metres per second, a value that defines the metre in the International System of Units. The visible band occupies only a small region of the electromagnetic spectrum, conventionally extending from roughly 380 to 700 nanometres, although the limits vary with the observer and the definition used.¹

Classical wave theory accounts for wavelength, frequency, interference, diffraction, and polarization. Quantum theory adds that light is exchanged in discrete packets called photons. A photon's energy is proportional to frequency, expressed as E = hf; higher-frequency ultraviolet, X-ray, and gamma radiation therefore carries more energy per photon than red or infrared radiation.²

Light changes direction, speed, or intensity when it encounters matter. Reflection sends radiation back from a surface, refraction changes its path when it enters a medium, and absorption transfers photon energy to atoms, molecules, or electrons. Dispersion separates wavelengths because a material's refractive index depends on frequency, producing effects such as the colors of a prism or rainbow.¹

In empty space, all wavelengths travel at the same speed, but in transparent materials their phase velocities are lower and wavelength-dependent. Light can also interfere with itself: constructive and destructive interference underlie thin-film colors, holography, and the resolving limits of optical instruments. The wave and photon descriptions are complementary rather than competing pictures; experiments reveal whichever aspect the measurement makes relevant.

Light carries momentum as well as energy. Radiation pressure is weak in everyday settings but becomes significant for highly reflective spacecraft sails, microscopic particles, and stars. In atoms, absorption and emission occur at characteristic frequencies, so spectra can identify elements in distant stars and hot gases.

Human color vision begins when photons are absorbed by three classes of cone cells in the retina, with sensitivity peaks broadly associated with short-, medium-, and long-wavelength light. The brain compares their responses rather than reading wavelength directly, which is why perceived color depends on illumination, surrounding colors, and adaptation. Rod cells provide greater sensitivity in dim conditions but do not support normal color vision.

Natural sources include the Sun, lightning, flames, bioluminescent organisms, and thermal radiation from matter. Artificial sources range from incandescent filaments and gas-discharge lamps to light-emitting diodes and lasers. A laser produces highly directional, coherent radiation through stimulated emission; LEDs use semiconductor junctions to convert electrical energy into light efficiently.

Optical fibers transmit information as pulses or modulated patterns of light, while cameras, microscopes, telescopes, barcode readers, and medical imaging systems convert optical signals into measurements. Visible light is also used in photolithography, spectroscopy, solar cells, and photodynamic therapies, where its interaction with matter is deliberately controlled.

Light can reveal objects that emit no visible radiation. Infrared observations trace cool dust and molecular clouds, radio observations map cold gas and pulsars, and X-ray observations expose extremely hot plasma around black holes and neutron stars; together these bands form astronomy's multiwavelength view of the universe.²

Not every visible-light effect is directly visible to the eye. Polarization records the orientation of an electromagnetic field and can expose stress in glass, surface textures, or magnetic structures. Scattering also explains why the sky is blue and sunsets appear red: shorter wavelengths are preferentially scattered by molecules in Earth's atmosphere, while long paths through the atmosphere remove more blue light from the direct solar beam.¹

Ultraviolet light is outside human vision but has both useful and harmful roles. It drives vitamin D production and can disinfect water and surfaces, yet excessive exposure damages DNA and increases the risk of skin cancer; protection depends on wavelength, intensity, duration, and biological sensitivity.⁴ The photoelectric effect, in which light ejects electrons from a material, was a decisive clue for quantum physics and earned Albert Einstein the 1921 Nobel Prize in Physics.³

Electromagnetic spectrum

The complete range of electromagnetic radiation, ordered by frequency, wavelength, or photon energy.

Photon

A quantum of electromagnetic radiation carrying energy proportional to its frequency.

Wavelength

The distance between corresponding points on successive cycles of a wave.

Polarization

The orientation pattern of the oscillating electric field in electromagnetic radiation.

Refraction

The change in direction and phase velocity that occurs when light enters a different medium.