The term light (from Latin lux) refers to the portion of the electromagnetic spectrum visible to the human eye, between 400 and 700 nanometers in wavelength, or between 790 and 434 THz in frequency. This interval coincides with the center of the spectral region of light emitted by the Sun that manages to reach the ground through the atmosphere.

The limits of the spectrum visible to the human eye are not the same for all people, but vary subjectively and can reach 720 nanometers, approaching the infrared, and 380 nanometers approaching the ultraviolet. The simultaneous presence of all visible wavelengths, in amounts proportional to those of sunlight, forms white light.

Light, like all electromagnetic waves, interacts with matter. The phenomena that most commonly affect or prevent the transmission of light through matter are: absorption, scattering, specular or diffuse reflection, refraction and diffraction. Diffuse reflection from surfaces, alone or combined with absorption, is the main mechanism by which objects reveal themselves to our eyes, while scattering from the atmosphere is responsible for the brightness of the sky.

Although in classical electromagnetism light is described as a wave, the advent of quantum mechanics at the beginning of the twentieth century has allowed us to understand that light also possesses properties typical of particles and to explain phenomena such as the photoelectric effect. In modern physics light (and all electromagnetic radiation) is composed by quanta, fundamental units of electromagnetic field, also called photons.

Physical agent that makes possible the sensation of vision as well as the content of the sensation itself. The ambiguity of the definition arises from the fact that the same term is used to indicate two very different entities that, however, in scientific Latin were once referred to as lumen, the physical agent, and lux, the visual sensation. The confusion between lumen and lux (similar confusion exists for the term “sound” which indicates both a physical agent and a sensation), rooted in the scientific thought of the eighteenth century, after the work of Newton, remained throughout the last century. Both entities were considered one thing and indicated with a single term, corresponding in practice to the Italian term light.

The definition and the study of quantities related to light as a visual sensation are the subject of photometry. The physical agent that causes the sensation of vision is constituted by electromagnetic radiation of wavelength between approx. 400 and approx. 700 nm. The only element that differentiates this interval of the electromagnetic spectrum (interval called the visible spectrum) is precisely the ability of the radiation included in it (called visible radiation) to stimulate the retina of the human eye so as to produce visual sensations in the brain. The quantities related to light (understood in the sense of visible spectrum radiations), their units of measurement and methods to measure them are the subject of radiometry as all other radiations of the electromagnetic spectrum. Although, sometimes, the radiations of the visible spectrum are spoken of as visible light, in fact this term is improper and must be replaced with others of the type “visible radiation”, meaning the adjective visible in the sense “that can be seen” and not in the sense “that can be seen”.

Speaking of visible light derives from the fact that, at the same time, we speak, and just as improperly, of invisible light, including in this diction the ultraviolet light, band of electromagnetic radiation of wavelength less than that of the visible spectrum, and the infrared light, band of electromagnetic radiation of wavelength greater than that of the visible. It is preferable to use the term ultraviolet radiation, or ultraviolet, infrared radiation, or infrared.

Based on the composition of light radiation, we speak of monochromatic light, to indicate visible radiation in which there are electromagnetic radiation of a single wavelength, and polychromatic light, to indicate visible radiation in which there are radiations of multiple wavelengths.

White light is an important light radiation of particular composition (because of the characteristics of light as a physical agent and the phenomena that affect it, see absorption, diffusion, dispersion, emission, interference, radiation, polarization, reflection, refraction, spectrum).

Sunlight, daylight; natural light, artificial light; living light, glaring, fixed; intermittent; electric light, in the same sense as electricity; direct light, coming directly from a light source, not reflected. Specifically: light source, a body or entity emitting light radiation; may be point or extended, real or virtual; light ray, an entity considered by geometrical optics based on the assumption of rectilinear propagation of light; light beam, a bundle of light rays, understood in the geometrical sense; light brush, a very thin bundle of parallel light rays; cold light, luminescence that occurs at extremely low temperatures of the source; ashy light; anthelial light; zodiacal light; quantity of light, in photometry, product of luminous flux times time; the unit of measurement is the lumensecond; coherent light, set of light radiations that are composed of elementary radiations of equal frequency and phase. This condition is not obtainable with the usual light sources, because they are made up of atoms that emit elementary waves in an absolutely random way and therefore not coordinated with each other. In interferometric techniques, to obtain two coherent beams of light, a single beam is split, by means of mirrors or other devices, so that each elementary radiation of the first beam corresponds to an elementary radiation of the second, coherent with it. Coherent light sources are obtained artificially by laser techniques.

Historical notes

For Pythagoras and his school the light consisted in a fluid emitted by the eyes that returned later with the image of the object, while for Democritus and the atomists were the simulacra (or idola) emitted by objects to cause the luminous sensation in the eyes. These theories merged in the conception of Plato, who believed that the light derived from the meeting of two fluids, one coming from the eye and the other from the object. In the third century BC the study of light was addressed by Euclid developing a specific discipline, geometric optics. On the basis of the two postulates that “the rays emitted by the eye proceed directly” and “the figure included by the visual rays is a cone with the vertex in the eye and the base at the edge of the object,” Euclid made a systematic study of the images produced by small openings, shadows, the apparent size of objects and formulated the laws of reflection.

Euclid’s optics dominated unchallenged until the Middle Ages, except for a few contributions by Ptolemy and Eronerelativi especially to new experimental data on the phenomenon of refraction. Around the year one thousand, the Arab Alhazen rejected the thesis of Galen, of Platonic formulation, according to which vision was due to the encounter between the light coming from outside and the light which, secreted by the brain, is led by the optic nerve to the retina and from there to the vitreous humor and the crystalline lens. In the Opticae Thesaurus, Alhazen reintroduced the simple idea of the light ray, no longer considered, however, as the vehicle of the image of the object, but of a single point of it. Each point of light emits an infinite number of rays, but only those that enter the cone having the object as a base and the eye as a vertex determine the vision. Alhazen also studied phenomena related to the use of dark rooms and the magnification produced by glass, the latter considered as a phenomenon of optical illusion. His studies were known in the West through the eclectic work of Vitellione (thirteenth century) to which is substantially linked R. Bacone, who realized the finite speed of light.

In the Renaissance there was a deepening of experimental investigation intended to undermine the conceptual structure of traditional theories on light phenomena, especially by F. Maurolico and G. Della Porta. The latter, in De Magia, offered a sufficiently systematic exposition of old and new experiments, among which the application of the camera obscura to the execution of drawings and the analogy of the camera obscura with the eye to explain the mechanism of vision stand out. These studies made increasingly evident the inadequacy of the old theories, which confused the physiological problem of vision with the physical phenomenon that produces it.

The use in astronomy of the telescope, by Galilei, contributed to link the need for a more rigorous optics to the general change that was taking place in mechanics. The first attempt in this direction was made by G. Kepler who, in the work Ad Vitellionem Paralipomena (1604), made a broad review of all studies of optics in view of the application to astronomy. As part of a geometric theory of light, Kepler gave new importance to the studies conducted using auxiliary instruments, discovering that, in refraction through a diaphragm sphere, an object point corresponds to an image point and that a beam of parallel rays converges in a point, which he called for the first time focus, and developing a precise theory of the telescope.

The close link between optics and mechanics was established in a systematic way by Descartes, as part of his mechanistic conception. He attempted to explain the various optical phenomena on the basis of the hypothesis that light consists of corpuscles in rapid linear movement; among other things, he attributed colors to the different speeds of rotation of matter that transmits the action of light. Starting from these assumptions, Descartes was able to establish the laws of refraction, already discovered experimentally by W. Snellius, and to give a sufficiently valid explanation of the rainbow.

Descartes’ conceptions were opposed by P. Fermat, who made a first attempt to mathematize geometric optics, deriving all his propositions, including Descartes’ law, from the general principle that the light ray travels between two points the path of minimum time. In the meantime, new interesting phenomena were discovered, such as diffraction, by F.M. Grimaldi, and double refraction, by E. Bartholin. G.D. Cassini then observed certain delays in the occultation of the innermost satellite of Jupiter, which he linked to its variable distance from Earth. O. Römer then explained the phenomenon as due to the finite speed of light. The first general theoretical framework of the numerous experimental results was made by I. Newton starting from a series of works of 1675 that, merged and expanded, formed the content of the famous Optics (1704). Newton expounded his corpuscular or emissionistic theory of Cartesian derivation, according to which light is constituted by corpuscles emitted by the luminous body and traveling according to rectilinear trajectories with velocity dependent on the density of the medium in which they move. He also assumed that the motion of the corpuscles aroused vibratory movements in the surrounding ether, such as to strengthen or hinder the light rays.

Newton’s corpuscular conception contradicted the wave hypothesis, exposed for the first time in an organic way by C. Huygens in Traité de la lumière (1690). Huygens argued that light consists in the undulatory motion of the ether, conceived as an elastic matter that penetrates space; each point of the light source communicates a wave motion to the particles of the surrounding ether, which in turn become the center of a tiny wave with a longitudinal plane of oscillation in the direction of propagation. The possibility that light could, in this way, to cover large distances, was explained by Huygens with the envelope principle. The undulatory theory explained very well reflection, refraction and double refraction, but did not give an exhaustive account of the rectilinear propagation of light and other phenomena, such as dispersion and decomposition of white light. The corpuscular theory remained throughout the eighteenth century the only accepted theory on the nature of light. Only in 1801 T. Young, studying interference phenomena, credited the undulatory theory, but only in the years 1808-21, with the study of the complex problem of polarization of light waves, the controversy was resolved.

The research of E.L. Malus (1808) and D. sir Brewster led at first to accredit the corpuscular theory that explained the phenomenon of polarization based on the hypothesis that in natural light the light particles are oriented in all directions, while in crossing birefringent crystals and reflections they are oriented in a particular direction, that is polarized. However, later A. Fresnel, after having solved the question left open by Huygens on the rectilinear propagation of light through an appropriate generalization of the envelope principle, was able to explain the phenomenon of polarization on the basis of the wave theory, identifying the weak point of Huygens’ theory in the assumed longitudinality of light waves; according to Fresnel, it was necessary instead to introduce the hypothesis of the transversality of these waves to fully explain all the experimental results.

Contemporary and partly subsequent to Fresnel’s work were the various attempts to formulate optics in mathematical terms, outside any hypothesis about the nature of light. This mathematical work, started by R.W. sir Hamilton, who rationalized the geometric optics starting from an original minimum principle, was completed by K.G.J. Jacobi in 1842.

The undisputed success of Fresnel’s wave theory did not stop the experimental researches that, through the discovery of light effects in phenomena usually considered of no optical interest (thermometers, thermoelectric batteries, bolometers, emulsions with silver salts and, later, the thermionic and photoelectric effects), made evident the links between optics and the various branches of physics. An attempt to solve this problem was first made by J.C. Maxwell, who, with the electromagnetic theory of light, brought the treatment of optics in the field of electromagnetism.

This solution of the problem was only partial: even the matter, elementary particles (electrons, protons, neutrons etc..) in appropriate experimental conditions presented, in fact, a behavior identical to that of light, giving rise to phenomena of diffraction, reflection, refraction, interference. On the other hand, the problem of the existence of the ether, which became increasingly important as a result of attempts to overcome the “dissymmetries” found between Maxwellian electromagnetism, which assumed for the waves a rigid ether, and Newtonian mechanics, which required a fluid ether, pushed A.A. Michelson and E.W. Morley in 1886 to perform a famous experience, later explained with the abolition of the concept of ether. The efforts of H.A. Lorentz and other physicists of the time were, in fact, directed to overcome the paradoxical results of the experience, explaining with appropriate physical hypothesis on the structure of matter the impossibility to ascertain experimentally the existence of ether.

A radical modification of the traditional conceptions took place, finally, with the theory of relativity of A. Einstein (1905), which eliminated the hypothesis of the ether, attributing to the speed of light the value of a universal constant. Moreover, reconnecting to concepts developed by M. Planck, Einstein introduced the hypothesis that electromagnetic radiations are not emitted and absorbed as continuous waves, but in “packets” of energy, light quanta or photons. In fact, based on the experiences and studies developed in quantum mechanics, especially by L.V. de Broglie and E. Schrödinger, it has come to the unification of the two opposite theories undulatory and corpuscular: it is wrong to attribute to electromagnetic radiation only the undulatory or corpuscular aspect, but both can occur in different conditions.


Light from the Sun provides 2 calories per cm2 per minute. Only 67% of it reaches the Earth’s surface at the rate of 1.34 calories per cm2 per minute at noon in summer. Sunlight is one of the main factors that influence both directly and indirectly every vital activity of the biosphere; it is fundamental in the complex mechanism of photosynthesis and therefore it is the source of energy for the whole trophic chain of living beings both in the terrestrial and in the aqueous environment. The alternation of a luminous phase and a dark phase conditions then a very complex series of biological phenomena: in plants, in addition to photosynthesis, phototropisms and phototactisms, photoperiodisms, some biorhythms, respiration, seed germination, flowering, etc..

In heterotrophic organisms, light is essential for vision and for the development of particular rhythms of activity or rest, the so-called “biological clocks” (see biorhythm), for the reproductive activity related to photoperiodism and other phenomena related to biorhythms such as migration of birds or the change of livery of some mammals, facts that, in the middle and high latitudes, depend on hormonal mechanisms activated by nerve centers sensitive to seasonal variations in the duration of the daily period of solar illumination. According to the necessity of strong or poor illumination to carry out their vital functions, we speak respectively of photophilic and photophobic (or scotophilic) organisms.

For plants it is more precise the term “heliophilic plants”, which need a strong illumination, or “scapophilic plants”, if they prefer a weak and shielded illumination, as, for example, the vegetal associations of an underwood. Truly photophobic organisms are the components of the endogean fauna, but in the same way can be considered those of abyssal marine areas, strictly cave animals (very often depigmented and completely blind) or internal parasites of other animals. See alsoecological (rule of light).


Light has offered food for thought even when the first elements of a cosmogony were derived from mythology: in this sense light was considered a radiation of the divine substance. The concept of light, however, remained anchored to matter; Heraclitus himself, defining light as the “mind of the world”, gave it a more noble position, but without yet detaching it from the concept of matter. Plato clarified the misunderstanding, for whom the Sun, the astro-source of sensitive light, is a symbol and imitation of the intelligible Sun, the supreme idea of Good, from which every other intelligible entity draws life and reality, just as sensitive things draw their life and strength from the sensitive Sun, which is the source of light and energy.

Aristotle, while addressing the problem differently, left to the light its value of “eternal essence” and entity belonging to the celestial world, but in relation to the earthly world and regulating its rhythms and changes; finally Plotinus and Neoplatonism developed a real “metaphysics of light”, which, continuing to distinguish Platonically between intelligible light and sensitive light, made the one and the other different degrees of emanation from the One: higher degree the intelligible light, direct emanation of the One, and lower the sensitive light, proper to the world of bodies.

In the Middle Ages, thanks mainly to the optical-mathematical speculations of the Oxford School, especially Robert Grossatesta, the metaphysics of light took a decisive role in the development of physical sciences and mathematics in Europe. The light, considered a direct emanation of God, was studied in order to understand the divine laws, established for the regulation of the Universe. In the modern period the concept of light has resumed its physical meaning with the exception of a momentary metaphysical affirmation in the “philosophy of Nature” by Schelling.

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