AskDefine | Define light

Dictionary Definition

light adj
1 of comparatively little physical weight or density; "a light load"; "magnesium is a light metal--having a specific gravity of 1.74 at 20 degrees C" [ant: heavy]
2 (used of color) having a relatively small amount of coloring agent; "light blue"; "light colors such as pastels"; "a light-colored powder" [syn: light-colored] [ant: dark]
3 of the military or industry; using (or being) relatively small or light arms or equipment; "light infantry"; "light cavalry"; "light industry"; "light weapons" [ant: heavy]
4 not great in degree or quantity or number; "a light sentence"; "a light accent"; "casualties were light"; "light snow was falling"; "light misty rain"; "light smoke from the chimney" [ant: heavy]
5 psychologically light; especially free from sadness or troubles; "a light heart" [ant: heavy]
6 characterized by or emitting light; "a room that is light when the shutters are open"; "the inside of the house was airy and light" [ant: dark]
7 used of vowels or syllables; pronounced with little or no stress; "a syllable that ends in a short vowel is a light syllable"; "a weak stress on the second syllable" [syn: unaccented, weak]
8 easily assimilated in the alimentary canal; not rich or heavily seasoned; "a light diet"
9 (used of soil) loose and large-grained in consistency; "light sandy soil" [syn: friable, sandy]
10 (of sound or color) free from anything that dulls or dims; "efforts to obtain a clean bass in orchestral recordings"; "clear laughter like a waterfall"; "clear reds and blues"; "a light lilting voice like a silver bell" [syn: clean, clear, unclouded]
11 moving easily and quickly; nimble; "the dancer was light and graceful"; "a lightsome buoyant step"; "walked with a light tripping step" [syn: lightsome, tripping]
12 demanding little effort; not burdensome; "light housework"; "light exercise"
13 of little intensity or power or force; "the light touch of her fingers"; "a light breeze" [ant: heavy]
14 (physics, chemistry) not having atomic weight greater than average; "light water is ordinary water" [ant: heavy]
15 weak and likely to lose consciousness; "suddenly felt faint from the pain"; "was sick and faint from hunger"; "felt light in the head"; "a swooning fit"; "light-headed with wine"; "light-headed from lack of sleep" [syn: faint, swooning, light-headed, lightheaded]
16 very thin and insubstantial; "thin paper"; "flimsy voile"; "light summer dresses" [syn: flimsy]
17 marked by temperance in indulgence; "abstemious meals"; "a light eater"; "a light smoker"; "ate a light supper" [syn: abstemious, light(a)]
18 less than the correct or legal or full amount often deliberately so; "a light pound"; "a scant cup of sugar"; "regularly gives short weight" [syn: scant(p), short]
19 having little importance; "losing his job was no light matter"
20 intended primarily as entertainment; not serious or profound; "light verse"; "a light comedy"
21 silly or trivial; "idle pleasure"; "light banter"; "light idle chatter" [syn: idle]
22 having a spongy or flaky texture; well-leavened; "light pastries"
23 designed for ease of movement or to carry little weight; "light aircraft"; "a light truck"
24 having relatively few calories; "diet cola"; "light (or lite) beer"; "lite (or light) mayonnaise"; "a low-cal diet" [syn: lite, low-cal]
25 (of sleep) easily disturbed; "in a light doze"; "a light sleeper"; "a restless wakeful night" [syn: wakeful]
26 casual and unrestrained in sexual behavior; "her easy virtue"; "he was told to avoid loose (or light) women"; "wanton behavior" [syn: easy, loose, promiscuous, sluttish, wanton]


1 (physics) electromagnetic radiation that can produce a visual sensation; "the light was filtered through a soft glass window" [syn: visible light, visible radiation]
2 any device serving as a source of illumination; "he stopped the car and turned off the lights" [syn: light source]
3 a particular perspective or aspect of a situation; "although he saw it in a different light, he still did not understand"
4 the quality of being luminous; emitting or reflecting light; "its luminosity is measured relative to that of our sun" [syn: luminosity, brightness, brightness level, luminance, luminousness]
5 an illuminated area; "he stepped into the light"
6 a condition of spiritual awareness; divine illumination; "follow God's light" [syn: illumination]
7 the visual effect of illumination on objects or scenes as created in pictures; "he could paint the lightest light and the darkest dark" [syn: lightness]
8 a person regarded very fondly; "the light of my life"
9 mental understanding as an enlightening experience; "he finally saw the light"; "can you shed light on this problem?"
10 having abundant light or illumination; "they played as long as it was light"; "as long as the lighting was good" [syn: lighting] [ant: dark]
11 public awareness; "it brought the scandal to light"
12 brightness and animation of countenance; "he had a sparkle in his eye" [syn: sparkle, spark]
13 a divine presence believed by Quakers to enlighten and guide the soul [syn: Inner Light, Light Within, Christ Within]
14 a visual warning signal; "they saw the light of the beacon"; "there was a light at every corner"
15 a device for lighting or igniting fuel or charges or fires; "do you have a light?" [syn: lighter, igniter, ignitor] adv : with few burdens; "experienced travellers travel light" [syn: lightly]


1 make lighter or brighter; "This lamp lightens the room a bit" [syn: illume, illumine, light up, illuminate]
2 begin to smoke; "After the meal, some of the diners lit up" [syn: light up, fire up]
3 to come to rest, settle; "Misfortune lighted upon him" [syn: alight, perch]
4 cause to start burning; subject to fire or great heat; "Great heat can ignite almost any dry matter"; "Light a cigarette" [syn: ignite] [ant: snuff out]
5 fall to somebody by assignment or lot; "The task fell to me"; "It fell to me to notify the parents of the victims" [syn: fall]
6 get off (a horse) [syn: unhorse, dismount, get off, get down] [also: lit]

User Contributed Dictionary



  • , /laɪt/, /laIt/
  • Rhymes: -aɪt


Etymology 1



  1. To start a fire.
  2. To illuminate.
Usage notes
In the UK, as a predicative adjective, lighted is more usual than lit. So, for example, "a lighted candle" is more usual than "a lit candle". Conversely, in the US, lit is more common.
to start a fire
to illuminate

Etymology 2

lēoht. Cognate with Dutch licht, German Licht.


  1. The natural medium emanating from the sun and other very hot sources (now recognised as electromagnetic radiation with a wavelength of 400-750 nm), within which vision is possible.
    As you can see, this spacious dining-room gets a lot of light in the mornings.
  2. A source of illumination.
    Put that light out!
  3. Spiritual or mental illumination; enlightenment, useful information.
    Can you throw any light on this problem?
  4. A notable person within a specific field or discipline.
    Picasso was one of the leading lights of the cubist movement.
  5. A point of view, or aspect from which a concept, person or thing is regarded.
    I'm really seeing you in a different light today.
  6. A flame or something used to create fire.
    Hey, buddy, you got a light?
  7. A window, or space for a window in architecture
    This facade has eight south-facing lights.
  8. The series of squares reserved for the answer to a crossword clue
    The average length of a light on a 15x15 grid is 7 or 8.
electromagnetic wave
a source of light
a spiritual truth
a point of view
a flame or something used to create fire.


en-adj er
  1. having light
  2. pale in colour
  3. (coffee) served with extra milk or cream
  • (having light): bright
  • (pale in colour): pale
  • (coffee: served with extra milk or cream): with milk, with cream
having light
pale in colour
coffee: served with extra milk or cream

Etymology 4

lēocht. Cognate with Dutch licht, German leicht, Swedish lätt, Norwegian lett.


en-adj er
  1. Of low weight; not heavy.
    My bag was much lighter once I had dropped off the books.
  2. Lightly-built; designed for speed or small loads.
    We took a light aircraft down to the city.
  3. Gentle; having little force or momentum.
    This artist clearly had a light, flowing touch.
  4. Low in fat, calories, alcohol, salt, etc.
    This light beer still gets you drunk if you have enough of it.
  5. Unimportant, trivial, having little value or significance.
    I made some light comment, and we moved on.
of low weight
low in fat, calories, alcohol, salt, etc.
of little significance


  1. A stone that is not thrown hard enough


  1. To unload a ship, ot to jettison material to make it lighter
  • Italian: allegiare, allibare

Etymology 5



  1. To come by chance.
    I lit upon a rare book in a second-hand bookseller's.
  2. To alight.
    She fell out of the window but luckily lit on her feet.
Derived terms
to come by chance
to alight
  • Vietnamese: xuống

Extensive Definition

Light, or visible light, is electromagnetic radiation of a wavelength that is visible to the human eye (about 400–700 nm). In a scientific context, the word light is sometimes used to refer to the entire electromagnetic spectrum. Moreover, in optics, the term "visible light" refers to electromagnetic radiation with wavelengths of ~300 nm (near UV) through ~1400 nm (near infrared). Light is composed of elementary particles called photons.
Three primary properties of light are:
Light can exhibit properties of both waves and particles. This property is referred to as wave–particle duality. The study of light, known as optics, is an important research area in modern physics.

Speed of light

The speed of light in a vacuum is exactly 299,792,458 m/s (about 186,282.397 miles per second). The speed of light depends upon the medium in which it is traveling, and the speed will be lower in a transparent medium. Although commonly called the "velocity of light", technically the word velocity is a vector quantity, having both magnitude and direction. Speed refers only to the magnitude of the velocity vector. This fixed definition of the speed of light is a result of the modern attempt, in physics, to define the basic unit of length in terms of the speed of light, rather than defining the speed of light in terms of a length.
Different physicists have attempted to measure the speed of light throughout history. Galileo attempted to measure the speed of light in the seventeenth century. A good early experiment to measure the speed of light was conducted by Ole Rømer, a Danish physicist, in 1676. Using a telescope, Ole observed the motions of Jupiter and one of its moons, Io. Noting discrepancies in the apparent period of Io's orbit, Rømer calculated that light takes about 18 minutes to traverse the diameter of Earth's orbit. Unfortunately, this was not a value that was known at that time. If Ole had known the diameter of the earth's orbit, he would have calculated a speed of 227,000,000 m/s.
Another, more accurate, measurement of the speed of light was performed in Europe by Hippolyte Fizeau in 1849. Fizeau directed a beam of light at a mirror several kilometers away. A rotating cog wheel was placed in the path of the light beam as it traveled from the source, to the mirror and then returned to its origin. Fizeau found that at a certain rate of rotation, the beam would pass through one gap in the wheel on the way out and the next gap on the way back. Knowing the distance to the mirror, the number of teeth on the wheel, and the rate of rotation, Fizeau was able to calculate the speed of light as 313,000,000 m/s.
Léon Foucault used an experiment which used rotating mirrors to obtain a value of 298,000,000 m/s in 1862. Albert A. Michelson conducted experiments on the speed of light from 1877 until his death in 1931. He refined Foucault's methods in 1926 using improved rotating mirrors to measure the time it took light to make a round trip from Mt. Wilson to Mt. San Antonio in California. The precise measurements yielded a speed of 299,796,000 m/s.
Some scientists were able to bring light to a complete standstill by passing it through a Bose-Einstein Condensate of the element rubidium.


Note, n = 1 in a vacuum and n > 1 in a transparent medium, where n is the index of refraction.
When a beam of light crosses the boundary between a vacuum and another medium, or between two different mediums, the wavelength of the light changes, but the frequency remains constant. If the beam of light is not orthogonal to the boundary, the change in wavelength results in a change in the direction of the beam. This change of direction is known as refraction.
The refraction quality of lenses is frequently used to manipulate light in order to change the apparent size of images. Magnifying glasses, spectacles, contact lenses, microscopes and refracting telescopes are all examples of this manipulation.


The study of light and the interaction of light and matter is termed optics. The observation and study of optical phenomena such as rainbows and the aurora borealis offer many clues as to the nature of light as well as much enjoyment.

Light sources

seealso List of light sources
There are many sources of light. The most common light sources are thermal: a body at a given temperature emits a characteristic spectrum of black-body radiation. Examples include sunlight (the radiation emitted by the chromosphere of the Sun at around 6,000 K peaks in the visible region of the electromagnetic spectrum), incandescent light bulbs (which emit only around 10% of their energy as visible light and the remainder as infrared), and glowing solid particles in flames. The peak of the blackbody spectrum is in the infrared for relatively cool objects like human beings. As the temperature increases, the peak shifts to shorter wavelengths, producing first a red glow, then a white one, and finally a blue color as the peak moves out of the visible part of the spectrum and into the ultraviolet. These colors can be seen when metal is heated to "red hot" or "white hot". The blue color is most commonly seen in a gas flame or a welder's torch.
Atoms emit and absorb light at characteristic energies. This produces "emission lines" in the spectrum of each atom. Emission can be spontaneous, as in light-emitting diodes, gas discharge lamps (such as neon lamps and neon signs, mercury-vapor lamps, etc.), and flames (light from the hot gas itself—so, for example, sodium in a gas flame emits characteristic yellow light). Emission can also be stimulated, as in a laser or a microwave maser.
Acceleration of a free charged particle, such as an electron, can produce visible radiation: cyclotron radiation, synchrotron radiation, and bremsstrahlung radiation are all examples of this. Particles moving through a medium faster than the speed of light in that medium can produce visible Cherenkov radiation.
Certain chemicals produce visible radiation by chemoluminescence. In living things, this process is called bioluminescence. For example, fireflies produce light by this means, and boats moving through water can disturb plankton which produce a glowing wake.
Certain substances produce light when they are illuminated by more energetic radiation, a process known as fluorescence. This is used in fluorescent lights. Some substances emit light slowly after excitation by more energetic radiation. This is known as phosphorescence.
Phosphorescent materials can also be excited by bombarding them with subatomic particles. Cathodoluminescence is one example of this. This mechanism is used in cathode ray tube televisions.
When the concept of light is intended to include very-high-energy photons (gamma rays), additional generation mechanisms include:

Theories about light

Indian theories

In ancient India, the philosophical schools of Samkhya and Vaisheshika, from around the 6th5th century BC, developed theories on light. According to the Samkhya school, light is one of the five fundamental "subtle" elements (tanmatra) out of which emerge the gross elements. The atomicity of these elements is not specifically mentioned and it appears that they were actually taken to be continuous.
On the other hand, the Vaisheshika school gives an atomic theory of the physical world on the non-atomic ground of ether, space and time. (See Indian atomism.) The basic atoms are those of earth (prthivı), water (apas), fire (tejas), and air (vayu), that should not be confused with the ordinary meaning of these terms. These atoms are taken to form binary molecules that combine further to form larger molecules. Motion is defined in terms of the movement of the physical atoms and it appears that it is taken to be non-instantaneous. Light rays are taken to be a stream of high velocity of tejas (fire) atoms. The particles of light can exhibit different characteristics depending on the speed and the arrangements of the tejas atoms. Around the first century BC, the Vishnu Purana correctly refers to sunlight as the "the seven rays of the sun".
Later in 499, Aryabhata, who proposed a heliocentric solar system of gravitation in his Aryabhatiya, wrote that the planets and the Moon do not have their own light but reflect the light of the Sun.
The Indian Buddhists, such as Dignāga in the 5th century and Dharmakirti in the 7th century, developed a type of atomism that is a philosophy about reality being composed of atomic entities that are momentary flashes of light or energy. They viewed light as being an atomic entity equivalent to energy, similar to the modern concept of photons, though they also viewed all matter as being composed of these light/energy particles.

Greek and Hellenistic theories

In the fifth century BC, Empedocles postulated that everything was composed of four elements; fire, air, earth and water. He believed that Aphrodite made the human eye out of the four elements and that she lit the fire in the eye which shone out from the eye making sight possible. If this were true, then one could see during the night just as well as during the day, so Empedocles postulated an interaction between rays from the eyes and rays from a source such as the sun.
In about 300 BC, Euclid wrote Optica, in which he studied the properties of light. Euclid postulated that light travelled in straight lines and he described the laws of reflection and studied them mathematically. He questioned that sight is the result of a beam from the eye, for he asks how one sees the stars immediately, if one closes one's eyes, then opens them at night. Of course if the beam from the eye travels infinitely fast this is not a problem.
In 55 BC, Lucretius, a Roman who carried on the ideas of earlier Greek atomists, wrote:
"The light & heat of the sun; these are composed of minute atoms which, when they are shoved off, lose no time in shooting right across the interspace of air in the direction imparted by the shove." - On the nature of the Universe
Despite being similar to later particle theories, Lucretius's views were not generally accepted and light was still theorized as emanating from the eye.
Ptolemy (c. 2nd century) wrote about the refraction of light, and developed a theory of vision that objects are seen by rays of light emanating from the eyes.

Optical theory

The Muslim scientist Ibn al-Haytham (c. 965-1040), known as Alhacen in the West, in his Book of Optics, developed a broad theory that explained vision, using geometry and anatomy, which stated that each point on an illuminated area or object radiates light rays in every direction, but that only one ray from each point, which strikes the eye perpendicularly, can be seen. The other rays strike at different angles and are not seen. He described the pinhole camera and invented the camera obscura, which produces an inverted image, and used it as an example to support his argument. This contradicted Ptolemy's theory of vision that objects are seen by rays of light emanating from the eyes. Alhacen held light rays to be streams of minute particles that travelled at a finite speed. He improved Ptolemy's theory of the refraction of light, and went on to discover the laws of refraction.
He also carried out the first experiments on the dispersion of light into its constituent colors. His major work Kitab al-Manazir was translated into Latin in the Middle Ages, as well his book dealing with the colors of sunset. He dealt at length with the theory of various physical phenomena like shadows, eclipses, the rainbow. He also attempted to explain binocular vision, and gave a correct explanation of the apparent increase in size of the sun and the moon when near the horizon. Because of his extensive research on optics, Al-Haytham is considered the father of modern optics.
Al-Haytham also correctly argued that we see objects because the sun's rays of light, which he believed to be streams of tiny particles travelling in straight lines, are reflected from objects into our eyes. He understood that light must travel at a large but finite velocity, and that refraction is caused by the velocity being different in different substances. He also studied spherical and parabolic mirrors, and understood how refraction by a lens will allow images to be focused and magnification to take place. He understood mathematically why a spherical mirror produces aberration.

The 'plenum'

René Descartes (1596-1650) held that light was a disturbance of the plenum, the continuous substance of which the universe was composed. In 1637 he published a theory of the refraction of light that assumed, incorrectly, that light travelled faster in a denser medium than in a less dense medium. Descartes arrived at this conclusion by analogy with the behaviour of sound waves. Although Descartes was incorrect about the relative speeds, he was correct in assuming that light behaved like a wave and in concluding that refraction could be explained by the speed of light in different media. As a result, Descartes' theory is often regarded as the forerunner of the wave theory of light.

Particle theory

Pierre Gassendi (1592-1655), an atomist, proposed a particle theory of light which was published posthumously in the 1660s. Isaac Newton studied Gassendi's work at an early age, and preferred his view to Descartes' theory of the plenum. He stated in his Hypothesis of Light of 1675 that light was composed of corpuscles (particles of matter) which were emitted in all directions from a source. One of Newton's arguments against the wave nature of light was that waves were known to bend around obstacles, while light travelled only in straight lines. He did, however, explain the phenomenon of the diffraction of light (which had been observed by Francesco Grimaldi) by allowing that a light particle could create a localised wave in the aether.
Newton's theory could be used to predict the reflection of light, but could only explain refraction by incorrectly assuming that light accelerated upon entering a denser medium because the gravitational pull was greater. Newton published the final version of his theory in his Opticks of 1704. His reputation helped the particle theory of light to hold sway during the 18th century. The particle theory of light led Laplace to argue that a body could be so massive that light could not escape from it. In other words it would become what is now called a black hole. Laplace withdrew his suggestion when the wave theory of light was firmly established. A translation of his essay appears in The large scale structure of space-time, by Stephen Hawking and George F. R. Ellis.

Wave theory

In the 1660s, Robert Hooke published a wave theory of light. Christiaan Huygens worked out his own wave theory of light in 1678, and published it in his Treatise on light in 1690. He proposed that light was emitted in all directions as a series of waves in a medium called the Luminiferous ether. As waves are not affected by gravity, it was assumed that they slowed down upon entering a denser medium.
The wave theory predicted that light waves could interfere with each other like sound waves (as noted around 1800 by Thomas Young), and that light could be polarized. Young showed by means of a diffraction experiment that light behaved as waves. He also proposed that different colors were caused by different wavelengths of light, and explained color vision in terms of three-colored receptors in the eye.
Another supporter of the wave theory was Leonhard Euler. He argued in Nova theoria lucis et colorum (1746) that diffraction could more easily be explained by a wave theory.
Later, Augustin-Jean Fresnel independently worked out his own wave theory of light, and presented it to the Académie des Sciences in 1817. Simeon Denis Poisson added to Fresnel's mathematical work to produce a convincing argument in favour of the wave theory, helping to overturn Newton's corpuscular theory.
The weakness of the wave theory was that light waves, like sound waves, would need a medium for transmission. A hypothetical substance called the luminiferous aether was proposed, but its existence was cast into strong doubt in the late nineteenth century by the Michelson-Morley experiment.
Newton's corpuscular theory implied that light would travel faster in a denser medium, while the wave theory of Huygens and others implied the opposite. At that time, the speed of light could not be measured accurately enough to decide which theory was correct. The first to make a sufficiently accurate measurement was Léon Foucault, in 1850. His result supported the wave theory, and the classical particle theory was finally abandoned.

Electromagnetic theory

In 1845, Michael Faraday discovered that the angle of polarization of a beam of light as it passed through a polarizing material could be altered by a magnetic field, an effect now known as Faraday rotation. This was the first evidence that light was related to electromagnetism. Faraday proposed in 1847 that light was a high-frequency electromagnetic vibration, which could propagate even in the absence of a medium such as the ether.
Faraday's work inspired James Clerk Maxwell to study electromagnetic radiation and light. Maxwell discovered that self-propagating electromagnetic waves would travel through space at a constant speed, which happened to be equal to the previously measured speed of light. From this, Maxwell concluded that light was a form of electromagnetic radiation: he first stated this result in 1862 in On Physical Lines of Force. In 1873, he published A Treatise on Electricity and Magnetism, which contained a full mathematical description of the behaviour of electric and magnetic fields, still known as Maxwell's equations. Soon after, Heinrich Hertz confirmed Maxwell's theory experimentally by generating and detecting radio waves in the laboratory, and demonstrating that these waves behaved exactly like visible light, exhibiting properties such as reflection, refraction, diffraction, and interference. Maxwell's theory and Hertz's experiments led directly to the development of modern radio, radar, television, electromagnetic imaging, and wireless communications.

The special theory of relativity

The wave theory was wildly successful in explaining nearly all optical and electromagnetic phenomena, and was a great triumph of nineteenth century physics. By the late nineteenth century, however, a handful of experimental anomalies remained that could not be explained by or were in direct conflict with the wave theory. One of these anomalies involved a controversy over the speed of light. The constant speed of light predicted by Maxwell's equations and confirmed by the Michelson-Morley experiment contradicted the mechanical laws of motion that had been unchallenged since the time of Galileo, which stated that all speeds were relative to the speed of the observer. In 1905, Albert Einstein resolved this paradox by revising the Galilean model of space and time to account for the constancy of the speed of light. Einstein formulated his ideas in his special theory of relativity, which radically altered humankind's understanding of space and time. Einstein also demonstrated a previously unknown fundamental equivalence between energy and mass with his famous equation
E = mc^2 \,
where E is energy, m is rest mass, and c is the speed of light.

Particle theory revisited

Another experimental anomaly was the photoelectric effect, by which light striking a metal surface ejected electrons from the surface, causing an electric current to flow across an applied voltage. Experimental measurements demonstrated that the energy of individual ejected electrons was proportional to the frequency, rather than the intensity, of the light. Furthermore, below a certain minimum frequency, which depended on the particular metal, no current would flow regardless of the intensity. These observations clearly contradicted the wave theory, and for years physicists tried in vain to find an explanation. In 1905, Einstein solved this puzzle as well, this time by resurrecting the particle theory of light to explain the observed effect. Because of the preponderance of evidence in favor of the wave theory, however, Einstein's ideas were met initially by great skepticism among established physicists. But eventually Einstein's explanation of the photoelectric effect would triumph, and it ultimately formed the basis for wave–particle duality and much of quantum mechanics.

Quantum theory

A third anomaly that arose in the late 19th century involved a contradiction between the wave theory of light and measurements of the electromagnetic spectrum emitted by thermal radiators, or so-called black bodies. Physicists struggled with this problem, which later became known as the ultraviolet catastrophe, unsuccessfully for many years. In 1900, Max Planck developed a new theory of black-body radiation that explained the observed spectrum correctly. Planck's theory was based on the idea that black bodies emit light (and other electromagnetic radiation) only as discrete bundles or packets of energy. These packets were called quanta, and the particle of light was given the name photon, to correspond with other particles being described around this time, such as the electron and proton. A photon has an energy, E, proportional to its frequency, f, by
E = hf = \frac \,\!
where h is Planck's constant, \lambda is the wavelength and c is the speed of light. Likewise, the momentum p of a photon is also proportional to its frequency and inversely proportional to its wavelength:
p = = = .
As it originally stood, this theory did not explain the simultaneous wave- and particle-like natures of light, though Planck would later work on theories that did. In 1918, Planck received the Nobel Prize in Physics for his part in the founding of quantum theory.

Wave–particle duality

The modern theory that explains the nature of light includes the notion of wave–particle duality, described by Albert Einstein in the early 1900s, based on his study of the photoelectric effect and Planck's results. Einstein asserted that the energy of a photon is proportional to its frequency. More generally, the theory states that everything has both a particle nature and a wave nature, and various experiments can be done to bring out one or the other. The particle nature is more easily discerned if an object has a large mass, and it was not until a bold proposition by Louis de Broglie in 1924 that the scientific community realized that electrons also exhibited wave–particle duality. The wave nature of electrons was experimentally demonstrated by Davission and Germer in 1927. Einstein received the Nobel Prize in 1921 for his work with the wave–particle duality on photons (especially explaining the photoelectric effect thereby), and de Broglie followed in 1929 for his extension to other particles.

Quantum electrodynamics

The quantum mechanical theory of light and electromagnetic radiation continued to evolve through the 1920's and 1930's, and culminated with the development during the 1940's of the theory of quantum electrodynamics, or QED. This so-called quantum field theory is among the most comprehensive and experimentally successful theories ever formulated to explain a set of natural phenomena. QED was developed primarily by physicists Richard Feynman, Freeman Dyson, Julian Schwinger, and Shin-Ichiro Tomonaga. Feynman, Schwinger, and Tomonaga shared the 1965 Nobel Prize in Physics for their contributions.

Light pressure

Light pushes on objects in its way, just as the wind would do. This pressure is most easily explainable in particle theory: photons hit and transfer their momentum. Light pressure can cause asteroids to spin faster, acting on their irregular shapes as on the vanes of a windmill. The possibility to make solar sails that would accelerate spaceships in space is also under investigation.
Although the motion of the Crookes radiometer was originally attributed to light pressure, this interpretation is incorrect; the characteristic Crookes rotation is the result of a partial vacuum. This should not be confused with the Nichols radiometer, in which the motion is directly caused by light pressure.


The sensory perception of light plays a central role in spirituality (vision, enlightenment, darshan, Tabor Light), and the presence of light as opposed to its absence (darkness) is a common Western metaphor of good and evil, knowledge and ignorance, and similar concepts.


See also

light in Dutch: Licht
light in Dutch Low Saxon: Locht (straoling)
light in Newari: जः
light in Japanese: 光
light in Neapolitan: Luce
light in Norwegian: Lys
light in Norwegian Nynorsk: Lys
light in Narom: Lumyire
light in Novial: Lume
light in Occitan (post 1500): Lutz
light in Uzbek: Yorugʻlik
light in Polish: Światło
light in Portuguese: Luz
light in Romanian: Lumină
light in Quechua: Achkiy
light in Russian: Свет
light in Albanian: Drita
light in Sicilian: Luci
light in Simple English: Light
light in Slovak: Svetlo
light in Slovenian: Svetloba
light in Serbian: Светлост
light in Finnish: Valo
light in Swedish: Ljus
light in Tamil: ஒளி
light in Telugu: కాంతి
light in Thai: แสง
light in Vietnamese: Ánh sáng
light in Tajik: Нӯр
light in Turkish: Işık
light in Ukrainian: Світло (фізика)
light in Urdu: روشنی
light in Walloon: Loumire
light in Yiddish: ליכטיגקייט
light in Yoruba: Imole
light in Contenese: 光
light in Chinese: 光

Synonyms, Antonyms and Related Words

ASA scale, Anschauung, British candle, Hefner candle, Mickey Mouse, Paphian, Scheiner scale, Very flare, abuse, accented, account, acquaintance, active, aerial, aeriform, aery, agile, airish, airlike, airy, alabaster, alabastrine, albescent, alfresco, alight, allegorization, alveolar, amplitude, amusing, angle, angle of vision, ankle-deep, announcement, antelope, antinode, apical, apico-alveolar, apico-dental, appear, arrow, articulated, asinine, aspect, assail, assault, assimilated, atmospheric, attack, attenuate, attenuated, aurora, back, balefire, bank, barytone, basis, bay, bay window, be exposed, be revealed, beacon, beacon fire, beacons, beaming, bearable, beat, belabor, berate, bilabial, birdbrained, birdwitted, blond, blue book, blue darter, blue streak, bougie decimale, bow window, boyish, brand, break, break of day, breakable, breezy, briefing, bright, brighten, brighten up, brightening, brightness, brilliant, bring to light, brittle, broad, bubbly, bulletin, bump, buoyant, burn, butane lighter, cacuminal, camp, campy, candle, candle lumen, candle power, candle-foot, candle-hour, candle-meter, candlelight, cannonball, carefree, casement, casement window, casual, catchpenny, central, cerebral, chambering, chance, changeable, chanticleer, cheap-jack, checked, cheer up, cheerful, cigarette lighter, clarification, clarify, clear, climb down, clobber, close, cloudless, cobwebby, cockcrow, cocklight, come across, come down, come in, come out, come to light, comic, communication, communique, configuration, conflagrate, considering, consonant, consonantal, continuant, coquettish, corky, counterglow, courser, crack of dawn, cracking, crash-land, cream, creamy, crest, crumbly, cursory, cushy, dainty, dart, data, datum, dawn, dawning, day-peep, daybreak, daylight, dayspring, de Broglie wave, debark, debonair, decimal candle, decipherment, decoding, delicate, delicately weak, demonstration, demythologization, dental, deplane, depthless, descend, detrain, develop, diaphane, diaphanous, diffraction, diluted, dim, directory, disclose, discover, disembark, dismiss, dismount, dispatch, display case, dissimilated, ditch, diverting, dizzy, dorsal, downwind, downy, dun-white, eagle, easy, easy as pie, easygoing, editing, effect, effeminate, effortless, effulgent, eggshell, eidolon, electric light bulb, electricity, electromagnetic radiation, electromagnetic wave, elucidate, elucidation, emendation, emerge, empty, empty-headed, encounter, endurable, enkindle, enlighten, enlightenment, entertaining, epidermal, eternity, ethereal, euhemerism, evanescent, evidence, evolve, exegesis, exemplification, explain, explanation, explication, expose, exposed, exposition, exposure meter, expounding, express train, eye, facet, facile, facts, factual information, faddish, faded, faint, fair, familiarization, fan the flame, fan window, fanlight, farcical, fashion, fatuous, featherbrained, featherweight, feathery, featly, feature, feed, feed the fire, fickle, figure, find, fine, fine-drawn, finespun, fire, firebrand, firelight, first brightening, flambeau, flame, flare, flare-up, flash, flashlight, flat, fleet, flighty, flimsy, flint, flint and steel, flirtatious, flood with light, floodlight, fluffy, fluorescence, fluorescent, flux, foamy, foolish, foot-candle, footing, form, fragile, frail, frame of reference, framework, frangible, free and easy, frequency, frequency band, frequency spectrum, fribble, fribbling, frivolous, front, frothy, fusee, futile, gaslight, gauzy, gay, gazelle, gen, general information, gentle, gestalt, get down, get off, giddy, gimcrack, gimcracky, girlish, glare, glass, glassware, glaucescent, glaucous, gleam, glib, glide, glim, glory, gloss, glossal, glottal, glow, go down, gossamer, gossamery, graceful, gracile, gray-white, greased lightning, greyhound, grille, grow bright, grow light, guidebook, guided wave, guise, guttural, handout, happy, happy-go-lucky, harangue, hard, hard information, hare, headlight, heavy, high, highlight, hit, holiness, idle, ignite, igniter, ignition, illume, illuminant, illuminate, illumination, illuminator, illumine, illustration, image, imago, immutability, imperceptible, imponderous, impression, in consideration of, in phase, in view of, inane, incandescence, incandescent, incandescent body, incidental information, inconsequential, inconsiderable, inconstant, indistinct, infinite goodness, infinite justice, infinite love, infinite mercy, infinite power, infinite wisdom, infinity, inflame, info, information, insight, insignificant, instruction, insubstantial, intelligence, intensity, interference, international candle, intonated, iridescent, irradiate, ivory, ivory-white, jaunty, jejune, jerry, jerry-built, jet plane, jolly, jovial, joyful, kindle, knee-deep, knowledge, labial, labiodental, labiovelar, lacy, lambaste, lamp, lamp-hour, lancet window, land, lantern, lateral, lattice, lax, leger, level off, light as air, light bulb, light into, light meter, light quantum, light source, light up, light upon, light-colored, light-footed, light-headed, light-hearted, light-hued, lighten, lighter, lighter than vanity, lighthearted, lighthouse, lightish, lightning, lightship, lightsome, lightweight, likeness, limber, lineaments, lingual, lint-white, liquid, lissome, lithe, little, longitudinal wave, look, loose, loose-moraled, louver window, low, lucent, lucid, luck, luculent, lumen, lumen meter, lumen-hour, lumeter, luminant, luminary, luminate, luminescence, luminosity, luminous, luminous flux, luminous intensity, luminous power, lux, magnesium flare, majesty, make a light, make known, make light of, manner, match, meager, mechanical wave, meet, meet up with, mellow, mental outlook, mention, mercurial, mercury, merry, message, mid, mild, misty, moderate, monophthongal, moon, moonlight, morn, morning, mother-of-pearl, mousse, muted, nacreous, namby-pamby, narrow, nasal, nasalized, neat-fingered, neat-handed, nimble, nimble-footed, node, not deep, nothing to it, notice, notification, nugacious, nugatory, obscure, occlusive, occulting light, of easy virtue, of loose morals, off-white, omnipotence, omnipotency, omnipresence, omniscience, omnisciency, on the surface, opalescent, open, open the light, open-air, oriel, otiose, out of phase, outlook, overshine, overshoot, oxytone, painless, palatal, palatalized, pale, pancake, pane, papery, pasteboardy, pastel, patinaed, pearl, pearly, pearly-white, peart, peep of day, pellucid, perch, period, periodic wave, perky, petty, pharos, pharyngeal, pharyngealized, phase, phasis, phonemic, phonetic, phonic, phosphorescence, phosphorescent, photon, picture window, pitch, pitched, place, plain, pneumatic, point of view, port, portable, portfire, porthole, position, posttonic, presentation, prime, promiscuous, promotional material, proof, publication, publicity, puny, put on, quantum, quicksilver, quiet, radiance, radiation, radio wave, rare, rarefied, rationale, ray, reason, reference, reference system, reflection, regard, reinforcement, rekindle, release, relight, relucent, relume, relumine, report, resilient, resonance, resonance frequency, respect, retroflex, reveal, ridicule, rocket, roomy, roost, rose window, rounded, sad, scared rabbit, scatterbrained, scintillation, scold, seeming, seismic wave, semblance, semigloss, semivowel, serene, set down, set fire to, set on fire, settle, settle down, shallow, shallow-headed, shallow-minded, shallow-pated, shallow-rooted, shallow-witted, shape, shattery, shed light upon, shine, shine a light, shine upon, shining, shoal, shock wave, shopwindow, shot, showcase, shrug off, side, sidelight, sight, signal beacon, signal flare, signal lamp, signal rocket, silly, simple, simple as ABC, simplification, simplify, simulacrum, sissified, sit down, situation, skin-deep, skinny, skittish, skylight, skyrocket, slack, slant, slapstick, sleazy, slender, slenderish, slight, slight-made, slim, slimmish, slinky, small, smooth, sober, soft, soft-colored, soft-hued, softened, solution, somber, sonant, souffle, sound wave, source of light, sovereignty, sparker, sparkle, spill, spotlight, sprightly, spry, stand, standpoint, starlight, stars, statement, stemware, stir the fire, stoke, stoke the fire, stopped, straightforward, streak, streak of lightning, stressed, strike a light, striped snake, strong, stumble, stumble on, style, subdued, subtle, sun, sunlight, sunny, sunrise, sunup, superficial, supportable, surd, sure-footed, surface, surface wave, svelte, swallow, sweet, swift, switch on, syllabic, sylphlike, system, tacky, taking into account, talk down, taper, tender, tense, tenuous, the dope, the goods, the know, the scoop, thick, thin, thin-bodied, thin-set, thin-spun, thinnish, thought, threadlike, throaty, throw light upon, thunderbolt, tidal wave, tolerable, tonal, tongue-lash, tonic, torch, torrent, total effect, touch down, touch off, toying, tragicomic, translucent, translucid, transmission, transom, transparent, transparent substance, transpire, transverse wave, trifling, trite, trivial, trivialize, tropospheric, trough, tumble, turn on, turn up, twangy, twist, ubiquity, unaccented, unburdensome, unclear, unclouded, uncomplicated, uncover, undemanding, undependable, understanding, underweight, unearth, unheavy, unhorse, unimportant, unit of flux, unit of light, unity, universe, unlocking, unobscured, unpredictable, unprofound, unreliable, unrounded, unstressed, unsubstantial, untaxing, unveil, upbraid, upwind, vacant, vacillating, vacuous, vague, vain, vapid, velar, versatile, vertiginous, vestibule of Day, view, viewpoint, vitrics, vitrine, vocalic, vocoid, voiced, voiceless, volatile, vowel, vowellike, wanton, watch crystal, watch fire, watch glass, watered, watered-down, watery, wave, wave equation, wave motion, wave number, wavelength, wayward, weak, weightless, white book, white paper, whitish, whity, whorish, wicket, wide, willowy, wind, window, window bay, window glass, windowlight, windowpane, windy, wiredrawn, wise, wispy, witty, womanish, word, write off, yeasty
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