The visible portion of the electromagnetic spectrum, corresponding to electromagnetic waves ranging in wavelength from approximately 3·9 × 10?7 m (violet) to 7·8 × 10?7 m (red) (corresponding frequencies 7·7 × 1014 Hz and 3·8 × 1014 Hz, respectively). Different wavelengths of light are perceived by humans as different colours. White light is composed of light of different wavelengths (colours), as may be seen by dispersing the beam through a prism. Light of a single colour is called monochromatic, a term sometimes taken to mean a single wavelength (which is in practice unachievable). The best source of monochromatic light is the laser.
Light as an electromagnetic wave was deduced by James Clerk Maxwell: he derived an expression for the velocity of electromagnetic waves, using electric and magnetic quantities only, which equals the velocity of light (2·998 × 108 m/s; approximately 186 000 mi/s). Interactions with matter (in particular, the photoelectric effect) show that light can also be viewed as composed of particles (photons) of definite energy; the polarization of light as a wave effect translates into the spin properties of photons. The modern description of light is as particles whose behaviour is governed by wave principles according to the rules of quantum theory. In geometrical optics (eg lens systems), light is thought of as rays, travelling in straight lines, causing shadows for opaque objects; these rays change direction when passing between regions of different refractive index.
Light comes from many sources. Light bulbs produce light from electrically heated filaments; thermal energy causes the motion of atoms and electrons in the filament, producing thermal radiation with a frequency spectrum related to the temperature of the filament; for filament temperatures of a few hundred degrees Celsius, the radiation produced is mostly infrared; for higher temperatures (c.3000°C for household bulbs), light is produced. Mercury arc lamps produce light by electrical discharge (the arc) in mercury vapour. Fluorescent lamps rely on electrical excitation in a gas to produce ultraviolet radiation, which in turn causes the phosphor coating on a glass tube to emit visible light by fluorescence. Electrical discharge in gases (eg neon signs) or the application of heat to substances often produces light having specific spectra; this is related to the atomic structure of the substance, and is explained in terms of quantum theory.
Light is electromagnetic radiation with a wavelength that is visible to the eye (visible light) or, in a technical or scientific context, electromagnetic radiation of any wavelength . The three basic dimensions of light (i.e., all electromagnetic radiation) are:
Intensity (or amplitude), which is related to the human perception of brightness of the light, Frequency (or wavelength), perceived by humans as the colour of the light, and Polarization (or angle of vibration), which is only weakly perceptible by humans under ordinary circumstances.Due to the wave-particle duality of matter, light simultaneously exhibits properties of both waves and particles. Electromagnetic radiation in this range of wavelengths is called visible light or simply light.
The optical spectrum includes not only visible light, but also infrared and ultraviolet.
Speed of light
The speed of light in a vacuum is exactly 299,792,458 metres per second (fixed by definition).
The speed of light has been measured many times, by many physicists.
The first successful measurement of the speed of light in Europe using an earthbound apparatus was carried out by Hippolyte Fizeau in 1849. Knowing the distance to the mirror, the number of teeth on the wheel, and the rate of rotation, Fizeau measured the speed of light as 313,000 kilometres per second.
Refraction
All light propagates at a finite speed. Even moving observers always measure the same value of c, the speed of light in vacuum, as c = 299,792,458 metres per second (186,282.397 miles per second). The reduction of the speed of light in a denser material can be indicated by the refractive index, n, which is defined as:
Thus, n = 1 in a vacuum and n >
When a beam of light enters a medium from vacuum or another medium, it keeps the same frequency and changes its wavelength. Refraction of light by lenses is used to focus light in magnifying glasses, spectacles and contact lenses, microscopes and refracting telescopes.
Optics
The study of light and the interaction of light and matter is termed optics.
UV radiation is not normally directly perceived by humans except in a very delayed fashion, as overexposure of the skin to UV light can cause sunburn, or skin cancer, and underexposure can cause vitamin D deficiency. However, because UV is a higher frequency radiation than visible light, it very easily can cause materials to fluoresce visible light.
Cameras that can detect IR and convert it to light are called, depending on their application, night-vision cameras or infrared cameras.
Measurement of light
The following quantities and units are used to measure the quantity or "brightness" of light.
| Notes | |||||
|---|---|---|---|---|---|
| Luminous energy | Qv | lumen second | lm·s | units are sometimes called Talbots | |
| Luminous flux | F | lumen (= cd·sr) | lm | also called luminous power | |
| Luminous intensity | Iv | candela (= lm/sr) | cd | an SI base unit | |
| Luminance | Lv | candela per square metre | cd/m2 | units are sometimes called nits | |
| Illuminance | Ev | lux (= lm/m2) | lx | Used for light incident on a surface | |
| Luminous emittance | Mv | lux (= lm/m2) | lx | Used for light emitted from a surface | |
| Luminous efficacy | lumen per watt | lm/W | ratio of luminous flux to radiant flux; | Notes | |
| Radiant energy | Q | joule | J | energy | |
| Radiant flux | Φ | watt | W | radiant energy per unit time, also called radiant power | |
| Radiant intensity | I | watt per steradian | W·sr−1 | power per unit solid angle | |
| Radiance | L | watt per steradian per square metre | W·sr |
power per unit solid angle per unit projected source area. Sometimes confusingly called "intensity". |
|
| Irradiance | E | watt per square metre | W·m−2 |
power incident on a surface. Sometimes confusingly called "intensity". |
|
| Radiant exitance / Radiant emittance | M | watt per square metre | W·m−2 |
power emitted from a surface. Sometimes confusingly called "intensity". |
|
| Spectral radiance |
Lλ or Lν |
watt per steradian per metre3 or watt per steradian per square metre per hertz |
W·sr or W·sr·Hz−1 |
commonly measured in W·sr·nm−1 | |
| Spectral irradiance |
Eλ or Eν |
watt per metre3 or watt per square metre per hertz |
W·m−3 or W·m |
commonly measured in W·m |
Light can also be characterised by:
amplitude, colour, wavelength, or frequency, and polarization (or angle of vibration).Light sources
See also: List of light sourcesThere are many sources of light. 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.
Atoms emit and absorb light at characteristic energies. 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). Particles moving through a medium faster than the speed of light in that medium can produce visible Cherenkov radiation.
Certain substances produce light when they are illuminated by more energetic radiation, a process known as fluorescence.
Certain other mechanisms can produce light:
scintillation scintillator electroluminescence sonoluminescence triboluminescence radioactive decay particle-antiparticle annihilationTheories about light
Indian theories
In ancient India, the philosophical schools of Samkhya and Vaisheshika, from around the 6th–5th century BC, developed theories on light.
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.
In about 300 BC, Euclid wrote Optica, in which he studied the properties of light.
In 55 BC, Lucretius, a Roman who carried on the ideas of earlier Greek atomists, wrote:
"The light and 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 remarkably similar to how we think of light today, Lucretius's views were not generally accepted and light was still theorized as emanating from the eye. 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. 965-1040), also known as Alhazen in the West, 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. This contradicted Ptolemy's theory of vision that objects are seen by rays of light emanating from the eyes.
He also carried out the first experiments on the dispersion of light into its constituent colours.
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.
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. Although Descarte's was incorrect about the relative speeds, he was on the right track in terms of 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. 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. His reputation helped the particle theory of light to dominate physics during the 18th century.
Wave theory
In the 1660s, Robert Hooke published a wave theory of light. Christian 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 aether.
The wave theory predicted that light waves could interfere with each other like sound waves (as noted in the 18th century by Thomas Young), and that light could be polarized. He also proposed that different colours were caused by different wavelengths of light, and explained colour vision in terms of three-coloured receptors in the eye.
Later, Augustin-Jean Fresnel independently worked out his own wave theory of light, and presented it to the Académie des Sciences in 1817.
The weakness of the wave theory was that light waves, like sound waves, would need a medium for transmission.
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.
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.
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. 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. 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. Einstein also demonstrated a previously unknown fundamental equivalence between energy and mass with his famous equation
where E is energy, m is 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. In 1905, Einstein solved this puzzle as well, this time by resurrecting the particle theory of light to explain the observed effect.
Quantum theory
A third anomaly that arose in the late nineteenth 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. 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
where h is Planck's constant, λ 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:
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.
Wave-particle duality
The modern theory that explains the nature of light is wave-particle duality, described by Albert Einstein in the early 1900s, based on his work on the photoelectric effect and Planck's results.
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.
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