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Light

2007 Schools Wikipedia Selection. Related subjects: General Physics

   Prism splitting light
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   Prism splitting light

   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 elementary particle
   that defines light is the photon. 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. The precise nature of
   light is one of the key questions of modern physics.

Visible electromagnetic radiation

   The visible spectrum is the portion of the electromagnetic spectrum
   that is visible to the human eye, referred to as the Balmer series.
   Electromagnetic radiation in this range of wavelengths is called
   visible light or simply light. There are no exact bounds to the visible
   spectrum; a typical human eye will respond to wavelengths from 400 to
   700 nm, although some people may be able to perceive wavelengths from
   380 to 780  nm. A light-adapted eye typically has its maximum
   sensitivity at around 555  nm, in the green region of the optical
   spectrum (see: luminosity function). The spectrum does not, however,
   contain all the colours that the human eyes and brain can distinguish.
   Brown and pink are absent, for example. See Colour to understand why.

   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). Although some people speak of the "velocity of
   light", the word velocity is usually reserved for vector quantities,
   which have a direction.

   The speed of light has been measured many times, by many physicists.
   The best early measurement in Europe is by Ole Rømer, a Danish
   physicist, in 1676. By observing the motions of Jupiter and one of its
   moons, Io, with a telescope, and 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. If he had known the
   diameter of the orbit in kilometres (which he didn't) he would have
   deduced a speed of 227,000  kilometres per second (approximately
   141,050  miles per second).

   The first successful measurement of the speed of light in Europe using
   an earthbound apparatus was carried out by Hippolyte Fizeau in 1849.
   Fizeau directed a beam of light at a mirror several thousand metres
   away, and placed a rotating cog wheel in the path of the beam from the
   source to the mirror and back again. At a certain rate of rotation, the
   beam could 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 measured
   the speed of light as 313,000 kilometres per second.

   Léon Foucault used rotating mirrors to obtain a value of 298,000 km/s
   (about 185,000 miles/s) in 1862. Albert A. Michelson conducted
   experiments on the speed of light from 1877 until his death in 1931. He
   refined Foucault's results 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 186,285 mi/s (299,796 km/s [1,079,265,600 km/h]). In daily use, the
   figures are rounded off to 300,000 km/s and 186,000 miles/s)

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). When
   light passes through a transparent substance, such as air, water or
   glass, its speed is reduced, and it undergoes refraction. The reduction
   of the speed of light in a denser material can be indicated by the
   refractive index, n, which is defined as:

          n = \frac{c}{v} \;\!

   Thus, n = 1 in a vacuum and n > 1 in matter.

   When a beam of light enters a medium from vacuum or another medium, it
   keeps the same frequency and changes its wavelength. If the incident
   beam is not orthogonal to the edge between the media, the direction of
   the beam will change. 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. The observation and study of optical phenomena such as rainbows
   offers many clues as to the nature of light as well as much enjoyment.

Colour and wavelength

   The different wavelengths are detected by the human eye and then
   interpreted by the brain as colours, ranging from red at the longest
   wavelengths of about 700 nm to violet at the shortest wavelengths of
   about 400 nm. The intervening frequencies are seen as orange, yellow,
   green, and blue.

   The wavelengths of the electromagnetic spectrum immediately outside the
   range that the human eye is able to perceive are called ultraviolet
   (UV) at the short wavelength (high frequency) end and infrared (IR) at
   the long wavelength (low frequency) end. Some animals, such as bees,
   can see UV radiation while others, such as pit viper snakes, can see
   infrared light.

   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. These are different from image intensifier cameras, which only
   amplify available visible light.

   When intense radiation (of any frequency) is absorbed in the skin, it
   causes heating that can be felt. Since hot objects are strong sources
   of infrared radiation, IR radiation is commonly associated with this
   sensation. Any intense radiation that can be absorbed in the skin will
   have the same effect, however.

Measurement of light

   The following quantities and units are used to measure the quantity or
   " brightness" of light.

   CAPTION: SI photometry units

   Quantity Symbol SI unit Abbr. Notes
   Luminous energy Q[v] lumen second lm· s units are sometimes called
   Talbots
   Luminous flux F lumen (=  cd· sr) lm also called luminous power
   Luminous intensity I[v] candela (=  lm/ sr) cd an SI base unit
   Luminance L[v] candela per square metre cd/ m^2 units are sometimes
   called nits
   Illuminance E[v] lux (= lm/ m^2) lx Used for light incident on a
   surface
   Luminous emittance M[v] lux (= lm/ m^2) lx Used for light emitted from
   a surface
   Luminous efficacy   lumen per watt lm/ W ratio of luminous flux to
   radiant flux; maximum possible is 683.002

   CAPTION: SI radiometry units

   Quantity Symbol SI unit Abbr. 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^−1·m^−2 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 L[λ]
   or
   L[ν] watt per steradian per metre^3 or

   watt per steradian per square metre per hertz
   W·sr^−1·m^−3
   or

   W·sr^−1·m^−2·Hz^−1
   commonly measured in W·sr^−1·m^−2·nm^−1
   Spectral irradiance E[λ]
   or
   E[ν] watt per metre^3 or
   watt per square metre per hertz W·m^−3
   or
   W·m^−2·Hz^−1 commonly measured in W·m^−2·nm^−1

   Light can also be characterised by:
     * amplitude,
     * colour, wavelength, or frequency, and
     * polarization (or angle of vibration).

Theories 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.
   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, the
   Vishnu Purana 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
   ones 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 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.

   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 Abu Ali al-Hasan ibn al-Haytham (c. 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. The other rays strike at different angles
   and are not seen. He invented the pinhole camera, 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. Alhazen 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 colours. His major work Kitab al-Manazir was
   translated into Latin in the Middle Ages, as well his book dealing with
   the colours 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. Through these extensive researches 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 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. 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 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. As waves are not affected by gravity, it was
   assumed that they slowed down upon entering a denser medium.
   Thomas Young's sketch of the two-slit experiment showing the
   diffraction of light. Young's experiments supported the theory that
   light consists of waves.
   Enlarge
   Thomas Young's sketch of the two-slit experiment showing the
   diffraction of light. Young's experiments supported the theory that
   light consists of waves.

   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. Young showed by means of a
   diffraction experiment that light behaved as waves. 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.

   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

   A linearly-polarized light wave frozen in time and showing the two
   oscillating components of light; an electric field and a magnetic field
   perpendicular to each other and to the direction of motion (a
   transverse wave).
   Enlarge
   A linearly-polarized light wave frozen in time and showing the two
   oscillating components of light; an electric field and a magnetic field
   perpendicular to each other and to the direction of motion (a
   transverse wave).

   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 phenomna, 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 Newton's laws of motion 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 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 favour 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 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. 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{hc}{\lambda} \,\!

   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:

          p = { E \over c } = { hf \over c } = { h \over \lambda }.

   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 is wave-particle
   duality, described by Albert Einstein in the early 1900s, based on his
   work on the photoelectric effect and Planck's results. Einstein
   determined 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, so it took until an experiment
   by Louis de Broglie in 1924 to realise that electrons also exhibited
   wave-particle duality. Einstein received the Nobel Prize in 1921 for
   his work with the wave-particle duality on photons, 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 Sin-Itiro Tomonaga. Feynman, Schwinger, and Tomonaga
   shared the 1965 Nobel Prize in Physics for their contributions.

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