'Light' is
electromagnetic radiation with a
wavelength that is visible to the
eye ('visible light') or, in a
technical or
scientific context, the word is sometimes used to mean
electromagnetic radiation of all
wavelengths.
[1] The
elementary particle that defines light is the
photon. The three basic properties of light (i.e., all electromagnetic radiation) are:
★
Intensity, or alternatively
amplitude, which is related to the perception of
brightness of the light,
★
Frequency, or alternatively
wavelength, perceived by humans as the
color of the light, and
★
Polarization (angle of vibration), which is only
weakly perceptible by humans under ordinary circumstances.
Due to its
wave–particle duality, light can exhibit properties of both
waves and
particles. The study of light, known as '
optics', is an important research area in modern
physics.
Speed of light
Main articles: Speed of light
The speed of light in a
vacuum is exactly 299,792,458
m/
s (fixed by definition). Although this quantity is sometimes referred to as the "velocity of light", the word ''
velocity'' refers to a
vector quantity, which has a direction (and ''speed'' refers to the magnitude of the velocity vector).
The speed of light has been measured many times, by many physicists. Though
Galileo attempted to measure the speed of light in the 1600s, the best early measurement in Europe was 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 (which he did not) he would have deduced a speed of 227,000
km/s.
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 km/s.
Léon Foucault used rotating mirrors to obtain a value of 298,000 km/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 299 796 km/s. This was close to the modern value of 299,792,458 m/s.
Refraction
Main articles: Refraction
All light propagates at a finite speed, a speed usually denoted as ''c''. Light travels fastest in vacuum (which is the reference for the definition of ''c'') and slower in any other transparent medium. 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'' > 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; this change of direction is known as refraction.
Refraction of light by
lenses is used to focus light in
magnifying glasses,
spectacles and
contact lenses,
microscopes and
refracting telescopes.
Optics
Main articles: 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 and the
Aurora Borealis offer many clues as to the nature of light as well as much enjoyment.
Light sources
Mist illuminated by sunlight
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.
Certain other mechanisms can produce light:
★
scintillation
★
electroluminescence
★
sonoluminescence
★
triboluminescence
★
Cherenkov radiation
When the concept of light is intended to include very-high-energy photons (gamma rays), additional generation mechanisms include:
★
radioactive decay
★ particle–
antiparticle annihilation
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 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
DignÄga 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 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 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.
[1] 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.
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 ether''. 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.
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
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
:
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 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 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
:
where ''h'' is
Planck's constant,
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. 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, so it took until a bold proposition by
Louis de Broglie in 1924 to realise 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
Main articles: Radiation 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,
[2] 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.
[3] This should not be confused with the
Nichols radiometer, in which the motion ''is'' directly caused by light pressure.
[4]
Spirituality
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 universal metaphor of
good and evil,
knowledge and
ignorance, and similar concepts.
References
1. What Is a Light Source?
2. Asteroids Get Spun By the Sun Kathy A.
3. P. Lebedev, Untersuchungen über die Druckkräfte des Lichtes, Ann. Phys. 6, 433 (1901).
4. Nichols, E.F & Hull, G.F. (1903) The Pressure due to Radiation, ''The Astrophysical Journal'',Vol.17 No.5, p.315-351
See also
★
Automotive lighting
★
Color temperature
★
Corpuscular theory of light
★
Electromagnetic spectrum
★
Huygens' principle
★
Fermat's principle
★
International Commission on Illumination
★
Light beam - in particular about light beams visible from the side
★
Light pollution
★
Lighting
★
Photic sneeze reflex
★
Photometry
★
Rights of Light
★
Spectrometry
External links
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