Light is electromagnetic radiation within a certain portion of the
electromagnetic spectrum. The word usually refers to visible light,
which is the visible spectrum that is visible to the human eye and is
responsible for the sense of sight. Visible light is usually
defined as having wavelengths in the range of 400–700 nanometres
(nm), or 4.00 × 10−7 to 7.00 × 10−7 m, between the infrared
(with longer wavelengths) and the ultraviolet (with shorter
wavelengths). This wavelength means a frequency range of roughly
430–750 terahertz (THz).
Beam of sun light inside the cavity of Rocca ill'Abissu at Fondachelli
The main source of light on
Earth is the Sun.
Sunlight provides the
energy that green plants use to create sugars mostly in the form of
starches, which release energy into the living things that digest
them. This process of photosynthesis provides virtually all the energy
used by living things. Historically, another important source of light
for humans has been fire, from ancient campfires to modern kerosene
lamps. With the development of electric lights and power systems,
electric lighting has effectively replaced firelight. Some species of
animals generate their own light, a process called bioluminescence.
For example, fireflies use light to locate mates, and vampire squids
use it to hide themselves from prey.
The primary properties of visible light are intensity, propagation
direction, frequency or wavelength spectrum, and polarization, while
its speed in a vacuum, 299,792,458 metres per second, is one of the
fundamental constants of nature. Visible light, as with all types of
electromagnetic radiation (EMR), is experimentally found to always
move at this speed in a vacuum.
In physics, the term light sometimes refers to electromagnetic
radiation of any wavelength, whether visible or not. In this
sense, gamma rays, X-rays, microwaves and radio waves are also light.
Like all types of EM radiation, visible light propagates as waves.
However, the energy imparted by the waves is absorbed at single
locations the way particles are absorbed. The absorbed energy of the
EM waves is called a photon, and represents the quanta of light. When
a wave of light is transformed and absorbed as a photon, the energy of
the wave instantly collapses to a single location, and this location
is where the photon "arrives." This is what is called the wave
function collapse. This dual wave-like and particle-like nature of
light is known as the wave–particle duality. The study of light,
known as optics, is an important research area in modern physics.
Electromagnetic spectrum and visible light
2 Speed of light
5 Units and measures
7 Historical theories about light, in chronological order
7.1 Classical Greece and Hellenism
7.2 Classical India
7.4 Particle theory
7.5 Wave theory
7.6 Electromagnetic theory
7.7 Quantum theory
8 See also
11 External links
Electromagnetic spectrum and visible light
Main article: Electromagnetic spectrum
Electromagnetic spectrum with light highlighted
Generally, EM radiation, or EMR (the designation "radiation" excludes
static electric and magnetic and near fields), is classified by
wavelength into radio, microwave, infrared, the visible region that we
perceive as light, ultraviolet, X-rays and gamma rays.
The behavior of EMR depends on its wavelength. Higher frequencies have
shorter wavelengths, and lower frequencies have longer wavelengths.
When EMR interacts with single atoms and molecules, its behavior
depends on the amount of energy per quantum it carries.
EMR in the visible light region consists of quanta (called photons)
that are at the lower end of the energies that are capable of causing
electronic excitation within molecules, which leads to changes in the
bonding or chemistry of the molecule. At the lower end of the visible
light spectrum, EMR becomes invisible to humans (infrared) because its
photons no longer have enough individual energy to cause a lasting
molecular change (a change in conformation) in the visual molecule
retinal in the human retina, which change triggers the sensation of
There exist animals that are sensitive to various types of infrared,
but not by means of quantum-absorption.
Infrared sensing in snakes
depends on a kind of natural thermal imaging, in which tiny packets of
cellular water are raised in temperature by the infrared radiation.
EMR in this range causes molecular vibration and heating effects,
which is how these animals detect it.
Above the range of visible light, ultraviolet light becomes invisible
to humans, mostly because it is absorbed by the cornea below 360
nanometers and the internal lens below 400. Furthermore, the rods and
cones located in the retina of the human eye cannot detect the very
short (below 360 nm) ultraviolet wavelengths and are in fact
damaged by ultraviolet. Many animals with eyes that do not require
lenses (such as insects and shrimp) are able to detect ultraviolet, by
quantum photon-absorption mechanisms, in much the same chemical way
that humans detect visible light.
Various sources define visible light as narrowly as 420 to 680
to as broadly as 380 to 800 nm. Under ideal laboratory
conditions, people can see infrared up to at least 1050 nm;
children and young adults may perceive ultraviolet wavelengths down to
about 310 to 313 nm.
Plant growth is also affected by the color spectrum of light, a
process known as photomorphogenesis.
Speed of light
Main article: Speed of light
The speed of light in a vacuum is defined to be exactly
299,792,458 m/s (approx. 186,282 miles per second). The fixed
value of the speed of light in SI units results from the fact that the
metre is now defined in terms of the speed of light. All forms of
electromagnetic radiation move at exactly this same speed in vacuum.
Different physicists have attempted to measure the speed of light
throughout history. Galileo attempted to measure the speed of light in
the seventeenth century. An early experiment to measure the speed of
light was conducted by Ole Rømer, a Danish physicist, in 1676. Using
a telescope, Rømer observed the motions of
Jupiter and one of its
moons, Io. Noting discrepancies in the apparent period of Io's orbit,
he calculated that light takes about 22 minutes to traverse the
diameter of Earth's orbit. However, its size was not known at that
time. If Rømer 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 carried out 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
Mount Wilson to
Mount San Antonio
Mount San Antonio in California. The precise
measurements yielded a speed of 299,796,000 m/s.
The effective velocity of light in various transparent substances
containing ordinary matter, is less than in vacuum. For example, the
speed of light in water is about 3/4 of that in vacuum.
Two independent teams of physicists were said to bring light to a
"complete standstill" by passing it through a Bose–Einstein
condensate of the element rubidium, one team at
Harvard University and
Rowland Institute for Science
Rowland Institute for Science in Cambridge, Massachusetts, and the
other at the Harvard–Smithsonian Center for Astrophysics, also in
Cambridge. However, the popular description of light being
"stopped" in these experiments refers only to light being stored in
the excited states of atoms, then re-emitted at an arbitrary later
time, as stimulated by a second laser pulse. During the time it had
"stopped" it had ceased to be light.
Main article: 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
Main article: Refraction
An example of refraction of light. The straw appears bent, because of
refraction of light as it enters liquid from air.
A cloud illuminated by sunlight
Refraction is the bending of light rays when passing through a surface
between one transparent material and another. It is described by
displaystyle n_ 1 sin theta _ 1 =n_ 2 sin theta _ 2 .
where θ1 is the angle between the ray and the surface normal in the
first medium, θ2 is the angle between the ray and the surface normal
in the second medium, and n1 and n2 are the indices of refraction, n =
1 in a vacuum and n > 1 in a transparent substance.
When a beam of light crosses the boundary between a vacuum and another
medium, or between two different media, the wavelength of the light
changes, but the frequency remains constant. If the beam of light is
not orthogonal (or rather normal) 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 refractive 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.
Further information: List of light sources
There are many sources of light. A body at a given temperature emits a
characteristic spectrum of black-body radiation. A simple thermal
source is sunlight, the radiation emitted by the chromosphere of the
Sun at around 6,000 kelvins (5,730 degrees Celsius; 10,340 degrees
Fahrenheit) peaks in the visible region of the electromagnetic
spectrum when plotted in wavelength units and roughly 44% of
sunlight energy that reaches the ground is visible. Another
example is incandescent light bulbs, which emit only around 10% of
their energy as visible light and the remainder as infrared. A common
thermal light source in history is the glowing solid particles in
flames, but these also emit most of their radiation in the infrared,
and only a fraction in the visible spectrum.
The peak of the blackbody spectrum is in the deep infrared, at about
10 micrometre wavelength, 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-white colour as the peak moves out of the visible part of the
spectrum and into the ultraviolet. These colours can be seen when
metal is heated to "red hot" or "white hot". Blue-white thermal
emission is not often seen, except in stars (the commonly seen
pure-blue colour in a gas flame or a welder's torch is in fact due to
molecular emission, notably by CH radicals (emitting a wavelength band
around 425 nm, and is not seen in stars or pure thermal
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.
Deceleration 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. 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. This mechanism is used in cathode
ray tube television sets and computer monitors.
A city illuminated by colorful artificial lighting
Certain other mechanisms can produce light:
When the concept of light is intended to include very-high-energy
photons (gamma rays), additional generation mechanisms include:
Units and measures
Photometry (optics) and Radiometry
Light is measured with two main alternative sets of units: radiometry
consists of measurements of light power at all wavelengths, while
photometry measures light with wavelength weighted with respect to a
standardised model of human brightness perception. Photometry is
useful, for example, to quantify
Illumination (lighting) intended for
human use. The SI units for both systems are summarised in the
Table 1. SI radiometry units
Energy of electromagnetic radiation.
Radiant energy density
joule per cubic metre
Radiant energy per unit volume.
W = J/s
Radiant energy emitted, reflected, transmitted or received, per unit
time. This is sometimes also called "radiant power".
watt per hertz
watt per metre
Radiant flux per unit frequency or wavelength. The latter is commonly
measured in W⋅nm−1.
watt per steradian
Radiant flux emitted, reflected, transmitted or received, per unit
solid angle. This is a directional quantity.
watt per steradian per hertz
watt per steradian per metre
Radiant intensity per unit frequency or wavelength. The latter is
commonly measured in W⋅sr−1⋅nm−1. This is a directional
watt per steradian per square metre
Radiant flux emitted, reflected, transmitted or received by a surface,
per unit solid angle per unit projected area. This is a directional
quantity. This is sometimes also confusingly called "intensity".
watt per steradian per square metre per hertz
watt per steradian per square metre, per metre
Radiance of a surface per unit frequency or wavelength. The latter is
commonly measured in W⋅sr−1⋅m−2⋅nm−1. This is a
directional quantity. This is sometimes also confusingly called
watt per square metre
Radiant flux received by a surface per unit area. This is sometimes
also confusingly called "intensity".
Spectral flux density
watt per square metre per hertz
watt per square metre, per metre
Irradiance of a surface per unit frequency or wavelength. This is
sometimes also confusingly called "spectral intensity". Non-SI units
of spectral flux density include jansky (1 Jy =
10−26 W⋅m−2⋅Hz−1) and solar flux unit (1 sfu =
10−22 W⋅m−2⋅Hz−1 = 104 Jy).
watt per square metre
Radiant flux leaving (emitted, reflected and transmitted by) a surface
per unit area. This is sometimes also confusingly called "intensity".
watt per square metre per hertz
watt per square metre, per metre
Radiosity of a surface per unit frequency or wavelength. The latter is
commonly measured in W⋅m−2⋅nm−1. This is sometimes also
confusingly called "spectral intensity".
watt per square metre
Radiant flux emitted by a surface per unit area. This is the emitted
component of radiosity. "Radiant emittance" is an old term for this
quantity. This is sometimes also confusingly called "intensity".
watt per square metre per hertz
watt per square metre, per metre
Radiant exitance of a surface per unit frequency or wavelength. The
latter is commonly measured in W⋅m−2⋅nm−1. "Spectral
emittance" is an old term for this quantity. This is sometimes also
confusingly called "spectral intensity".
joule per square metre
Radiant energy received by a surface per unit area, or equivalently
irradiance of a surface integrated over time of irradiation. This is
sometimes also called "radiant fluence".
joule per square metre per hertz
joule per square metre, per metre
Radiant exposure of a surface per unit frequency or wavelength. The
latter is commonly measured in J⋅m−2⋅nm−1. This is sometimes
also called "spectral fluence".
Radiant exitance of a surface, divided by that of a black body at the
same temperature as that surface.
Spectral hemispherical emissivity
Spectral exitance of a surface, divided by that of a black body at the
same temperature as that surface.
Radiance emitted by a surface, divided by that emitted by a black body
at the same temperature as that surface.
Spectral directional emissivity
Spectral radiance emitted by a surface, divided by that of a black
body at the same temperature as that surface.
Radiant flux absorbed by a surface, divided by that received by that
surface. This should not be confused with "absorbance".
Spectral hemispherical absorptance
Spectral flux absorbed by a surface, divided by that received by that
surface. This should not be confused with "spectral absorbance".
Radiance absorbed by a surface, divided by the radiance incident onto
that surface. This should not be confused with "absorbance".
Spectral directional absorptance
Spectral radiance absorbed by a surface, divided by the spectral
radiance incident onto that surface. This should not be confused with
Radiant flux reflected by a surface, divided by that received by that
Spectral hemispherical reflectance
Spectral flux reflected by a surface, divided by that received by that
Radiance reflected by a surface, divided by that received by that
Spectral directional reflectance
Spectral radiance reflected by a surface, divided by that received by
Radiant flux transmitted by a surface, divided by that received by
Spectral hemispherical transmittance
Spectral flux transmitted by a surface, divided by that received by
Radiance transmitted by a surface, divided by that received by that
Spectral directional transmittance
Spectral radiance transmitted by a surface, divided by that received
by that surface.
Hemispherical attenuation coefficient
Radiant flux absorbed and scattered by a volume per unit length,
divided by that received by that volume.
Spectral hemispherical attenuation coefficient
Spectral radiant flux absorbed and scattered by a volume per unit
length, divided by that received by that volume.
Directional attenuation coefficient
Radiance absorbed and scattered by a volume per unit length, divided
by that received by that volume.
Spectral directional attenuation coefficient
Spectral radiance absorbed and scattered by a volume per unit length,
divided by that received by that volume.
See also: SI · Radiometry · Photometry ·
Table 2. SI photometry quantities
Qv [nb 8]
The lumen second is sometimes called the talbot.
Luminous flux / luminous power
Φv [nb 8]
lumen (= cd⋅sr)
Luminous energy per unit time
candela (= lm/sr)
Luminous flux per unit solid angle
candela per square metre
Luminous flux per unit solid angle per unit projected source area. The
candela per square metre is sometimes called the nit.
lux (= lm/m2)
Luminous flux incident on a surface
Luminous exitance / luminous emittance
Luminous flux emitted from a surface
Luminous energy density
lumen second per cubic metre
η [nb 8]
lumen per watt
Ratio of luminous flux to radiant flux or power consumption, depending
Luminous efficiency / luminous coefficient
Luminous efficacy normalized by the maximum possible efficacy
See also: SI · Photometry · Radiometry ·
The photometry units are different from most systems of physical units
in that they take into account how the human eye responds to light.
The cone cells in the human eye are of three types which respond
differently across the visible spectrum, and the cumulative response
peaks at a wavelength of around 555 nm. Therefore, two sources of
light which produce the same intensity (W/m2) of visible light do not
necessarily appear equally bright. The photometry units are designed
to take this into account, and therefore are a better representation
of how "bright" a light appears to be than raw intensity. They relate
to raw power by a quantity called luminous efficacy, and are used for
purposes like determining how to best achieve sufficient illumination
for various tasks in indoor and outdoor settings. The illumination
measured by a photocell sensor does not necessarily correspond to what
is perceived by the human eye, and without filters which may be
costly, photocells and charge-coupled devices (CCD) tend to respond to
some infrared, ultraviolet or both.
Main article: Radiation pressure
Light exerts physical pressure on objects in its path, a phenomenon
which can be deduced by Maxwell's equations, but can be more easily
explained by the particle nature of light: photons strike and transfer
Light pressure is equal to the power of the light beam
divided by c, the speed of light. Due to the magnitude of c, the
effect of light pressure is negligible for everyday objects. For
example, a one-milliwatt laser pointer exerts a force of about 3.3
piconewtons on the object being illuminated; thus, one could lift a
U.S. penny with laser pointers, but doing so would require about 30
billion 1-mW laser pointers. However, in nanometre-scale
applications such as nanoelectromechanical systems (NEMS), the effect
of light pressure is more significant, and exploiting light pressure
to drive NEMS mechanisms and to flip nanometre-scale physical switches
in integrated circuits is an active area of research. At larger
scales, light pressure can cause asteroids to spin faster, acting
on their irregular shapes as on the vanes of a windmill. The
possibility of making 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
(slight) motion caused by torque (though not enough for full rotation
against friction) is directly caused by light pressure. As a
consequence of light pressure, Einstein in 1909 predicted the
existence of "radiation friction" which would oppose the movement of
matter. He wrote, “radiation will exert pressure on both sides of
the plate. The forces of pressure exerted on the two sides are equal
if the plate is at rest. However, if it is in motion, more radiation
will be reflected on the surface that is ahead during the motion
(front surface) than on the back surface. The backwardacting force of
pressure exerted on the front surface is thus larger than the force of
pressure acting on the back. Hence, as the resultant of the two
forces, there remains a force that counteracts the motion of the plate
and that increases with the velocity of the plate. We will call this
resultant 'radiation friction' in brief.”
Historical theories about light, in chronological order
Classical Greece and Hellenism
This section needs additional citations for verification. Please help
improve this article by adding citations to reliable sources.
Unsourced material may be challenged and removed. (May 2011) (Learn
how and when to remove this template message)
In the fifth century BC,
Empedocles postulated that everything was
composed of four elements; fire, air, earth and water. He believed
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. 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 that "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." (from On the nature of the Universe). Despite
being similar to later particle theories, Lucretius's views were not
Ptolemy (c. 2nd century) wrote about the
refraction of light in his book Optics.
In ancient India, the
Hindu schools of
Samkhya and Vaisheshika, from
around the early centuries AD developed theories on light. According
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 (prthivi), water
(pani), fire (agni), and air (vayu)
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. The Vishnu Purana
refers to sunlight as "the seven rays of the sun".[citation
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.
René Descartes (1596–1650) held that light was a mechanical
property of the luminous body, rejecting the "forms" of Ibn al-Haytham
Witelo as well as the "species" of Bacon, Grosseteste, and
Kepler. 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.[citation
needed] 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
Descartes is not the first to use the mechanical analogies but because
he clearly asserts that light is only a mechanical property of the
luminous body and the transmitting medium, Descartes' theory of light
is regarded as the start of modern physical optics.
Main article: Corpuscular theory of light
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
Opticks of 1704. His reputation helped the particle theory of
light to hold sway during the 18th century. The particle theory of
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 later, after a
wave theory of light became firmly established as the model for light
(as has been explained, neither a particle or wave theory is fully
correct). A translation of Newton's essay on light appears in The
large scale structure of space-time, by
Stephen Hawking and George F.
The fact that light could be polarized was for the first time
qualitatively explained by Newton using the particle theory.
Étienne-Louis Malus in 1810 created a mathematical particle theory of
Jean-Baptiste Biot in 1812 showed that this theory
explained all known phenomena of light polarization. At that time the
polarization was considered as the proof of the particle theory.
To explain the origin of colors,
Robert Hooke (1635-1703) developed a
"pulse theory" and compared the spreading of light to that of waves in
water in his 1665 work
Micrographia ("Observation IX"). In 1672 Hooke
suggested that light's vibrations could be perpendicular to the
direction of propagation.
Christiaan Huygens (1629-1695) worked out a
mathematical wave theory of light in 1678, and published it in his
Treatise on light
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 a double-slit experiment showing diffraction.
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 around 1800 by Thomas Young). 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. In 1816
André-Marie Ampère gave
Augustin-Jean Fresnel an
idea that the polarization of light can be explained by the wave
theory if light were a transverse wave.
Later, Fresnel independently worked out his own wave theory of light,
and presented it to the
Académie des Sciences
Académie des Sciences in 1817. Siméon 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. By the year 1821, Fresnel was able to show via
mathematical methods that polarisation could be explained by the wave
theory of light and only if light was entirely transverse, with no
longitudinal vibration whatsoever.
The weakness of the wave theory was that light waves, like sound
waves, would need a medium for transmission. The existence of the
hypothetical substance luminiferous aether proposed by Huygens in 1678
was cast into strong doubt in the late nineteenth century by the
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, only to partly re-emerge in the
Main article: Electromagnetic radiation
A 3–dimensional rendering of linearly polarised 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).
Michael Faraday discovered that the plane of polarisation of
linearly polarised light is rotated when the light rays travel along
the magnetic field direction in the presence of a transparent
dielectric, an effect now known as Faraday rotation. This was the
first evidence that light was related to electromagnetism. In 1846 he
speculated that light might be some form of disturbance propagating
along magnetic field lines. 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
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
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.
In the quantum theory, photons are seen as wave packets of the waves
described in the classical theory of Maxwell. The quantum theory was
needed to explain effects even with visual light that Maxwell's
classical theory could not (such as spectral lines).
In 1900 Max Planck, attempting to explain black body radiation
suggested that although light was a wave, these waves could gain or
lose energy only in finite amounts related to their frequency. Planck
called these "lumps" of light energy "quanta" (from a Latin word for
"how much"). In 1905,
Albert Einstein used the idea of light quanta to
explain the photoelectric effect, and suggested that these light
quanta had a "real" existence. In 1923
Arthur Holly Compton
Arthur Holly Compton showed
that the wavelength shift seen when low intensity X-rays scattered
from electrons (so called Compton scattering) could be explained by a
particle-theory of X-rays, but not a wave theory. In 1926 Gilbert N.
Lewis named these light quanta particles photons.
Eventually the modern theory of quantum mechanics came to picture
light as (in some sense) both a particle and a wave, and (in another
sense), as a phenomenon which is neither a particle nor a wave (which
actually are macroscopic phenomena, such as baseballs or ocean waves).
Instead, modern physics sees light as something that can be described
sometimes with mathematics appropriate to one type of macroscopic
metaphor (particles), and sometimes another macroscopic metaphor
(water waves), but is actually something that cannot be fully
imagined. As in the case for radio waves and the X-rays involved in
Compton scattering, physicists have noted that electromagnetic
radiation tends to behave more like a classical wave at lower
frequencies, but more like a classical particle at higher frequencies,
but never completely loses all qualities of one or the other. Visible
light, which occupies a middle ground in frequency, can easily be
shown in experiments to be describable using either a wave or particle
model, or sometimes both.
In February 2018, scientists reported, for the first time, the
discovery of a new form of light, which may involve polaritons, that
could be useful in the development of quantum computers.
Incandescent light bulb
International Commission on Illumination
Journal of Luminescence
Light beam – in particular about light beams visible from the side
Light Fantastic (TV series)
List of light sources
Luminescence: The Journal of Biological and Chemical Luminescence
Photic sneeze reflex
Right to light
Risks and benefits of sun exposure
^ Standards organizations recommend that radiometric quantities should
be denoted with suffix "e" (for "energetic") to avoid confusion with
photometric or photon quantities.
^ a b c d e Alternative symbols sometimes seen: W or E for radiant
energy, P or F for radiant flux, I for irradiance, W for
^ a b c d e f g Spectral quantities given per unit frequency are
denoted with suffix "ν" (Greek)—not to be confused with suffix "v"
(for "visual") indicating a photometric quantity.
^ a b c d e f g Spectral quantities given per unit wavelength are
denoted with suffix "λ" (Greek).
^ a b Directional quantities are denoted with suffix "Ω" (Greek).
^ Standards organizations recommend that photometric quantities be
denoted with a suffix "v" (for "visual") to avoid confusion with
radiometric or photon quantities. For example: USA Standard Letter
Symbols for Illuminating Engineering USAS Z7.1-1967, Y10.18-1967
^ The symbols in this column denote dimensions; "L", "T" and "J" are
for length, time and luminous intensity respectively, not the symbols
for the units litre, tesla and joule.
^ a b c Alternative symbols sometimes seen: W for luminous energy, P
or F for luminous flux, and ρ or K for luminous efficacy.
^ CIE (1987). International
Lighting Vocabulary. Number 17.4. CIE, 4th
edition. ISBN 978-3-900734-07-7.
By the International
Lighting Vocabulary, the definition of light is:
“Any radiation capable of causing a visual sensation directly.”
^ Pal, G. K.; Pal, Pravati (2001). "chapter 52". Textbook of Practical
Physiology (1st ed.). Chennai: Orient Blackswan. p. 387.
ISBN 978-81-250-2021-9. Retrieved 11 October 2013. The human eye
has the ability to respond to all the wavelengths of light from
400–700 nm. This is called the visible part of the spectrum.
^ Buser, Pierre A.; Imbert, Michel (1992). Vision. MIT Press.
p. 50. ISBN 978-0-262-02336-8. Retrieved 11 October 2013.
Light is a special class of radiant energy embracing wavelengths
between 400 and 700 nm (or mμ), or 4000 to 7000 Å.
^ Uzan, J-P; Leclercq, B (2008). The Natural Laws of the Universe:
Understanding Fundamental Constants. Springer. pp. 43–4.
^ Gregory Hallock Smith (2006). Camera lenses: from box camera to
digital. SPIE Press. p. 4. ISBN 978-0-8194-6093-6.
^ Narinder Kumar (2008). Comprehensive
Physics XII. Laxmi
Publications. p. 1416. ISBN 978-81-7008-592-8.
^ Laufer, Gabriel (13 July 1996). Introduction to
Optics and Lasers in
Engineering. Cambridge University Press. p. 11.
ISBN 978-0-521-45233-5. Retrieved 20 October 2013.
^ Bradt, Hale (2004). Astronomy Methods: A Physical Approach to
Astronomical Observations. Cambridge University Press. p. 26.
ISBN 978-0-521-53551-9. Retrieved 20 October 2013.
^ Ohannesian, Lena; Streeter, Anthony (9 November 2001). Handbook of
Pharmaceutical Analysis. CRC Press. p. 187.
ISBN 978-0-8247-4194-5. Retrieved 20 October 2013.
^ Ahluwalia, V. K.; Goyal, Madhuri (1 January 2000). A Textbook of
Organic Chemistry. Narosa. p. 110. ISBN 978-81-7319-159-6.
Retrieved 20 October 2013.
^ Sliney, David H.; Wangemann, Robert T.; Franks, James K.; Wolbarsht,
Myron L. (1976). "Visual sensitivity of the eye to infrared laser
radiation". Journal of the Optical Society of America. 66 (4):
339–341. doi:10.1364/JOSA.66.000339. (Subscription required (help)).
The foveal sensitivity to several near-infrared laser wavelengths was
measured. It was found that the eye could respond to radiation at
wavelengths at least as far as 1064 nm. A continuous 1064 nm laser
source appeared red, but a 1060 nm pulsed laser source appeared green,
which suggests the presence of second harmonic generation in the
^ Lynch, David K.; Livingston, William Charles (2001).
Color and Light
in Nature (2nd ed.). Cambridge, UK: Cambridge University Press.
p. 231. ISBN 978-0-521-77504-5. Retrieved 12 October 2013.
Limits of the eye's overall range of sensitivity extends from about
310 to 1050 nanometers
^ Dash, Madhab Chandra; Dash, Satya Prakash (2009). Fundamentals Of
Ecology 3E. Tata McGraw-Hill Education. p. 213.
ISBN 978-1-259-08109-5. Retrieved 18 October 2013. Normally the
human eye responds to light rays from 390 to 760 nm. This can be
extended to a range of 310 to 1,050 nm under artificial
^ Saidman, Jean (15 May 1933). "Sur la visibilité de l'ultraviolet
jusqu'à la longueur d'onde 3130" [The visibility of the ultraviolet
to the wave length of 3130]. Comptes rendus de l'Académie des
sciences (in French). 196: 1537–9.
^ "Scientific Method, Statistical Method and the Speed of Light".
Statistical Science. 15 (3): 254–278. 2000.
doi:10.1214/ss/1009212817. MR 1847825.
^ Michelson,, A. A. (January 1927). "Measurements of the velocity of
light between Mount Wilson and Mount San Antonio". Astrophysical
Journal. 65: 1. Bibcode:1927ApJ....65....1M. doi:10.1086/143021.
Retrieved 12 March 2014.
^ Harvard News Office (2001-01-24). "Harvard Gazette: Researchers now
able to stop, restart light". News.harvard.edu. Archived from the
original on 28 October 2011. Retrieved 2011-11-08.
Spectrum and the
Color Sensitivity of the Eye" (PDF).
Thulescientific.com. Retrieved 2017-08-29.
^ "Reference Solar Spectral Irradiance: Air Mass 1.5". Retrieved
^ Tang, Hong (1 October 2009). "May The Force of
Light Be With You".
IEEE Spectrum. 46 (10): 46–51. doi:10.1109/MSPEC.2009.5268000.
^ See, for example, nano-opto-mechanical systems research at Yale
^ Kathy A. (2004-02-05). "Asteroids Get Spun By the Sun". Discover
^ "Solar Sails Could Send Spacecraft 'Sailing' Through Space". NASA.
NASA team successfully deploys two solar sail systems". NASA.
^ P. Lebedev, Untersuchungen über die Druckkräfte des Lichtes, Ann.
Phys. 6, 433 (1901).
^ Nichols, E.F; Hull, G.F. (1903). "The Pressure due to Radiation".
The Astrophysical Journal. 17 (5): 315–351.
^ Einstein, A. (1909). On the development of our views concerning the
nature and constitution of radiation. Translated in: The Collected
Papers of Albert Einstein, vol. 2 (Princeton University Press,
Princeton, 1989). Princeton, NJ: Princeton University Press.
^ Singh, S. (2009). "Fundamentals of Optical Engineering". Discovery
Publishing House. ISBN 9788183564366. Missing or empty
^ O'Connor, J J; Robertson, E F (August 2002). "
Light through the
ages: Ancient Greece to Maxwell".
Ptolemy and A. Mark Smith (1996). Ptolemy's Theory of Visual
Perception: An English Translation of the
Optics with Introduction and
Commentary. Diane Publishing. p. 23.
^ a b c "Shastra Pratibha 2015 Seniors Booklet" (PDF). Sifuae.com.
^ a b Theories of light, from Descartes to Newton A. I. Sabra CUP
Archive,1981 pg 48 ISBN 0-521-28436-8,
^ Fokko Jan Dijksterhuis, Lenses and Waves:
Christiaan Huygens and the
Mathematical Science of
Optics in the 17th Century, Kluwer Academic
Publishers, 2004, ISBN 1-4020-2697-8
^ James R. Hofmann, André-Marie Ampère: Enlightenment and
Electrodynamics, Cambridge University Press, 1996, p. 222.
^ David Cassidy; Gerald Holton; James Rutherford (2002). Understanding
Physics. Birkhäuser. ISBN 0-387-98756-8.
^ a b Longair, Malcolm (2003). Theoretical Concepts in Physics.
^ Barrow, Gordon M. (1962). Introduction to Molecular Spectroscopy
(Scanned PDF). McGraw-Hill. LCCN 62-12478.
^ Hignett, Katherine (16 February 2018). "
Physics Creates New Form Of
Light That Could Drive The Quantum Computing Revolution". Newsweek.
Retrieved 17 February 2018.
^ Liang, Qi-Yu; et al. (16 February 2018). "Observation of
three-photon bound states in a quantum nonlinear medium". Science. 359
(6377): 783–786. doi:10.1126/science.aao7293. Retrieved 17 February
2018. CS1 maint: Explicit use of et al. (link)
Media related to
Light at Wikimedia Commons
The dictionary definition of light at Wiktionary
Quotations related to
Light at Wikiquote
On Vision and Colors
Spectral power distribution
Lüscher color test
Tertiary color (intermediate)
Aggressive color (warm)
Receding color (cool)
Achromatic colors (Neutral)
Tinctures in heraldry
Color analysis (art)
Color realism (art style)
Linguistic relativity and the color naming debate
Blue–green distinction in language
Color in Chinese culture
Traditional colors of Japan
Human skin color
Colorfulness (chroma and saturation)
Tints and shades
Lightness (tone and value)
Color Marketing Group
Color Association of the United States
International Colour Authority
International Commission on Illumination
International Commission on Illumination (CIE)
International Colour Association
List of colors: A–F
List of colors: G–M
List of colors: N–Z
List of colors
List of colors (compact)
List of colors
List of colors by shade
List of color palettes
List of color spaces
List of Crayola crayon colors
List of fictional colors
List of RAL colors
List of web colors
Multi-primary color display
Local color (visual art)
Index of color-related articles