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).
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
==
Generally, EM radiation (the designation "radiation"
excludes static electric, magnetic, and near
fields), or EMR, is classified by wavelength
into radio waves, microwaves, infrared, the
visible spectrum 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 vision.
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 nm and the internal lens below 400 nm.
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 420–680 nm to as broadly as 380–800
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 310–313 nm.Plant
growth is also affected by the color spectrum
of light, a process known as photomorphogenesis.
== 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 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 the 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.
== 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.
=== Refraction ===
Refraction is the bending of light rays when
passing through a surface between one transparent
material and another. It is described by Snell's
Law:
n
1
sin
⁡
θ
1
=
n
2
sin
⁡
θ
2
.
{\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.
== 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 radiation).
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.
Certain other mechanisms can produce light:
Bioluminescence
Cherenkov radiation
Electroluminescence
Scintillation
Sonoluminescence
TriboluminescenceWhen the concept of light
is intended to include very-high-energy photons
(gamma rays), additional generation mechanisms
include:
Particle–antiparticle annihilation
Radioactive decay
== 
Units and measures ==
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 following tables.
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.
== Light 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 their momentum.
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 ===
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. 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 generally accepted.
Ptolemy (c. 2nd century) wrote about the refraction
of light in his book Optics.
=== Classical India ===
In ancient India, the Hindu schools of Samkhya
and Vaisheshika, from around the early centuries
AD 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 (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".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.
=== Descartes ===
René Descartes (1596–1650) held that light
was a mechanical property of the luminous
body, rejecting the "forms" of Ibn al-Haytham
and 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. 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.
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.
=== Particle theory ===
Pierre Gassendi (1592–1655), an atomist,
proposed a particle theory of light which
was published posthumously in the 1660s. Isaac
Newton studied Gassendi's work at an early
age, and preferred his view to Descartes'
theory of the plenum. He stated in his Hypothesis
of Light of 1675 that light was composed of
corpuscles (particles of matter) which were
emitted in all directions from a source. One
of Newton's arguments against the wave nature
of light was that waves were known to bend
around obstacles, while light travelled only
in straight lines. He did, however, explain
the phenomenon of the diffraction of light
(which had been observed by Francesco Grimaldi)
by allowing that a light particle could create
a localised wave in the aether.
Newton's theory could be used to predict the
reflection of light, but could only explain
refraction by incorrectly assuming that light
accelerated upon entering a denser medium
because the gravitational pull was greater.
Newton published the final version of his
theory in his Opticks of 1704. His reputation
helped the particle theory of light to hold
sway during the 18th century. The particle
theory of light led Laplace to argue that
a body could be so massive that light could
not escape from it. In other words, it would
become what is now called a black hole. Laplace
withdrew his suggestion 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. R. Ellis.
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 polarization. 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.
=== Wave 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 in 1690. He proposed
that light was emitted in all directions as
a series of waves in a medium called the Luminiferous
ether. As waves are not affected by gravity,
it was assumed that they slowed down upon
entering a denser medium.
The wave theory predicted that light waves
could interfere with each other like sound
waves (as noted around 1800 by Thomas Young).
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
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 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, only to partly
re-emerge in the 20th century.
=== Electromagnetic theory ===
In 1845, 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 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.
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).
=== Quantum theory ===
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 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.
== See also ==
== 
Notes
