## what colors

#### Why do we see colors ?

Colors of objects

We usually view objects when they are illuminated by white light, usually sunlight or ordinary room light.  White light is a mixture of all colors, in roughly equal proportions.   White objects look white because they reflect back all the visible wavelengths of light that shine on them - so the light still looks white to us.  Colored objects, on the other hand, reflect back only some of the wavelengths; the rest they absorb.  For example, if white light shines on a red ball, the ball reflects back mostly red light, and so we see red.  Most of the greens and blues that are part of white light are absorbed by the ball so we cannot see them.  Likewise, a blue book is reflecting the blue part of the white light spectrum.  The red and green parts are absorbed by the book.

What happens when red light shines on a red ball? It continues to reflect the red light, and so it is still red -- but a white ball would also look red in red light, because it reflects all colors. If instead we shine blue light on a red ball, it will look dark, because it does not reflect blue light. It cannot look red unless there is red light coming to it from the light source.  And it cannot look blue because the red ball absorbs blue light.  So when we ask what color an object is, the answer is not simple - it depends on what color light we are using to see the object.

One consequence of the fact that different colored objects absorb different wavelengths of light is that darker objects heat up faster in the sun than white ones do - because they absorb many of the different wavelengths of light energy, while white objects reflect most of the wavelengths.

## visible light

Additional InformationAdditional Information "Visible light" redirects here. For light that cannot be seen with human eye, see Electromagnetic radiation. For other uses, see Light (disambiguation) and Visible light (disambiguation). For other uses, see Light (disambiguation). A triangular prism dispersing a beam of white light. The longer wavelengths (red) and the shorter wavelengths (blue) are separated. Modern physics       H ^     |   ψ  n   ( t ) ⟩ = i ℏ   ∂  ∂ t     |   ψ  n   ( t ) ⟩   {\displaystyle {\hat {H}}|\psi _{n}(t)\rangle =i\hbar {\frac {\partial }{\partial t}}|\psi _{n}(t)\rangle }
1   c   2          ∂   2     ϕ   n      ∂ t   2     −    ∇   2     ϕ   n    +    (    m c  ℏ   )    2     ϕ   n   = 0   {\displaystyle {\frac  {1}{{c}^{2}}}{\frac {{\partial }^{2}{\phi }_{n}}{{\partial  t}^{2}}}-{{\nabla }^{2}{\phi }_{n}}+{\left({\frac {mc}{\hbar  }}\right)}^{2}{\phi }_{n}=0}  Manifold dynamics: Schrödinger and Klein–Gordon equations Founders[show]  Concepts[show]  Branches[show]  Scientists[show]

Light is electromagnetic radiation within a certain portion of the electromagnetic spectrum. The word usually refers to visible light, which is the portion of the spectrum that can be perceived by the human eye.[1] Visible light is usually defined as having wavelengths in the range of 400–700 nanometers (nm), or 4.00 × 10−7 to 7.00 × 10−7 m, between the infrared (with longer wavelengths) and the ultraviolet (with shorter wavelengths).[2][3] 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 Fantina, Sicily

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 meters 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.[4]

In physics, the term light sometimes refers to electromagnetic radiation of any wavelength, whether visible or not.[5][6] 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

Electromagnetic spectrum and visible light

The electromagnetic spectrum, with the visible portion highlighted Main article: Electromagnetic spectrum

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[7][8] to as broadly as 380–800 nm.[9][10] Under ideal laboratory conditions, people can see infrared up to at least 1050 nm;[11] children and young adults may perceive ultraviolet wavelengths down to about 310–313 nm.[12][13][14]

Plant growth is also affected by the color spectrum of light, a process known as photomorphogenesis

## Speed of light

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 meter 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.[15] 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.[16]

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.[17] 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.

## Site Content

"Lightsource" redirects here. For the solar energy developer named Lightsource, see Lightsource Renewable Energy. 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[18] and roughly 44% of sunlight energy that reaches the ground is visible.[19] 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 black-body spectrum is in the deep infrared, at about 10 micrometer 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 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". Blue-white thermal emission is not often seen, except in stars (the commonly seen pure-blue color 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

Hong Kong 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

Main articles: 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 standardized  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 summarized in the following tables.

Quantity  Unit  Dimension  Notes   Name  Symbol[nb 1]  Name  Symbol  Symbol   Radiant energy  Qe[nb 2]  joule  J  ML2⋅T−2  Energy of electromagnetic radiation.   Radiant energy density  we  joule per cubic metre  J/m3  ML−1⋅T−2  Radiant energy per unit volume.   Radiant flux  Φe[nb 2]  watt  W = J/s  ML2⋅T−3  Radiant energy emitted, reflected, transmitted or received, per unit time. This is sometimes also called "radiant power".   Spectral flux  Φe,ν[nb 3]
or
Φe,λ[nb 4]  watt per hertz
or
watt per metre  W/Hz
or
W/m  ML2⋅T−2
or
MLT−3  Radiant flux per unit frequency or wavelength. The latter is commonly measured in W⋅nm−1.   Radiant intensity  Ie,Ω[nb 5]  watt per steradian  W/sr  ML2⋅T−3  Radiant flux emitted, reflected, transmitted or received, per unit solid angle. This is a directional quantity.   Spectral intensity  Ie,Ω,ν[nb 3]
or
Ie,Ω,λ[nb 4]  watt per steradian per hertz
or
watt per steradian per metre  W⋅sr−1⋅Hz−1
or
W⋅sr−1⋅m−1  ML2⋅T−2
or
MLT−3  Radiant intensity per unit frequency or wavelength. The latter is commonly measured in W⋅sr−1⋅nm−1. This is a directional quantity.   Radiance  Le,Ω[nb 5]  watt per steradian per square metre  W⋅sr−1⋅m−2  MT−3  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".   Spectral radiance  Le,Ω,ν[nb 3]
or
Le,Ω,λ[nb 4]  watt per steradian per square metre per hertz
or
watt per steradian per square metre, per metre  W⋅sr−1⋅m−2⋅Hz−1
or
W⋅sr−1⋅m−3  MT−2
or
ML−1⋅T−3  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 "spectral intensity".   Irradiance
Flux density  Ee[nb 2]  watt per square metre  W/m2  MT−3  Radiant flux received by a surface per unit area. This is sometimes also confusingly called "intensity".   Spectral irradiance
Spectral flux density  Ee,ν[nb 3]
or
Ee,λ[nb 4]  watt per square metre per hertz
or
watt per square metre, per metre  W⋅m−2⋅Hz−1
or
W/m3  MT−2
or
ML−1⋅T−3  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).   Radiosity  Je[nb 2]  watt per square metre  W/m2  MT−3  Radiant flux leaving (emitted, reflected and transmitted by) a surface per unit area. This is sometimes also confusingly called "intensity".   Spectral radiosity  Je,ν[nb 3]
or
Je,λ[nb 4]  watt per square metre per hertz
or
watt per square metre, per metre  W⋅m−2⋅Hz−1
or
W/m3  MT−2
or
ML−1⋅T−3  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".   Radiant exitance  Me[nb 2]  watt per square metre  W/m2  MT−3  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".   Spectral exitance  Me,ν[nb 3]
or
Me,λ[nb 4]  watt per square metre per hertz
or
watt per square metre, per metre  W⋅m−2⋅Hz−1
or
W/m3  MT−2
or
ML−1⋅T−3  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".   Radiant exposure  He  joule per square metre  J/m2  MT−2  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".   Spectral exposure  He,ν[nb 3]
or
He,λ[nb 4]  joule per square metre per hertz
or
joule per square metre, per metre  J⋅m−2⋅Hz−1
or
J/m3  MT−1
or
ML−1⋅T−2  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".   Hemispherical emissivity  ε      1  Radiant exitance of a surface, divided by that of a black body at the same temperature as that surface.   Spectral hemispherical emissivity  εν
or
ελ      1  Spectral exitance of a surface, divided by that of a black body at the same temperature as that surface.   Directional emissivity  εΩ      1  Radiance emitted by a surface, divided by that emitted by a black body at the same temperature as that surface.   Spectral directional emissivity  εΩ,ν
or
εΩ,λ      1  Spectral radiance emitted by a surface, divided by that of a black body at the same temperature as that surface.   Hemispherical absorptance  A      1  Radiant flux absorbed by a surface, divided by that received by that surface. This should not be confused with "absorbance".   Spectral hemispherical absorptance  Aν
or
Aλ      1  Spectral flux absorbed by a surface, divided by that received by that surface. This should not be confused with "spectral absorbance".   Directional absorptance  AΩ      1  Radiance absorbed by a surface, divided by the radiance incident onto that surface. This should not be confused with "absorbance".   Spectral directional absorptance  AΩ,ν
or
AΩ,λ      1  Spectral radiance absorbed by a surface, divided by the spectral radiance incident onto that surface. This should not be confused with "spectral absorbance".   Hemispherical reflectance  R      1  Radiant flux reflected by a surface, divided by that received by that surface.   Spectral hemispherical reflectance  Rν
or
Rλ      1  Spectral flux reflected by a surface, divided by that received by that surface.   Directional reflectance  RΩ      1  Radiance reflected by a surface, divided by that received by that surface.   Spectral directional reflectance  RΩ,ν
or
RΩ,λ      1  Spectral radiance reflected by a surface, divided by that received by that surface.   Hemispherical transmittance  T      1  Radiant flux transmitted by a surface, divided by that received by that surface.   Spectral hemispherical transmittance  Tν
or
Tλ      1  Spectral flux transmitted by a surface, divided by that received by that surface.   Directional transmittance  TΩ      1  Radiance transmitted by a surface, divided by that received by that surface.   Spectral directional transmittance  TΩ,ν
or
TΩ,λ      1  Spectral radiance transmitted by a surface, divided by that received by that surface.   Hemispherical attenuation coefficient  μ  reciprocal metre  m−1  L−1  Radiant flux absorbed and scattered by a volume per unit length, divided by that received by that volume.   Spectral hemispherical attenuation coefficient  μν
or
μλ  reciprocal metre  m−1  L−1  Spectral radiant flux absorbed and scattered by a volume per unit length, divided by that received by that volume.   Directional attenuation coefficient  μΩ  reciprocal metre  m−1  L−1  Radiance absorbed and scattered by a volume per unit length, divided by that received by that volume.   Spectral directional attenuation coefficient  μΩ,ν
or
μΩ,λ  reciprocal metre  m−1  L−1  Spectral radiance absorbed and scattered by a volume per unit length, divided by that received by that volume.   See also: SI · Radiometry · Photometry ·  (Compare)

## Table 2. SI photometry quantities

Table 2. SI photometry quantities

Quantity  Unit  Dimension  Notes   Name  Symbol[nb 6]  Name  Symbol  Symbol[nb 7]   Luminous energy  Qv[nb 8]  lumen second  lm⋅s  TJ  The lumen second is sometimes called the talbot.   Luminous flux, luminous power  Φv[nb 8]  lumen (= candela steradians)  lm (= cd⋅sr)  J  Luminous energy per unit time   Luminous intensity  Iv  candela (= lumen per steradian)  cd (= lm/sr)  J  Luminous flux per unit solid angle   Luminance  Lv  candela per square metre  cd/m2  L−2⋅J  Luminous flux per unit solid angle per unit projected source area. The candela per square metre is sometimes called the nit.   Illuminance  Ev  lux (= lumen per square metre)  lx (= lm/m2)  L−2⋅J  Luminous flux incident on a surface   Luminous exitance, luminous emittance  Mv  lux  lx  L−2⋅J  Luminous flux emitted from a surface   Luminous exposure  Hv  lux second  lx⋅s  L−2⋅TJ  Time-integrated illuminance   Luminous energy density  ωv  lumen second per cubic metre  lm⋅s/m3  L−3⋅TJ     Luminous efficacy (of radiation)  K  lumen per watt  lm/W  M−1⋅L−2⋅T3⋅J  Ratio of luminous flux to radiant flux   Luminous efficacy (of a source)  η[nb 8]  lumen per watt  lm/W  M−1⋅L−2⋅T3⋅J  Ratio of luminous flux to power consumption   Luminous efficiency, luminous coefficient  V      1  Luminous efficacy normalized by the maximum possible efficacy   See also: SI · Photometry · Radiometry ·  (Compare)

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.[20]  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.[21] At larger scales, light pressure can cause asteroids to spin faster,[22] 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.[23][24]

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.[25] 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.[26]As a consequence of light pressure, Einstein[27] 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."

Usually light momentum is aligned with its direction of motion. However, for example in evanescent waves momentum is transverse to direction of propagation.[28]

## 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.[29]

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.[30]

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.[31]

### 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.[32]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.[citation needed]The Vishnu Purana refers to sunlight as "the seven rays of the sun".[32]

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.[32]

### 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.[33] 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 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.[33]

### Particle theory

Main article: Corpuscular theory of light Pierre Gassendi

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.[34]

Christiaan Huygens. 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 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. 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.[35]

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 favor of the wave theory, helping to overturn Newton's corpuscular  theory.[dubious  – discuss] By the year 1821, Fresnel was able to show via mathematical methods  that polarization could be explained by the wave theory of light if and  only if light was entirely transverse, with no longitudinal vibration  whatsoever.[citation needed

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.[36] 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

Main article: Electromagnetic radiation A 3–dimensional rendering of linearly polarized light wave frozen in time and showing the two oscillating components of light; an electric field and a magnetic field perpendicular to each other and to the direction of motion (a transverse wave).

In 1845, Michael Faraday discovered that the plane of polarization of linearly polarized 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.[37] 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.[37] 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.[citation needed

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 behavior 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.[38]

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.[39][40] This is a long form text area designed for your content that you can fill up with as many words as your heart desires. You can write articles, long mission statements, company policies, executive profiles, company awards/distinctions, office locations, shareholder reports, whitepapers, media mentions and other pieces of content that don’t fit into a shorter, more succinct space.

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## Optics

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 light.

### Refraction

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  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.

## Induced transparency

Electromagnetically induced transparency (EIT) is 'tuned' by two particles on the optical resonator. The different locations of particles control the propagation of light in either clockwise or counterclockwise directions, which switch on (upper configuration) or off (lower configuration) the interference of light, leading to controllable brightness (EIT) and darkness in the output. Credit: Yang Lab

In the quantum realm, under some circumstances and with the right interference patterns, light can pass through opaque media.

This feature of is more than a mathematical trick; optical quantum memory, optical storage and other systems that depend on interactions of just a few photons at a time rely on the process, called electromagnetically induced transparency, also known as EIT.

Because of its usefulness in existing and emerging quantum and optical technologies, researchers are interested in the ability to manipulate EIT without the introduction of an outside influence, such as additional photons that could perturb the already delicate system. Now, researchers at the McKelvey School of Engineering at Washington University in St. Louis have devised a fully contained optical system that can be used to turn transparency on and off, allowing for a measure of control that has implications across a wide variety of applications.

The group published the results of the research, conducted in the lab of Lan Yang, the Edwin H. & Florence G. Skinner Professor in the Preston M. Green Department of Electrical & Systems Engineering, in a paper titled Electromagnetically Induced Transparency at a Chiral Exceptional Point in the January 13 issue of Nature Physics.

An optical resonator system is analogous to an electronic resonant circuit but uses photons instead of electrons. Resonators come in different shapes, but they all involve reflective material that captures light for a period of time as it bounces back and forth between or around its surface. These components are found in anything from lasers to high precision measuring devices.

For their research, Yang's team used a type of resonator known as a whispering gallery mode resonator (WGMR). It operates in a manner similar to the whispering gallery at St. Paul's Cathedral, where a person on one side of the room can hear a person whispering on the other side. What the cathedral does with sound, however, WGMRs do with light—trapping light as it reflects and bounces along the curved perimeter.

In an idealized system, a fiber optic line intersects with a resonator, a ring made of silica, at a tangent. When a photon in the line meets the resonator, it swoops in, reflecting and propagating along the ring, exiting into the fiber in the same direction it was initially headed.

Reality, however, is rarely so neat.

"Fabrication in high quality resonators is not perfect," Yang said. "There is always some defect, or dust, that scatters the light." What actually happens is some of the scattered light changes direction, leaving the resonator and travelling back in the direction whence it came. The scattering effects disperse the light, and it doesn't exit the system.

Imagine a box around the system: If the light entered the box from the left, then exited out the right side, the box would appear transparent. But if the light that entered was scattered and didn't make it out, the box would seem opaque.

Because manufacturing imperfections in resonators are inconsistent and unpredictable, so too was transparency. Light that enters such systems scatters and ultimately loses its strength; it is absorbed into the resonator, rendering the system opaque.

In the system devised by co-first authors Changqing Wang, a Ph.D. candidate, and Xuefeng Jiang, a researcher in Yang's lab, there are two WGMRs indirectly coupled by a fiber optic line. The first resonator is higher in quality, having just one imperfection. Wang added a tiny pointed material that acts like a nanoparticle to the high-quality resonator. By moving the makeshift particle, Wang was able to "tune" it, controlling the way the light inside scatters.

Importantly, he was also able to tune the resonator to what's known as an "exceptional point," a point at which one and only one state can exist. In this case, the state is the direction of light in the resonator: clockwise or counter clockwise.

For the experiment, researchers directed light toward a pair of indirectly coupled resonators from the left (see illustration). The lightwave entered the first resonator, which was "tuned" to ensure light traveled clockwise. The light bounced around the perimeter, then exited, continuing along the fiber to the second, lower-quality resonator.

There, the light was scattered by the resonator's imperfections and some of it began traveling counter clockwise along the perimeter. The light wave then returned to the fiber, but headed back toward the first resonator.

Critically, researchers not only used the nanoparticle in the first resonator to make the lightwaves move clockwise, they also tuned it in a way that, as the light waves propagated back and forth between resonators, a special interference pattern would form. As a result of that pattern, the light in the resonators was cancelled out, so to speak, allowing the light traveling along the fiber to eek by, rendering the system transparent.

It would be as if someone shined a light on a brick wall—no light would get through. But then another person with another flashlight shined it in the same spot and, all of a sudden, that spot in the wall became transparent.

One of the more important—and interesting—functions of EIT is its ability to create "slow light." The speed of light is always constant, but the actual value of that speed can change based on the properties of the medium through which it moves. In a vacuum, light always travels at 300,000,000 meters per second.

With EIT, people have slowed light down to leight meters per second, Wang said. "That can have significant influence on the storage of light information. If light is slowed down, we have enough time to use the encoded information for optical quantum computing or optical communication." If engineers can better control EIT, they can more reliably depend on slow light for these applications.

Manipulating EIT could also be used in the development of long distance communication. A tuning resonator can be indirectly coupled to another resonator kilometers away along the same fiber optic cable. "You could change the transmitted light down the line," Yang said.

This could be critical for, among other things, quantum encryption.

More information: Electromagnetically induced transparency at a chiral exceptional point, Nature Physics (2020). DOI: 10.1038/s41567-019-0746-7 , https://nature.com/articles/s41567-019-0746-7

Journal information: Nature Physics

Provided by Washington University in St. Louis

This information was found on m.phys.org