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Physic
LIGHT
AHMED ZIKRI
1007133856
JURUSAN TEKNIK KIMIA
FAKULTAS TEKNIK UR
2011
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Properties of Light
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What is light
?
Light or visible light is the portion of electromagnetic radiation that is visible to the human eye,
responsible for the sense of sight. Visible light has a wavelength in a range from about 380 or
400 nanometres to about 760 or 780 nm,[1] with a frequency range of about 405 THz to 790
THz. In physics, the term light often comprises the adjacent radiation regions of infrared (at
lower frequencies) and ultraviolet (at higher), not visible to the human eye.[2][3]
Primary properties of light are intensity, propagation direction, frequency or wavelength
spectrum, and polarization, while its speed, about 300,000,000 meters per second (300,000
kilometers per second) in vacuum, is one of the fundamental constants of nature.
Light, which is emitted and absorbed in tiny "packets" called photons, exhibits properties of both
waves and particles. This property is referred to as the waveparticle duality. The study of light,
known as optics, is an important research area in modern physics.
When light waves, which travel in straight lines, encounter any substance, they are either
reflected, absorbed, transmitted, or refracted. This is illustrated in figure 2-2. Those substances
that transmit almost all the light waves falling upon them are said to be transparent. A
transparent substance is one through which you can see clearly.
Clear glass is transparent because it transmits light rays without diffusing them (view A of figure
2-3). There is no substance known that is perfectly transparent, but many substances are nearly
so. Substances through which some light rays can pass, but through which objects cannot be seen
clearly because the rays are diffused, are called translucent (view B of figure 2-3). The frosted
glass of a light bulb and a piece of oiled paper are examples of translucent materials. Those
substances that are unable to transmit any light rays are called opaque (view C of figure 2-3).
Opaque substances either reflect or absorb all the light rays that fall upon them.
Figure 2-2. - Light waves reflected, absorbed, and transmitted.
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Figure 2-3. - Substances: A. Transparent; B. Translucent; and C. Opaque.
All substances that are not light sources are visible only because they reflect all or some part of
the light reaching them from some luminous source.
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Examples of luminous sources include the sun, a gas flame, and an electric light filament,
because they are sources of light energy. If light is neither transmitted nor reflected, it is
absorbed or taken up by the medium. When light strikes a substance, some absorption and some
reflection always take place. No substance completely transmits, reflects, or absorbs all the light
rays that reach its surface.
Light and Matter
Why does matter interact with light? Remember, light is a manifestation of electromagnetic
force. Matter is made up of charged particles due to the nature of atoms, being composed of a
positively charged nucleus surrounded by electrons that are in motion. The nuclei in molecules
also move with respect to each other. In other words, these are charges that are in motion, and
everytime charges are in motion, there will be an electromagnetic force that will be changing
with time. Light is an electromagnetic field that is oscillating. It is a wave that can becharacterized by a frequency. But light is also a particle - its particle is called a photon and each
photon carries a packet of energy that is proportional to the frequency. Matter can absorb the
energy from a photon.
What does matter do with the energy from light? It depends on what kind of light. There is a
whole spectrum of light. Light with very low frequencies (or long wavelengths, such as
radiowaves) have photons that are not too energetic. This energy magnitude corresponds to
nuclear spin levels. Light with frequencies in the range of a gigahertz (109 per second or Hertz)
correspond to microwaves. These have enough energy to cause molecules to rotate faster. Even
higher frequencies (1012-1014 Hz) have enough energy to make molecules stretch and bendtheir bonds. Light that we see corresponds to a very narrow range of the spectrum, 400 - 750 nm
in wavelength. For some molecules, photons in this range have enough energy to excite
electrons, promoting them to higher energy levels. Since these molecules will only absorb a
specific frequency, what reaches our eye is no longer white light, but is now colored (It's white
minus the color that the molecule absorbed).
Color
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Color or colour (see spelling differences) is the visual perceptual property corresponding in
humans to the categories called red, green, blue and others. Color derives from the spectrum of
light (distribution of light energy versus wavelength) interacting in the eye with the spectral
sensitivities of the light receptors. Color categories and physical specifications of color are also
associated with objects, materials, light sources, etc., based on their physical properties such as
light absorption, reflection, or emission spectra. By defining a color space, colors can be
identified numerically by their coordinates.
Because perception of color stems from the varying spectral sensitivity of different types of cone
cells in the retina to different parts of the spectrum, colors may be defined and quantified by the
degree to which they stimulate these cells. These physical or physiological quantifications of
color, however, do not fully explain the psychophysical perception of color appearance.
The science of color is sometimes called chromatics. It includes the perception of color by the
human eye and brain, the origin of color in materials, color theory in art, and the physics of
electromagnetic radiation in the visible range (that is, what we commonly refer to simply aslight).
Color of objects
The upper disk and the lower disk have exactly the same objective color, and are in identical
gray surroundings; based on context differences, humans perceive the squares as having different
reflectances, and may interpret the colors as different color categories; see same color illusion.
The color of an object depends on both the physics of the object in its environment and the
characteristics of the perceiving eye and brain. Physically, objects can be said to have the color
of the light leaving their surfaces, which normally depends on the spectrum of the incidentillumination and the reflectance properties of the surface, as well as potentially on the angles of
illumination and viewing. Some objects not only reflect light, but also transmit light or emit light
themselves (see below), which contribute to the color also. And a viewer's perception of the
object's color depends not only on the spectrum of the light leaving its surface, but also on a host
of contextual cues, so that the color tends to be perceived as relatively constant: that is, relatively
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independent of the lighting spectrum, viewing angle, etc. This effect is known as color
constancy.
Some generalizations of the physics can be drawn, neglecting perceptual effects for now:
Light arriving at an opaque surface is either reflected "specularly" (that is, in the manner of amirror), scattered (that is, reflected with diffuse scattering), or absorbedor some combination
of these.
Opaque objects that do not reflect specularly (which tend to have rough surfaces) have their
color determined by which wavelengths of light they scatter more and which they scatter less
(with the light that is not scattered being absorbed). If objects scatter all wavelengths, they
appear white. If they absorb all wavelengths, they appear black.
Opaque objects that specularly reflect light of different wavelengths with different efficiencies
look like mirrors tinted with colors determined by those differences. An object that reflects some
fraction of impinging light and absorbs the rest may look black but also be faintly reflective;
examples are black objects coated with layers of enamel or lacquer.
Objects that transmit light are either translucent (scattering the transmitted light) or transparent
(not scattering the transmitted light). If they also absorb (or reflect) light of varying wavelengths
differentially, they appear tinted with a color determined by the nature of that absorption (or that
reflectance).
Objects may emit light that they generate themselves, rather than merely reflecting or
transmitting light. They may do so because of their elevated temperature (they are then said to be
incandescent), as a result of certain chemical reactions (a phenomenon calledchemoluminescence), or for other reasons (see the articles Phosphorescence and List of light
sources).
Objects may absorb light and then as a consequence emit light that has different properties. They
are then called fluorescent (if light is emitted only while light is absorbed) or phosphorescent (if
light is emitted even after light ceases to be absorbed; this term is also sometimes loosely applied
to light emitted because of chemical reactions).
For further treatment of the color of objects, see structural color, below.
To summarize, the color of an object is a complex result of its surface properties, its transmissionproperties, and its emission properties, all of which factors contribute to the mix of wavelengths
in the light leaving the surface of the object. The perceived color is then further conditioned by
the nature of the ambient illumination, and by the color properties of other objects nearby, via the
effect known as color constancy and via other characteristics of the perceiving eye and brain.
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Speed of light
The speed of light in a vacuum is defined to be exactly 299,792,458 m/s (approximately 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.
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 Rmer, a Danish physicist, in 1676. Using a
telescope, Ole observed the motions of Jupiter and one of its moons, Io. Noting discrepancies in
the apparent period of Io's orbit, Rmer calculated that light takes about 22 minutes to traverse
the diameter of Earth's orbit.[4] Unfortunately, its size was not known at that time. If Ole 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. Arotating 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.
Lon Foucault used 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 Mt. Wilson to Mt. San Antonio inCalifornia. The precise measurements yielded a speed of 299,796,000 m/s.
Two independent teams of physicists were able to bring light to a complete standstill by passing
it through a Bose-Einstein Condensate of the element rubidium, one team led by Dr. Lene
Vestergaard Hau of Harvard University and the Rowland Institute for Science in Cambridge,
Mass., and the other by Dr. Ronald L. Walsworth and Dr. Mikhail D. Lukin of the Harvard-
Smithsonian Center for Astrophysics, also in Cambridge.[5]
Electromagnetic spectrum
Although some radiations are marked as "N" for "no" in the diagram, some waves do in fact
penetrate the atmosphere, although extremely minimally compared to the other radiations.
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The electromagnetic spectrum is the range of all possible frequencies of electromagnetic
radiation.[1] The "electromagnetic spectrum" of an object is the characteristic distribution of
electromagnetic radiation emitted or absorbed by that particular object.
The electromagnetic spectrum extends from low frequencies used for modern radio to gamma
radiation at the short-wavelength end, covering wavelengths from thousands of kilometers downto a fraction of the size of an atom. The long wavelength limit is the size of the universe itself,
while it is thought that the short wavelength limit is in the vicinity of the Planck length, although
in principle the spectrum is infinite and continuous.
Range of the spectrum
EM waves are typically described by any of the following three physical properties: the
frequency f, wavelength , or photon energy E. Frequencies range from 2.41023 Hz (1 GeV
gamma rays) down to the local plasma
frequency of the ionized interstellar medium (~1 kHz). Wavelength is inversely proportional to
the wave frequency, so gamma rays have very short wavelengths that are fractions of the size of
atoms, whereas wavelengths can be as long as the universe. Photon energy is directly
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proportional to the wave frequency, so gamma rays have the highest energy (around a billion
electron volts) and radio waves have very low energy (around femto electron volts). These
relations are illustrated by the following equations:
where:
c = 299,792,458 m/s is the speed of light in vacuum and
h = 6.62606896(33)1034 J s = 4.13566733(10)10
15 eV s is Planck's
constant.[5]
Whenever electromagnetic waves exist in a medium with matter, their wavelength is decreased.
Wavelengths of electromagnetic radiation, no matter what medium they are traveling through,
are usually quoted in terms of the vacuum wavelength, although this is not always explicitly
stated.
Generally, EM radiation is classified by wavelength into radio wave, microwave, infrared, the
visible region we perceive as light, ultraviolet, X-rays and gamma rays. The behavior of EM
radiation depends on its wavelength. When EM radiation interacts with single atoms and
molecules, its behavior also depends on the amount of energy per quantum (photon) it carries.
Spectroscopy can detect a much wider region of the EM spectrum than the visible range of 400
nm to 700 nm. A common laboratory spectroscope can detect wavelengths from 2 nm to 2500
nm. Detailed information about the physical properties of objects, gases, or even stars can be
obtained from this type of device. Spectroscopes are widely used in astrophysics. For example,many hydrogen atoms emit a radio wave photon which has a wavelength of 21.12 cm. Also,
frequencies of 30 Hz and below can be produced by and are important in the study of certain
stellar nebulae[6] and frequencies as high as 2.91027 Hz have been detected from astrophysical
sources.[7]
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Types of radiation
While the classification scheme is generally accurate, in reality there is often some overlapbetween neighboring types of electromagnetic energy. For example, SLF radio waves at 60 Hz
may be received and studied by astronomers, or may be ducted along wires as electric power,
although the latter is, strictly speaking, not electromagnetic radiation at all (see near and far
field) The distinction between X and gamma rays is based on sources: gamma rays are the
photons generated from nuclear decay or other nuclear and subnuclear/particle process, whereas
X-rays are generated by electronic transitions involving highly energetic inner atomic electrons.
Generally, nuclear transitions are much more energetic than electronic transitions, so usually,
gamma-rays are more energetic than X-rays, but exceptions exist. By analogy to electronic
transitions, muonic atom transitions are also said to produce X-rays, even though their energy
may exceed 6 megaelectronvolts (0.96 pJ),[8] whereas there are many (77 known to be less than
10 keV (1.6 fJ)) low-energy nuclear transitions (e.g. the 7.6 eV (1.22 aJ) nuclear transition of
thorium-229), and despite being one million-fold less energetic than some muonic X-rays, the
emitted photons are still called gamma rays due to their nuclear origin.[9]
Also, the region of the spectrum of the particular electromagnetic radiation is reference-frame
dependent (on account of the Doppler shift for light) so EM radiation which one observer would
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say is in one region of the spectrum could appear to an observer moving at a substantial fraction
of the speed of light with respect to the first to be in another part of the spectrum. For example,
consider the cosmic microwave background. It was produced, when matter and radiation
decoupled, by the de-excitation of hydrogen atoms to the ground state. These photons were from
Lyman series transitions, putting them in the ultraviolet (UV) part of the electromagnetic
spectrum. Now this radiation has undergone enough cosmological red shift to put it into the
microwave region of the spectrum for observers moving slowly (compared to the speed of light)
with respect to the cosmos. However, for particles moving near the speed of light, this radiation
will be blue-shifted in their rest frame. The highest energy cosmic ray protons are moving such
that, in their rest frame, this radiation is blueshifted to high energy gamma rays which interact
with the proton to produce bound quark-antiquark pairs (pions). This is the source of the GZK
limit.
Refraction of Light
The direction of light propagation can be changed at the boundary of two media having differentdensities. This property is called refraction, and is illustrated in the following figure for the
boundary between air and water.
Refraction of light
The apparent and actual positions of the fish differ because the direction of light propagation has
been changed as light passes from the more dense water into the less dense air.
If we adopt the convention that the light passes from medium 1 into medium 2, the general rule
is that the refraction is
Away from the perpendicular if medium 2 is less dense than medium 1
Toward the perpendicular if medium 2 is more dense than medium 1
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Thus, in the above example the refraction is away from the perpendicular because air is less
dense than water. Such effects form the basis of the refracting telescope, and of optical devices
using lenses in general.
Diffraction of Light
Because light is a wave, it has the capability to "bend around corners". This is called diffraction,
and is illustrated in the adjacent image. The intensity of light behind the barrier is not zero in the
shadow region. diffractive effects occur generally when a part of a light wave is cut off by an
obstruction. Here is a Java applet illustrating diffraction of light by a single slit, and here is an
interactive demonstration of refraction and diffraction for ocean waves.
Diffraction has a number of consequences for astronomy. Two of the more important are that this
property is the basis for the diffraction grating that can be used to separate light into its
constituent colors, and that diffractive effects set an absolute limit on the quality of an image
observed through an optical instrument such as a telescope. This diffractive limit occurs because
the lenses of such objects are of finite size and diffract light because they cut off part of the light
wave.
Light sources
There are many sources of light. The most common light sources are thermal: a body at a given
temperature emits a characteristic spectrum of black-body radiation. Examples include sunlight
(the radiation emitted by the chromosphere of the Sun at around 6,000 Kelvin peaks in the
visible region of the electromagnetic spectrum when plotted in wavelength units [6] and roughly
40% of sunlight is visible), incandescent light bulbs (which emit only around 10% of their
energy as visible light and the remainder as infrared), and glowing solid particles in flames. The
peak of the blackbody spectrum is in the infrared for relatively cool objects like human beings.
As the temperature increases, the peak shifts to shorter wavelengths, producing first a red glow,
then a white one, and finally a blue color as the peak moves out of the visible part of the
spectrum and into the ultraviolet. These colors can be seen when metal is heated to "red hot" or
"white hot". Blue thermal emission is not often seen. The commonly seen blue colour in a gas
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flame or a welder's torch is in fact due to molecular emission, notably by CH radicals (emitting a
wavelength band around 425 nm).
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 fromthe hot gas itselfso, 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 movingthrough 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:
scintillation
electroluminescence
sonoluminescence
triboluminescence
Cherenkov radiation
When the concept of light is intended to include very-high-energy photons (gamma rays),additional generation mechanisms include:
Radioactive decay
Particleantiparticle annihilation
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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 standardized model of human brightness perception. Photometry is useful, for
example, to quantify Illumination (lighting) intended for human use. The SI units for bothsystems are summarized in the following tables.
SI radiometry units
Quantity Symbol SI unit Abbr. Notes
Radiant
energyQ oule J energy
Radiant flux watt Wradiant energy per unit time, also
called radiant power
Radiant
intensityI watt per steradian Wsr
1power per unit solid angle
Radiance Lwatt per steradian
per square metreWsr
1m
2
power per unit solid angle per unit
projectedsource area.
called intensity in some other fieldsof study.
Irradiance E, Iwatt per square
metreWm
2
power incident on a surface.
sometimes confusingly called
"intensity".
Radiantexitance/
Radiantemittance
Mwatt per square
metreWm
2power emitted from a surface.
Radiosity JorJwatt per square
metreWm
2
emitted plus reflected power leaving
a surface
Spectral
radiance
L
or
L
watt per steradianper metre3
or
watt per steradian
per squaremetre per hertz
Wsr1
m3
or
Wsr1
m2
Hz1
commonly measured inWsr
1m
2nm
1
Spectralirradiance
E
or
E
watt per metre3
orwatt per square
metre per hertz
Wm3
or
Wm2
Hz1
commonly measured in Wm2
nm1
or 10-22Wm-2Hz-1, known as a Solar
Flux Unit (SFU)[SI Radiometry units 1]
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SI photometry unitsv d e
Quantity Symbol SI unit Abbr. Notes
Luminous energy Qv lumen second lms units are sometimes called talbots
Luminous flux F lumen (= cdsr) lm also called luminous power
Luminous intensity Iv candela (= lm/sr) cd an SI base unit
Luminance
Lv
candela per square
metre cd/m2
units are sometimes called "nits"
Illuminance Ev lux (= lm/m2) lx Used for light incident on a surface
Luminous
emittance Mv lux (= lm/m
2) lxUsed for light emitted from a
surface
Luminous efficacy lumen per watt lm/W
ratio of luminous flux to radiant
flux
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 luminousefficacy, 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.
http://en.wikipedia.org/wiki/Template:SI_light_unitshttp://en.wikipedia.org/wiki/Template_talk:SI_light_unitshttp://en.wikipedia.org/w/index.php?title=Template:SI_light_units&action=edithttp://en.wikipedia.org/wiki/Luminous_energyhttp://en.wikipedia.org/wiki/Lumen_%28unit%29http://en.wikipedia.org/wiki/Secondhttp://en.wikipedia.org/wiki/Talbot_%28photometry%29http://en.wikipedia.org/wiki/Luminous_fluxhttp://en.wikipedia.org/wiki/Luminous_fluxhttp://en.wikipedia.org/wiki/Lumen_%28unit%29http://en.wikipedia.org/wiki/Steradianhttp://en.wikipedia.org/wiki/Luminous_intensityhttp://en.wikipedia.org/wiki/Candelahttp://en.wikipedia.org/wiki/SI_base_unithttp://en.wikipedia.org/wiki/Luminancehttp://en.wikipedia.org/wiki/Luminancehttp://en.wikipedia.org/wiki/Candela_per_square_metrehttp://en.wikipedia.org/wiki/Candela_per_square_metrehttp://en.wikipedia.org/wiki/Candela_per_square_metrehttp://en.wikipedia.org/wiki/Illuminancehttp://en.wikipedia.org/wiki/Illuminancehttp://en.wikipedia.org/wiki/Luxhttp://en.wikipedia.org/wiki/Luminous_emittancehttp://en.wikipedia.org/wiki/Luminous_emittancehttp://en.wikipedia.org/wiki/Luminous_emittancehttp://en.wikipedia.org/wiki/Luminous_efficacyhttp://en.wikipedia.org/wiki/Watthttp://en.wikipedia.org/wiki/Radiant_fluxhttp://en.wikipedia.org/wiki/Radiant_fluxhttp://en.wikipedia.org/wiki/Radiant_fluxhttp://en.wikipedia.org/wiki/Radiant_fluxhttp://en.wikipedia.org/wiki/Radiant_fluxhttp://en.wikipedia.org/wiki/Watthttp://en.wikipedia.org/wiki/Luminous_efficacyhttp://en.wikipedia.org/wiki/Luminous_emittancehttp://en.wikipedia.org/wiki/Luminous_emittancehttp://en.wikipedia.org/wiki/Luxhttp://en.wikipedia.org/wiki/Illuminancehttp://en.wikipedia.org/wiki/Candela_per_square_metrehttp://en.wikipedia.org/wiki/Candela_per_square_metrehttp://en.wikipedia.org/wiki/Luminancehttp://en.wikipedia.org/wiki/SI_base_unithttp://en.wikipedia.org/wiki/Candelahttp://en.wikipedia.org/wiki/Luminous_intensityhttp://en.wikipedia.org/wiki/Steradianhttp://en.wikipedia.org/wiki/Lumen_%28unit%29http://en.wikipedia.org/wiki/Luminous_fluxhttp://en.wikipedia.org/wiki/Talbot_%28photometry%29http://en.wikipedia.org/wiki/Secondhttp://en.wikipedia.org/wiki/Lumen_%28unit%29http://en.wikipedia.org/wiki/Luminous_energyhttp://en.wikipedia.org/w/index.php?title=Template:SI_light_units&action=edithttp://en.wikipedia.org/wiki/Template_talk:SI_light_unitshttp://en.wikipedia.org/wiki/Template:SI_light_units -
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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 isnegligible 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.[7] However, in
nanometer-scale applications such as NEMS, the effect of light pressure is more pronounced, and
exploiting light pressure to drive NEMS mechanisms and to flip nanometer-scale physical
switches in integrated circuits is an active area of research.[8]
At larger scales, light pressure can cause asteroids to spin faster,[9] acting on their irregular
shapes as on the vanes of a windmill. The possibility to make solar sails that would accelerate
spaceships in space is also under investigation.[10][11]
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.[12] This should not be confused with the Nichols radiometer, in which the motion is
directly caused by light pressure.
Historical theories about light, in chronological order
1. Hindu and Buddhist theories In ancient India, the Hindu schools of Samkhya and Vaisheshika,
from around the 6th5th century BC, developed theories on light. According to the Samkhya
school, light is one of the five fundamental "subtle" elements (tanmatra) out of which emerge the
gross elements. The atomicity of these elements is not specifically mentioned and it appears that
they were actually taken to be continuous.
2. Greek and Hellenistic theories In the fifth century BC, Empedocles postulated that everything
was composed of four elements; fire, air, earth and water. He believed that Aphrodite made the
human eye out of the four elements and that she lit the fire in the eye which shone out from the
eye making sight possible. If this were true, then one could see during the night just as well as
during the day, so Empedocles postulated an interaction between rays from the eyes and rays
from a source such as the sun.
3. Physical theories Ren Descartes (15961650) 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.[15] In 1637 he published a theory of the refraction of light that
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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 wavesAlthough
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.
4. Particle theory Pierre Gassendi (15921655), 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
Hypothesisfight 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.
5. Wave theory In the 1660s, Robert Hooke published a wave theory of light. Christiaan
Huygens worked out his own wave theory of light in 1678, and published it in his Treatise on
light in 1690. He proposed that light was emitted in all directions as a series of waves in a
medium called the Luminiferous ether. As waves are not affected by gravity, it was assumed that
they slowed down upon entering a denser medium.
6. Electromagnetic theory 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.[18] This was
the first evidence that light was related to electromagnetism. In 1846 he speculated that lightmight be some form of disturbance propagating along magnetic field lines.[19] 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 andmagnetic 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.
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7. The special theory of relativity The wave theory was wildly successful in explaining nearly all
optical and electromagnetic phenomena, and was a great triumph of nineteenth century physics.
By the late nineteenth century, however, a handful of experimental anomalies remained that
could not be explained by or were in direct conflict with the wave theory. One of these anomalies
involved a controversy over the speed of light. The constant speed of light predicted by
Maxwell's equations and confirmed by the Michelson-Morley experiment contradicted the
mechanical laws of motion that had been unchallenged since the time of Galileo, which stated
that all speeds were relative to the speed of the observer. In 1905, Albert Einstein resolved this
paradox by revising the Galilean model of space and time to account for the constancy of the
speed of light. Einstein formulated his ideas in his special theory of relativity, which advanced
humankind's understanding of space and time. Einstein also demonstrated a previously unknown
fundamental equivalence between energy and mass with his famous equation
where E is energy, m is, depending on the context, the rest mass or the relativistic mass, and c is
the speed of light in a vacuum.
Particle theory revisited
Another experimental anomaly was the photoelectric effect, by which light striking a metal
surface ejected electrons from the surface, causing an electric current to flow across an applied
voltage. Experimental measurements demonstrated that the energy of individual ejected electrons
was proportional to the frequency, rather than the intensity, of the light. Furthermore, below a
certain minimum frequency, which depended on the particular metal, no current would flowregardless of the intensity. These observations appeared to contradict the wave theory, and for
years physicists tried in vain to find an explanation. In 1905, Einstein solved this puzzle as well,
this time by resurrecting the particle theory of light to explain the observed effect. Because of the
preponderance of evidence in favor of the wave theory, however, Einstein's ideas were met
initially with great skepticism among established physicists. But eventually Einstein's
explanation of the photoelectric effect would triumph, and it ultimately formed the basis for
waveparticle duality and much of quantum mechanics.
8. Quantum theory A third anomaly that arose in the late 19th century involved a contradiction
between the wave theory of light and measurements of the electromagnetic spectrum emitted bythermal radiators, or so-called black bodies. Physicists struggled with this problem, which later
became known as the ultraviolet catastrophe, unsuccessfully for many years. In 1900, Max
Planck developed a new theory of black-body radiation that explained the observed spectrum.
Planck's theory was based on the idea that black bodies emit light (and other electromagnetic
radiation) only as discrete bundles or packets of energy. These packets were called quanta, and
the particle of light was given the name photon, to correspond with other particles being
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described around this time, such as the electron and proton. A photon has an energy, E,
proportional to its frequency, f, by
where h is Planck's constant, is the wavelength and c is the speed of light. Likewise, the
momentum p of a photon is also proportional to its frequency and inversely proportional to its
wavelength:
As it originally stood, this theory did not explain the simultaneous wave- and particle-like
natures of light, though Planck would later work on theories that did. In 1918, Planck receivedthe Nobel Prize in Physics for his part in the founding of quantum theory.
9. Waveparticle duality The modern theory that explains the nature of light includes the notion
of waveparticle duality, described by Albert Einstein in the early 1900s, based on his study of
the photoelectric effect and Planck's results. Einstein asserted that the energy of a photon is
proportional to its frequency. More generally, the theory states that everything has both a particle
nature and a wave nature, and various experiments can be done to bring out one or the other. The
particle nature is more easily discerned if an object has a large mass, and it was not until a bold
proposition by Louis de Broglie in 1924 that the scientific community realized that electrons also
exhibited waveparticle duality. The wave nature of electrons was experimentally demonstratedby Davisson and Germer in 1927. Einstein received the Nobel Prize in 1921 for his work with
the waveparticle duality on photons (especially explaining the photoelectric effect thereby), and
de Broglie followed in 1929 for his extension to other particles.
10.Quantum electrodynamics The quantum mechanical theory of light and electromagnetic
radiation continued to evolve through the 1920s and 1930s, and culminated with the
development during the 1940s of the theory of quantum electrodynamics, or QED. This so-called
quantum field theory is among the most comprehensive and experimentally successful theories
ever formulated to explain a set of natural phenomena. QED was developed primarily by
physicists Richard Feynman, Freeman Dyson, Julian Schwinger, and Shin-Ichiro Tomonaga.Feynman, Schwinger, and Tomonaga shared the 1965 Nobel Prize in Physics for their
contributions.
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