Tugas Fisika Light

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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.
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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.

Tugas Fisika Light

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