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LightLighthttp://jsu.ac.ir/~dvdtlb/
Brieflyf y• In physics, the term light sometimes refers to electromagnetic radiation of any wavelengthelectromagnetic radiation of any wavelength, whether visible or not.
• Primary properties of light are intensity, propagation y p p g y, p p gdirection, frequency or wavelength spectrum, and polarization.
• Its speed in a vacuum 299 792 458 meters per• Its speed in a vacuum, 299,792,458 meters per second, is one of the fundamental constants of nature.
• The study of light, and the interaction of light and matter is termed optics, is an important research area in modern physicsarea in modern physics.
• Light, which is emitted and absorbed in tiny g , y"packets" called photons, exhibits properties of both waves and particles. This property is referred to as the wave–particle duality.
Ray Theory – Light travelsRay Theory – Light travels along a straight line and b l f i lobeys laws of geometrical
optics. Ray theory is validoptics. Ray theory is valid when the objects are much l th th l thlarger than the wavelength (multimode fibers)( )
Dossier of History Archimedes of Syracuse(Greek: Ἀρχιμήδης)
Bornc. 287 BCSyracuse, Sicily; Magna Graecia
Diedc. 212 BC (aged around 75)Syracuse
Residence Syracuse, Sicily
FieldsMathematics, Physics, Engineering, Astronomy, Invention
A hi d ' P i i lKnown for
Archimedes' Principle, Archimedes' screw, Hydrostatics, Levers, Infinitesimals
Ptolemy (Batlamyus)
Born c. AD 90; Egypt
Diedc. AD 168 (aged 77–78)Alexandria, Egypt
Occupationmathematician, geographer, astronomer, astrologer
Law of reflectionLaw of reflection– the angle of incidence equals angle of reflection
7
angle of reflection
– angles are measured from normal
Refraction
• index of refraction, n, where c = speed of light in a vacuum and v = speed of light in that mediumvacuum and v = speed of light in that medium
– nair = 1
n = 1 5– nglass = 1.5
• Snell’s Law
Total Internal ReflectionTotal Internal Reflection• Incident angle where refracted angle (θ2) is 90 is the
i i l lcritical angle
• at incident angles greater than critical angle, light isat incident angles greater than critical angle, light is totally internally reflected
Geometric Optics
Guided Propagation Along the Optical Fiber
The structure of a typical single‐mode fiber.1. Core: 8 µm diameter2. Cladding: 125 µm dia.2. Cladding: 125 µm dia.3. Buffer: 250 µm dia.4. Jacket: 400 µm dia.
Plato’s fireS h f fi ld t b b tSo much of fire as would not burn, but gave gentle light, they formed into a substance related to the light of everyday life and the pure fire which iseveryday life, and the pure fire which is within us and related to it they made to flow through the eyes in a stream smooth and dense, compressing the
Full name Plato (Πλάτων)Born c. 428–427 BC; Athenssmooth and dense, compressing the
whole eye and specially the center part, so that it kept out everything of a coarser nature and allowed to pass
;
Diedc. 348–347 BC (age approx 80);Athens
Era Ancient philosophyi hil honly its pure element. Region Western Philosophy
School PlatonismInfluenced by: Socrates, Homer, Hesiod, Aesop, Protagoras, Parmenides, Pythagoras, Orphism, …g , , y g , p ,Influenced: Most of subsequent western philosophy, including Aristotle, Augustine, Neoplatonism, Cicero, Plutarch, Stoicism, Anselm, Machiavelli, Descartes, Hobbes, Leibniz, Mill, Schopenhauer, Nietzsche,Hobbes, Leibniz, Mill, Schopenhauer, Nietzsche, Heidegger, Arendt, Gadamer, Imam Khomeini, Russell and countless other philosophers and theologians.
End of visual rays“There is no vision unless something comes from
Alhazen (Ibn al‐Haytham)
Over‐confident about practical application ofThere is no vision unless something comes from
the visible object to the eye, whether or not anything goes out”
practical application of his mathematical knowledge, he assumed that he could regulate th fl d f th Nil
BornJuly 1, 965 CE (354 AH) Basra in present‐day Iraq, Buyid Persia
h ( d ) ( )
the floods of the Nile.
DiedMarch 6, 1040 (aged 74) (430 AH)Cairo, Egypt, Fatimid Caliphate
Fields physicist and Mathematician
Book of Optics, Doubts Concerning
Known for
Ptolemy, On the Configuration of the World, The Model of the Motions, Treatise on Light, Treatise on Place, scientific method, experimental science, experimental physics,
camera obscurascience, experimental physics, experimental psychology, visual perception, analytic geometry, non‐Ptolemaic astronomy, celestial mechanics
Influences Aristotle, Euclid, Ptolemy
Influenced Averroes, Witelo, Roger Bacon, Kepler
Measurements of the Speed of LightMeasurements of the Speed of Light
Date Author Method Result (km/s) Error( )
1676 Olaus Roemer Jupiter's satellites 214,000
1726 James Bradley Stellar aberration 301,000y ,
1849 Armand Fizeau Toothed wheel 315,000
1862 Leon Foucault Rotating mirror 298,000 +-500g
1879 Albert Michelson Rotating mirror 299,910 +-50
1907 Rosa, Dorsay Electromagnetic constants 299,788 +-30
1926 Albert Michelson Rotating mirror 299,796 +-4
1947 Essen, Gorden-Smith Cavity resonator 299,792 +-3
1958 K. D. Froome Radio interferometer 299,792.5 +-0.1
1973 Evanson et al Lasers 299,792.4574 +-0.001
1983 Adopted value 299,792.458
By watching the moons circling Jupiter through a telescope, he was able to observe the exact time that the moon moved behind Jupiter. After many years of observations, however, p y y , ,he noticed that the time intervals between eclipses were not always the same.
(position 1): The light from the eclipsing moon sometimes k l l h h htook a relatively short time to reach earth.
(position 2): when earth was in a farther part of its orbit, the light from Jupiter's moon would take longer to reach earth.
Roemer as able to se the time difference and theOle Rømer
Roemer was able to use the time difference and the diameter of Earth's orbit to calculate the speed of light. He determined it to be about 185,000 miles per second, or 296,000 kilometers per second, very close to the actual value.
Born25 Sept. 1644; Århus
Died19 Sept. 1710 (aged 65)Copenhagen
Nationality DanishNationality Danish
Light Speed measurementusing a pair of toothed wheels driven by a very fast motor. As the toothed wheels turned, a beam of light h h h h b h h f
Hippolyte Fizeau
BSept. 23, 1819
shone through the gaps between the teeth of one wheel would on ly pass through the gaps in the other wheel if they lined up. Knowing the speed that the wheels were Born
p ,Paris
DiedSept. 18, 1896Venteuil
Knowing the speed that the wheels were turning allowed him to calculate the speed of light with an accuracy similar to Roemer's.
Nationality French
Fields Physics
Known for
Doppler EffectFizeau‐Foucault apparatusCapacitorCapacitor
Michelson's method was to shine a light on the rotating hexagonal mirror, which then reflected to the other mirror 35
Albert Abraham Michelson
kilometres away.
The mirror needed to make one‐eighth of a rotation in the time it took the light to make the return trip. This meant that the octagonal mirror had to be turning at about 32 000 rpm.
From the round trip distance the light travelled, and the period of rotation of the octagonal mirror, the speed of light
d t i d it t l
BornDecember 19, 1852; Strzelno, Kingdom of Prussia
Died May 9, 1931; Pasadena, Californiawas determined quite accurately. United States; Physics
Alma mater: United States Naval Academy, University of Berlin; Doctoral advisor: Hermann Helmholtz; Doctoral students: Robert Millikane o t ; octo a stude ts: obe t a
KnownFor
Speed of lightMichelson‐Morley experiment
blNobel Prize for Physics (1907)
Notable awards
Nobel Prize for Physics (1907)Copley Medal (1907)Henry Draper Medal (1916)
The Search for AetherMichelson - Morley Experiment(1887)Michelson Morley Experiment(1887)
No aether drift was detected, despite repeated experiments!Conclusion:
jundi Shapur University, Dezful is the center of the Universe orThere is No Aether
Measuring Earth’s Movement through h hthe Aether
Rotating the apparatus would give the direction of the aether drift
Showed that there is no such thing as aether (nor any need for it). Light is perfectly happy traveling in a vacuum.
The speed of light is the same in any direction, which explains the null result of Michelson and Morley.
Wave Theory Light travels as aWave Theory – Light travels as a transverse electromagnetic wave
Magnet and MagnetismMagnet and Magnetism
certain iron oxides were discovered in various parts of the world, notably in Magnesiain Asia Minor, that had the property of attracting small pieces of iron.
Magnesia within Greece
Hans Christian Ørsteddiscovered that electric currents
t ti fi ldcreate magnetic fields.
Born14 August 1777 Rudkøbing, Denmark
d9 March 1851 ( d ) hDied (aged 73)Copenhagen, Denmark
Nationality Danish
Fields physics, chemistry
Known for electromagnetism
relates the integrated magnetic field around a closed loop to the electric
André‐Marie Ampère
around a closed loop to the electric current passing through the loop.
Born20 January 1775 Parish of St. Nizier, Lyon, France
Died10 June 1836 (aged 61)Marseille, France
Residence FranceResidence France
Nationality French
Fields Physics
Institutions
Bourg‐en‐BresseÉcole Polytechnique
Known for Ampere's Law
http://www.walter‐fendt.de/ph14e/mfwire.htm
Michael FaradayThe induced electromotive force (EMF) in any closed circuit is equal to the time rate of change of the magnetic flux through the circuit.
Born22 September 1791Newington Butts, England
Died25 August 1867 (aged 75) Hampton Court, Middlesex, England
Residence England
Nationality British
Fields Physics and chemistryy y
Institutions Royal Institution
Faraday's law of induction; Electrochemistry; Faraday effect; Faraday cage; Faraday constantFaraday cup; Faraday's laws of
Known forFaraday cup; Faraday s laws of electrolysis;Faraday paradox; Faraday rotator; Faraday‐efficiency effect; Faraday wave;Faraday wheel;Lines of force
flHumphry Davy
http://phet.colorado.edu/en/simulation/faraday
InfluencesHumphry DavyWilliam Thomas Brande
Notable awards
Royal Medal (1835 & 1846)Copley Medal (1832 & 1838)Rumford Medal (1846)
Heinrich Rudolf Hertz
BornFebruary 22, 1857 Hamburg, Germany
DiedJanuary 1, 1894 (aged 36) Bonn, Germany
Fields Physics; Electronic Engineering
University of KielInstitutions
University of KielUniversity of KarlsruheUniversity of Bonn
Doctoral advisor Hermann von Helmholtz
Electromagnetic radiationKnown for
Electromagnetic radiationPhotoelectric effect
NATURE OF WAVES• Waves (Def.) – A wave is a disturbance that transfers energy.• Medium – Substance or region through which a wave is transmitted.• Speed of Waves Depends on the properties of the medium• Speed of Waves – Depends on the properties of the medium.
Transverse WavesTransverse Waves
• Energy is perpendicular to direction of• Energy is perpendicular to direction of motion
• Moving photon creates electric & magnetic fieldmagnetic field
–Light has BOTH Electric & Magnetic g gfields at right angles!
So, What is Light?So, What is Light?
Light consists of a varying electric and magnetic field
http://www.walter‐fendt.de/ph14e/emwave.htm
http://www.phy.ntnu.edu.tw/ntnujava/index.php?topic=35
Electromagnetic SpectrumThe behavior of EM radiation depends on its wavelength. Higher frequencies have shorter wavelengths, and lower frequencies have longer wavelengths. A photon of ultra-violet radiation carries more energy than a photon of infrared radiation.
© 2000 Microsoft Clip Gallery
• Invisible SpectrumRadio WavesRadio WavesDef. – Longest wavelength & lowest frequency.
Uses – Radio & TV broadcastingUses – Radio & T.V. broadcasting.
Short Wavelength MicrowaveInfrared Rays– Infrared Rays
• Def – Light rays with longer wavelength than red light.• Uses: Cooking, Medicine, T.V. remote controlsg
© 2000 Microsoft Clip Gallery
• The shortest wavelengths in the visible spectrum are purple, and the longest wavelengths are red.the longest wavelengths are red.
• Visible light has wavelength in a range from about 380 nanometers to about 740 nm, with a frequency range of about 405 TH t 790 TH405 THz to 790 THz.
Electromagnetic Spectrum
• Visible Spectrum – Light p gwe can see
• Roy G. Biv – Acronym for R d O Y llRed, Orange, Yellow, Green, Blue, Indigo, & Violet.
– Largest to Smallest Wavelength.
Visible LightVisible Light• We now know what we see is part of the electromagnetic
spectrum We know that the light waves enter our eye andspectrum. We know that the light waves enter our eye, and stimulate parts of it that cause a electrical impulse to be sent to the brain which creates this visual image.
• But everything does not emit radiation. How do we see those things? And why cant we see a window?
37
Seeing colour• The colour an object appears depends on the colours of
light it reflects.g
For example, a red book only reflects red light:
White Only red light is
light reflected
A pair of purple trousers would reflect purple light (and red and blue, as purple is made up of red and blue):
Purple light
A white hat would reflect all seven colours:
White
li htlight
Using coloured lightUsing coloured light
• If we look at a coloured object in colouredIf we look at a coloured object in coloured light we see something different. For example consider a football kit:example, consider a football kit:
White
Shirt looks red
light
Sh t l k blShorts look blue
• In different colours of light this kit would look different:
RedShirt looks red
lightShirt looks red
Shorts look blackShorts look black
Blue
Shirt looks black
light
Sh t l k blShorts look blue
Seeing thingsSeeing things
• We know that when waves run into a boundary they y yare partially transmitted and partially reflected.
• Light behaves as a wave, so it to is reflected.• Therefore an object does not need to emit photons• Therefore, an object does not need to emit photons itself to be seen, it just has to reflect light back to our eyes where we can detect it.
• Objects that do not allow light to pass through them are called opaque.
• Objects that allow light to pass through them areObjects that allow light to pass through them are considered transparent.
• Objects in between are called translucent.
© 2003 Mike Maloney 42
Invisible spectrum (cont.).Ultraviolet rays.y
Def. – EM waves with frequencies slightly higher than visible lightUses: food processing & hospitals to kill germs’ cellsUses: food processing & hospitals to kill germs cellsHelps your body use vitamin D.
X‐RaysD f EM h h h UVDef. ‐ EM waves that are shorter than UV rays.Uses: Medicine – Bones absorb x‐rays; soft tissue does not. Lead absorbs X‐rays.
Light sources• Thermal: a body at a given temperature emits a characteristic spectrum of
black‐body radiation. As the temperature increases, the peak shifts to shorter wavelengths producing first a red glow then a white one and finally a bluewavelengths, producing first a red glow, then a white one, and finally a blue colour as the peak moves out of the visible part of the spectrum and into the ultraviolet.
• Atoms: emit and absorb light at characteristic energies. This produces "emission lines" in the spectrum of each atom. p
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 g g p gcharacteristic 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 radiationproduce 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 movingFor 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 televisionCathodoluminescence is one example. This mechanism is used in cathode ray tube television sets and computer monitors.
• Certain other mechanisms can produce light:
S i ill i l l i l i• Scintillation; electroluminescence; sonoluminescence; triboluminescence; Cherenkov radiation; BioluminescenceBioluminescence.
• When the concept of light is intended to include very‐high‐energy photons (gamma rays) additional generationhigh energy photons (gamma rays), additional generation mechanisms include: Radioactive decay; Particle–antiparticle annihilation
Kinds of Spectrap
PolarizationPolarization
• Polarization is a phenomenon of light that isPolarization is a phenomenon of light that is used in sun‐glasses and 3‐D movies.
• Play with the two polarizing filters for a few• Play with the two polarizing filters for a few minutes and note what is happening and see if you can think of any reasons for itif you can think of any reasons for it.
© 2003 Mike Maloney 50
Polarization HintPolarization Hint
• Light vibrates in all directionsLight vibrates in all directions.
• A polarizing filter acts like a picket fence. It only lets certain direction vibrations passonly lets certain direction vibrations pass through it.
Th f if li h h h f• Therefore, if you pass light through two of them you can completely block the light from
i h hpassing through.
• HOW?
© 2003 Mike Maloney 51
PolarizationPolarization
© 2003 Mike Maloney 52
http://www.phy.ntnu.edu.tw/ntnujava/index.php?topic=692.0
Light EnergyWhen EM radiation interacts with single atoms and molecules itsWhen EM radiation interacts with single atoms and molecules, its behaviour depends on the amount of energy per quantum it carries.
AtomsAtomsAs atoms absorb energy, electrons jump out to a higher energy level.
Electrons release light when falling down to the lower energy level.Photons bundles/packets of energy released when the electronsPhotons ‐ bundles/packets of energy released when the electrons fall.
Light: Stream of PhotonsLight: Stream of Photons
Quantum Theory – Light consistsQuantum Theory Light consists of small particles (photons)
Newton’s Corpuscular Theory of Light - light consists of small particles, because it:• travels in straight lines at great speeds• travels in straight lines at great speeds• is reflected from mirrors in a predictable way
Position x
Momentum p = mv
Why the Photon is NecessaryWhy the Photon is Necessary
Electron transitions in the Bohr model of the atom and the subsequent emission of qlight provides an example of when light should be viewed as a photon. There are t f th i f id f thitwo further pieces of evidence of this particle-like nature of light:
• photon scattering• photon scattering• photoelectric effect
ScatteringScattering
One experiment which provides conclusive proof of a particle nature of p p p pobjects is to scatter two objects off of each other, as in the collision of two billiard balls. This experiment with light and small atoms has been done, and is called Compton scattering.
The results of this experiment are completely at odds with predictions made if light is viewed only as a wave. Measurements show that the frequency of the scattered wave is changed, which does not come out of a wave picture of light. However when the light is viewed as a photon with energy proportional to theHowever, when the light is viewed as a photon with energy proportional to the associated light wave, excellent agreement with experiment is found.
Photoelectric EffectPhotoelectric EffectAnother compelling proof for the photon nature of light is the photoelectric effect. In this effect, light is shone at a metal plate and itphotoelectric effect. In this effect, light is shone at a metal plate and it is found that electrons are ejected. These electrons then get accelerated to a nearby plate by an external potential difference, and a photoelectric current is established.
This effect, which arises in devices such as automatic door openers, burglar alarms light detectors and photocopiers cannot be explainedburglar alarms, light detectors, and photocopiers, cannot be explained using a wave picture of light.
Einstein’s Photoelectric EffectEinstein s Photoelectric Effect
• Only light with a frequency greater than a certain threshold will produce a current
• Current begins almost instantaneously, even for light of very low intensity
• Current is proportional to the intensity of the incident light
Planck’s Quantum PostulatePlanck s Quantum Postulate
• Energy of radiation can only be emitted in discrete packets or quanta, i.e., in multiples of p q , , pthe minimum energy
E = hfE = hfwhere h is a new fundamental
constant of nature:h = 6.63 x 10-34 Joules sec
We Believe in PhotonsWe Believe in Photons
• Red light is used in photographic darkrooms because it is not energetic enough to break the halogen-silver bond in black and white films
• Ultraviolet light causes sunburn but visible light does not because UV photons are more energetic
• Our eyes detect color because photons of different energies trigger different chemical reactions in retina cells
Special Relativity: PostulatesSpecial Relativity: Postulates• Principle of Relativity
– All the laws of physics are the same in all inertial reference /frames, i.e. frames moving at constant velocities w/respect to
each other.
• Constant Speed of Light
– The speed of light, c = 3×108 m/s, is equal in all inertial frames, dl f h l f h b h l hregardless of the velocity of the observer or the light source.
• Consequences of Special relativityq p y
– “Slowing” down of clocks (time dilation) and length contraction in moving reference frames as measured by an observer in another f freference frame.
Galilean Transform: Predicts Preferred Ref FrameA di t th G lil t fGalilean Transform: Predicts Preferred Ref. Frame• According to the Galilean transform,the travel time t1 across & back a river
is shorter thant1
the travel time t2 up & down a river.
• Light moving in an “ether” is an analogous problem.
t2
analogous problem.– Can light have different travel times?
2 2
1
12 2 112 2 1L L v L vt
−⎛ ⎞
= = − ≈⎜ ⎟⎛ ⎞+ +⎜ ⎟L
2 212 22L L Lc L v L v
−⎛ ⎞⎛ ⎞⎛ ⎞⎛ ⎞
221 221
21
c cc vt
c c≈⎜ ⎟
− ⎝ ⎠+ +⎜ ⎟
⎝ ⎠
Phys 320 ‐ Baski Relativity I
22 22 2
2 2 12 1L L Lc L vc v c v cc v c
L vtc c
⎛ ⎞⎛ ⎞ + +⎜ ⎟⎜ ⎟⎝
⎛ ⎞⎛ ⎞= + = = − ≈⎜ ⎟⎜ ⎟+ − − ⎝ ⎠⎝ ⎝⎠ ⎠ ⎠L
Galilean Transform: NO Preferred Ref. Frame for • In 1800’s, scientists thought that light Light!propagated through some type of
“ether.”• Michelson‐Morley Experiment (1887)Michelson Morley Experiment (1887)
– Test if ether exists and sets “preferred” reference frame.
– Analogous to rowboat in river.
– Measure light speed relative to th’ ti (// d ⊥) iearth’s motion (// and ⊥) using an
interferometer (fringes).• Result: No detection of “ether”
– No detectable shift in interference fringes occurred, indicating that lightspeed DID NOT depend on direction
Phys 320 ‐ Baski Relativity I
speed DID NOT depend on direction.
Doppler Shift• Doppler shift causes change in measured frequency.Doppler Shift– When a light source moves towards an observer, the light frequency is shifted higher (i.e. blue shift).
Wh li ht f b th li ht– When a light source moves away from an observer, the light frequency is shifted lower (i.e. red shift).
– Only difference with “classical” Doppler shift for sound is the incorporation of time dilation (causes square root factor).
Approaching ‐ blue shiftpp oac g b ue s t
( )1 /v cf f
+=
( )1 /obs sourcef f
v c=
−
Phys 320 ‐ Baski Relativity I
Note: For a receding source, switch signs.
Cool Thing About LightCool Thing About Light
It can be thought of as both a particle and a wave, so called “particle-wave duality”
Lower energy (longer wavelength) light acts predominately like a wave
Hi h ( h t l th) li ht t d i t l likHigh energy (shorter wavelength) light acts predominately like a particle
Cool Things Light Can Tell Us
It can tell us what you are made out of
It can tell us if you are moving toward or away from usIt can tell us if you are moving toward or away from us
It can tell us how far away you are or (if we already know that) how energetic you are
It can tell us your temperature
h kAnother Way to Look at a Spectrum
Spectral LinesSpectral Lines
Lines from excited sodium gas in the laboratory
