2BrieflyIn physics, the term light sometimes refers to electromagnetic radiation of any wavelength, whether visible or not.Primary properties of light are intensity, propagation direction, frequency or wavelength spectrum, and polarization.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 physics.
3Light, which is emitted and absorbed in tiny "packets" called photons, exhibits properties of both waves and particles. This property is referred to as the wave–particle duality.
4Ray Theory – Light travels along a straight line and obeys laws of geometrical optics. Ray theory is valid when the objects are much larger than the wavelength (multimode fibers)
5Archimedes of Syracuse (Greek: Ἀρχιμήδης) Dossier of HistoryArchimedes of Syracuse (Greek: Ἀρχιμήδης)Bornc. 287 BC Syracuse, Sicily; Magna GraeciaDiedc. 212 BC (aged around 75) SyracuseResidenceSyracuse, SicilyFieldsMathematics, Physics, Engineering, Astronomy, InventionKnown forArchimedes' Principle, Archimedes' screw, Hydrostatics, Levers, Infinitesimals
6Ptolemy (Batlamyus)Bornc. AD 90; EgyptDiedc. AD 168 (aged 77–78) Alexandria, EgyptOccupationmathematician, geographer, astronomer, astrologer
7Law of reflection the angle of incidence equals angle of reflection angles are measured from normal
8Refraction Snell’s Law index of refraction, n, where c = speed of light in a vacuum and v = speed of light in that mediumnair = 1nglass = 1.5Snell’s Law
9Total Internal Reflection Incident angle where refracted angle (2) is 90 is the critical angleat incident angles greater than critical angle, light is totally internally reflectedGeometric Optics
10Guided Propagation Along the Optical Fiber The structure of a typical single-mode fiber. 1. Core: 8 µm diameter 2. Cladding: 125 µm dia. 3. Buffer: 250 µm dia. 4. Jacket: 400 µm dia.
11Plato’s fireFull namePlato (Πλάτων)Bornc. 428–427 BC; AthensDiedc. 348–347 BC (age approx 80); AthensEraAncient philosophyRegionWestern PhilosophySchoolPlatonismInfluenced by: Socrates, Homer, Hesiod, Aesop, Protagoras, Parmenides, Pythagoras, Orphism, …Influenced: Most of subsequent western philosophy, including Aristotle, Augustine, Neoplatonism, Cicero, Plutarch, Stoicism, Anselm, Machiavelli, Descartes, Hobbes, Leibniz, Mill, Schopenhauer, Nietzsche, Heidegger, Arendt, Gadamer, Imam Khomeini, Russell and countless other philosophers and theologians.So 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 is within us and related to it they made to flow through the eyes in a stream smooth 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 only its pure element.
12Alhazen (Ibn al-Haytham) End of visual raysAlhazen (Ibn al-Haytham)BornJuly 1, 965 CE (354 AH) Basra in present-day Iraq, Buyid PersiaDiedMarch 6, 1040 (aged 74) (430 AH) Cairo, Egypt, Fatimid CaliphateFieldsphysicist and MathematicianKnown forBook of Optics, Doubts Concerning Ptolemy, On the Configuration of the World, The Model of the Motions, Treatise on Light, Treatise on Place, scientific method, experimental science, experimental physics, experimental psychology, visual perception, analytic geometry, non-Ptolemaic astronomy, celestial mechanicsInfluencesAristotle, Euclid, PtolemyInfluencedAverroes, Witelo, Roger Bacon, KeplerOver-confident about practical application of his mathematical knowledge, he assumed that he could regulate the floods of the Nile.“There is no vision unless something comes from the visible object to the eye, whether or not anything goes out”camera obscura
15By 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, he noticed that the time intervals between eclipses were not always the same.(position 1): The light from the eclipsing moon sometimes took 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 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.Ole RømerBorn25 Sept. 1644; ÅrhusDied19 Sept (aged 65) CopenhagenNationalityDanish
16Light Speed measurement Hippolyte FizeauBornSept. 23, 1819 ParisDiedSept. 18, 1896 VenteuilNationalityFrenchFieldsPhysicsKnown forDoppler Effect Fizeau-Foucault apparatus Capacitorusing a pair of toothed wheels driven by a very fast motor.As the toothed wheels turned, a beam of light 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 turning allowed him to calculate the speed of light with an accuracy similar to Roemer's.
17Albert Abraham Michelson Michelson's method was to shine a light on the rotating hexagonal mirror, which then reflected to the other mirror 35 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 rpm. From the round trip distance the light travelled, and the period of rotation of the octagonal mirror, the speed of light was determined quite accurately.Albert Abraham MichelsonBornDecember 19, 1852; Strzelno, Kingdom of PrussiaDiedMay 9, 1931; Pasadena, CaliforniaUnited States; PhysicsAlma mater: United States Naval Academy, University of Berlin; Doctoral advisor: Hermann Helmholtz; Doctoral students: Robert MillikanKnown ForSpeed of light Michelson-Morley experimentNotable awardsNobel Prize for Physics (1907) Copley Medal (1907) Henry Draper Medal (1916)
18The Search for Aether 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
19Measuring Earth’s Movement through the Aether Rotating the apparatus would give the direction of the aether driftShowed 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.
20Wave Theory – Light travels as a transverse electromagnetic wave
21Magnet and Magnetismcertain iron oxides were discovered in various parts of the world, notably in Magnesia in Asia Minor, that had the property of attracting small pieces of iron.Magnesia within Greece
22discovered that electric currents create magnetic fields. Hans Christian ØrstedBorn14 August 1777 Rudkøbing, DenmarkDied9 March 1851 (aged 73)Copenhagen, DenmarkNationalityDanishFieldsphysics, chemistryKnown forelectromagnetismdiscovered that electric currents create magnetic fields.
23relates the integrated magnetic field around a closed loop to the electric current passing through the loop.André-Marie AmpèreBorn20 January 1775 Parish of St. Nizier, Lyon, FranceDied10 June 1836 (aged 61) Marseille, FranceResidenceFranceNationalityFrenchFieldsPhysicsInstitutionsBourg-en-Bresse École PolytechniqueKnown forAmpere's Law
24The induced electromotive force (EMF) in any closed circuit is equal to the time rate of change of the magnetic flux through the circuit.Michael FaradayBorn22 September 1791Newington Butts, EnglandDied25 August 1867 (aged 75) Hampton Court, Middlesex, EnglandResidenceEnglandNationalityBritishFieldsPhysics and chemistryInstitutionsRoyal InstitutionKnown forFaraday's law of induction; Electrochemistry; Faraday effect; Faraday cage; Faraday constant Faraday cup; Faraday's laws of electrolysis;Faraday paradox; Faraday rotator; Faraday-efficiency effect; Faraday wave;Faraday wheel;Lines of forceInfluencesHumphry Davy William Thomas BrandeNotable awardsRoyal Medal (1835 & 1846) Copley Medal (1832 & 1838) Rumford Medal (1846)
25Heinrich Rudolf HertzBornFebruary 22, 1857 Hamburg, GermanyDiedJanuary 1, 1894 (aged 36) Bonn, GermanyFieldsPhysics; Electronic EngineeringInstitutionsUniversity of Kiel University of Karlsruhe University of BonnDoctoral advisorHermann von HelmholtzKnown forElectromagnetic radiation Photoelectric effect
32The shortest wavelengths in the visible spectrum are purple, and 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 THz to 790 THz.
36Electromagnetic Spectrum Visible Spectrum – Light we can seeRoy G. Biv – Acronym for Red, Orange, Yellow, Green, Blue, Indigo, & Violet.Largest to Smallest Wavelength.
37Visible LightWe now know what we see is part of the electromagnetic spectrum. 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?
38Only red light is reflected Seeing colourThe colour an object appears depends on the colours of light it reflects.For example, a red book only reflects red light:HomeworkWhitelightOnly red light is reflected
39A white hat would reflect all seven colours: A pair of purple trousers would reflect purple light (and red and blue, as purple is made up of red and blue):Purple lightA white hat would reflect all seven colours:Whitelight
40Using coloured lightIf we look at a coloured object in coloured light we see something different. For example, consider a football kit:Shirt looks redWhitelightShorts look blue
41In different colours of light this kit would look different: RedlightShirt looks redShorts look blackShirt looks blackBluelightShorts look blue
43Invisible spectrum (cont.). Ultraviolet rays.Def. – EM waves with frequencies slightly higher than visible lightUses: food processing & hospitals to kill germs’ cellsHelps your body use vitamin D.X-RaysDef. - EM waves that are shorter than UV rays.Uses: Medicine – Bones absorb x-rays; soft tissue does not.Lead absorbs X-rays.
44Light sourcesThermal: 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 blue colour as the peak moves out of the visible part of the spectrum and into the ultraviolet.
45Atoms: emit and absorb light at characteristic energies Atoms: emit and absorb light at characteristic energies. This produces "emission lines" in the spectrum of each atom.Emission can be spontaneous, as in light-emitting diodes, gas discharge lamps (such as neon lamps and neon signs, mercury-vapor lamps, etc.), and flames (light from the hot gas itself—so, for example, sodium in a gas flame emits characteristic yellow light).Emission can also be stimulated, as in a laser or a microwave maser.
46Deceleration 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.
47Certain chemicals produce visible radiation by chemoluminescence. In living things, this process is called bioluminescence.For example, fireflies produce light by this means, and boats moving through water can disturb plankton which produce a glowing wake.Certain substances produce light when they are illuminated by more energetic radiation, a process known as fluorescence.Some substances emit light slowly after excitation by more energetic radiation. This is known as phosphorescence.Phosphorescent materials can also be excited by bombarding them with subatomic particles. Cathodoluminescence is one example. This mechanism is used in cathode ray tube television sets and computer monitors.
48Certain other mechanisms can produce light: Scintillation; electroluminescence; sonoluminescence; triboluminescence; Cherenkov radiation; Bioluminescence.When the concept of light is intended to include very-high-energy photons (gamma rays), additional generation mechanisms include: Radioactive decay; Particle–antiparticle annihilation
53Light EnergyWhen EM radiation interacts with single atoms and molecules, its behaviour depends on the amount of energy per quantum it carries.AtomsAs 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 electrons fall.Light: Stream of Photons
54Quantum 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• is reflected from mirrors in a predictable wayPosition xMomentum p = mv
55Why the Photon is Necessary Electron transitions in the Bohr model of the atom and the subsequent emission of light provides an example of when light should be viewed as a photon. There are two further pieces of evidence of this particle-like nature of light:• photon scattering• photoelectric effect
56ScatteringOne experiment which provides conclusive proof of a particle nature of objects 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 the associated light wave, excellent agreement with experiment is found.
57Photoelectric EffectAnother compelling proof for the photon nature of light is the photoelectric 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 explained using a wave picture of light.
58Einstein’s Photoelectric Effect Only light with a frequency greater than a certain threshold will produce a currentCurrent begins almost instantaneously, even for light of very low intensityCurrent is proportional to the intensity of the incident light
59Planck’s Quantum Postulate Energy of radiation can only be emitted in discrete packets or quanta, i.e., in multiples of the minimum energy E = hfwhere h is a new fundamentalconstant of nature: h = 6.63 x Joules sec
60We Believe in PhotonsRed light is used in photographic darkrooms because it is not energetic enough to break the halogen-silver bond in black and white filmsUltraviolet light causes sunburn but visible light does not because UV photons are more energeticOur eyes detect color because photons of different energies trigger different chemical reactions in retina cells
61Special Relativity: Postulates Principle of RelativityAll 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 LightThe speed of light, c = 3×108 m/s, is equal in all inertial frames, regardless of the velocity of the observer or the light source.Consequences of Special relativity“Slowing” down of clocks (time dilation) and length contraction in moving reference frames as measured by an observer in another reference frame.
62Galilean Transform: Predicts Preferred Ref. Frame According to the Galilean transform, the travel time t1 across & back a river is shorter than the travel time t2 up & down a river.Light moving in an “ether” is an analogous problem.Can light have different travel times?t1t2Phys BaskiRelativity I
63Galilean Transform: NO Preferred Ref. Frame for Light! In 1800’s, scientists thought that light propagated through some type of “ether.”Michelson-Morley Experiment (1887)Test if ether exists and sets “preferred” reference frame.Analogous to rowboat in river.Measure light speed relative to earth’s motion (// and ) using an interferometer (fringes).Result: No detection of “ether”No detectable shift in interference fringes occurred, indicating that light speed DID NOT depend on direction.Phys BaskiRelativity I
64Doppler Shift Doppler shift causes change in measured frequency. When a light source moves towards an observer, the light frequency is shifted higher (i.e. blue shift).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 shiftNote: For a receding source, switch signs.Phys BaskiRelativity I
65Cool Thing About LightIt 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 waveHigh energy (shorter wavelength) light acts predominately like a particle
66Cool Things Light Can Tell Us It can tell us what you are made out ofIt can tell us if you are moving toward or away from usIt can tell us how far away you are or (if we already know that) how energetic you areIt can tell us your temperature