What is Light?

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What is Light?. Light is a Wave!. Interference In phase = constructive Out of phase (180 degrees) = destructive Thin Film Interference. Light is a Wave!. Thin Film Interference. Light is a Wave!. Young’s Double Slit Experiment (1801) Java Applet - PowerPoint PPT Presentation

Transcript of What is Light?

  • What is Light?

  • Light is a Wave!InterferenceIn phase = constructiveOut of phase (180 degrees) = destructiveThin Film Interference

  • Light is a Wave!Thin Film Interference

  • Light is a Wave!Youngs Double Slit Experiment (1801)Java AppletImportant Formula: d*sin = m ; m = 1,2,3Electromagnetic (EM) Waves Discovered by James Clerk Maxwell (1864)Perpendicular E and B fields

  • Wait a minuteBlackbody RadiationThe Ultraviolet CatastropheClassical Mechanics predicts that a heated body should emit an infinite amount of energy, but experiment tells us otherwiseMax Planck proposed a theory that was able to mathematically reproduce the blackbody radiation graphs : E=nhf ; n=1,2,3Energy of vibration could only be some whole number multiple of hfEnergy is quantized not all energies possible.

  • Light is a Particle!Photoelectric EffectDiscovered by Heinrich Hertz (early 1900s)EM pulses produce spark between two metal knobsSpark increased when UV light present (electrons)Explained by Albert EinsteinExplained in 1905Nobel Prize 1921

  • Light is a Particle!Photoelectric Effect The ExperimentSpeed vs. LightProposed: Brighter light = more energy per electronFound: Brighter light cased more electrons, but at the same energyColorProposed: color has no/limited effectOnly the frequency (color) of light affects electron energiesHigher f = Higher EEven extremely dim light with a high frequency immediately ejected electrons w/ a high energy

  • Light is a Particle!Photoelectric Effect Explanation (A.E. 1905) Light is quantized it travels in packets and is absorbed in this form (think of 1 cent)Electrons in metal can only absorb light energy by absorbing one of these quanta (photon)E=hf h = Plancks ConstantIf light shines on a metal, a photon can give up its energy to the electronIf the energy of the photon is enough, the electron can be ejected from the metalImportant Formula: hf = KE + Wo

  • hf = KE + WoPhoton Energy = hfKE = max. ejected electron energyWo = work function - minimum work needed to eject e- (material specific)Fo (threshold frequency)

  • Energy of a PhotonPlancks Constant6.63 x 10-34 J*s4.136 x 10-15 eV/HzRed Light (f = 4.3 x 1014 Hz)E= (4.136 x 10-15) * (4.3 x 1014) = 1.78 eVBlue Light (f = 6.3 x 1014 Hz)E= (4.136 x 10-15) * (6.3 x 1014) = 2.61 eVUV Light (f = 5 x 1018 Hz)E= (4.136 x 10-15) * (5 x 1018) = 20700 eV (Ouch!!!)

  • Light is a Particle!(Part II)Compton ScatteringArthur H. Compton (1922)X-rays fired at electron targetScattered X-rays have lower frequency (lower E)Greater scatter angle greater the change in frequencyX-rays lose energy as they pass through matterX-rays (and therefore light) behave like particlesBy equating Einsteins two famous E equations:Photon mass = (hf/c2)Photon momentum = (hf/c)

  • So what is Light?WaveRefractionInterferenceParticlePhotoelectric effectCompton scatteringHow can it be both?Different sets of clues for recognition of the same person

  • Into The Atom!!Up And Atom!!

  • Into The Atom!!Background (or, what we knew early 1900s)Electrons discovered 1890sJ.J. Thomson uses cathode ray tube (TV tube) to determine q/m ratio of electron (1897)Robert Millikans oil drop experiment determines precise value of electron charge (1909)Atoms widely accepted as building block of matterActual makeup of the atom was still a matter of speculationProblemsMax Plancks quantized energy idea was in direct opposition to the predictions of classical mechanics, although the fact that it worked was irrefutableWave-Particle Duality still a troubling idea

  • Models of the AtomJ.J. Thomson Plum Pudding (1900)Atom is neutralNegatively charged electronsPositively charged pudding

    Ernest Rutherford Planetary (1909)Gold Foil scattering experiment (Geiger and Marsden)Fired newly discovered particles (2n, 2p+) at thin gold foilExpected (based on plum pudding model):particles to spread out due to weak electrical forceMost pass through un-deflectedFound:Most particles have small deflectionSome have large deflection, even reverse direction

  • Planetary Model particle: 8000x mass of e-Velocity = 2 x 107 m/s

    The model itselfStrong forces must be causing large deflectionsTiny nucleus with all of the positive charge and massElectrons outsideAtom mostly empty spacee.g. Gold AtomNucleus = 1 ft radius then outermost e- is 3.3 mi away

  • Atomic Spectra(Atomic Fingerprinting)Absorption SpectrumJosef Fraunhofer (1814)Optician testing high quality prismsFound dark lines in the spectrum produced by sunlight

    White (continuous spectrum) light directed through gas cloudLight is analyzed by spectroscope (prism)Most wavelengths pass through but a few are absorbed by gas (dark lines)Pattern of dark lines is discreteEmission Spectrum - Bunsen and Kirchoff (1859)Gas is stimulated (by heating or high voltage) to emit lightSpectroscopic pattern is opposite of absorption spectrumMatched spectra of earths elements to Fraunhofer lines (He)What is the makeup of distant objects?

  • Bohr Atomic Model (1912)Combined planetary model and quantum mechanicsElectrons orbit nucleus, but only certain orbits allowedAngular momentum quantized: L = (mr)v = n(h/2) n=1,2,3Fit mathematical prediction of H spectra by Balmer (1885)Electrons in atoms cannot lose/gain energy continuously (Planck/Einstein) but do so in jumpsLight emitted when e- jumps from higher to lower stateEu El = hfProblem: Orbiting electrons do not emit EM waves (energy)Normally charged particles do when accelerated (changing E field)Atoms shouldnt exist!Hydrogen AtomEn = -(13.6 eV)(Z2/n2)Z = # protons (Z = 1 for Hydrogen)n = principal quantum number of orbit

  • Energy LevelsHydrogen AtomE1 = -(13.6)(12/12) = -13.6 eVE2 = -(13.6)(12/22) = -3.40 eVE3 = -(13.6)(12/32) = -1.51 eVE = -(13.6)(12/) = 0 eVExcited states (E2, E3, etc) have less negative energyOrbit closest to the nucleus (E1) has the lowest total energyEnergy must be added to raise the electrons total energy

    Ionization (Binding) EnergyMinimum energy required to remove and electron from the ground stateI.E. = 13.6 eV for Hydrogen

  • Jumping Energy LevelsE2->E1E2-E1 = -3.4 (-13.6) = 10.2 eVUltraviolet (Lyman Series)

    E3->E2E3-E2 = -1.51 (-3.4) = 1.89 eVVisible (Balmer Series)

    E4->E3E4-E3 = -0.87 (-1.51) = 0.64 eVInfrared (Paschen Series)

  • Atomic Spectra Explained by BohrAbsorptionIn heated/charged atoms, electrons will jump up energy levelsAbsorb only those particular frequencies of light which give jumps to allowed energy statesEmissionAs atoms cool, electrons jump to a lower energy stateGive off photons in process only in allowed frequencies Bohr Model The Good Accurately predicts spectra photon wavelengths for HydrogenAccurately predicts ionization energy of Hydrogen and the BadAssumes (but does not explain) stability of atomsWhy quantization?Why is the ground state is the lowest state?Does not accurately predict line spectra for more complex atoms* * - (The solution to this problem we will not concern ourselves with)

  • Bring on Quantum Mechanics!Based on probabilities, not certaintiesLouis DeBroglie (1924)If waves behave like particles, can particles behave like waves?Why should there be two sets of rules one for small objects and one for large objects?Proposed DeBroglie Wavelength: = h/(mv)e.g. rock (50 g / 40 m/s) = h/(mv) = 6.63 x 10-34 /(.05*40) = 3.3 x 10-34 m (no wave behavior can be seen)e.g. electron (9.11 x 10-31 kg / 107 m/s) = h/(mv) = 6.63 x 10-34 /(9.11 x 10-31 * 107) = 7.28 x 10-11 m (X-rays in EM spectrum)Verified in 1927 by Davisson and GermerExperimentally determined electron wavelength

  • Quantum Mechanics to the Rescue!(well, kind of)deBroglie and the wave nature of particles (=h/mv)Each electron in orbit is a circular standing waveAny wavelength is possible, but only resonant (constructive interference) modes are sustainedOther wavelengths destructively interfere with themselves and die outSmallest radius is when circumference = 1 Explains ground state as lowest state.If 2r = n n = 1,2,3 then mvr = nh/2Angular Momentum is quantized.

  • Quantum Mechanics Gets WeirdWeird: Youngs Double Slit Experiment Mark II now with electrons which are particles? or waves?...both? (video)

    Really Weird: Quantum Eraser

    Try Not to Think Too Hard Or Youll Spontaneously Combust Weird: Delayed Choice Quantum Eraser

    Im Outta Here Weird: Entanglement (video)

  • Quantum MechanicsReality is affected by the observerMultiple states seem to coexist until measurement or observation takes placeSchrdingers Cat

  • Quantum MechanicsUncertainty Principle Werner Heisenberg (1927 N.P. 1932)Try to measure the position and velocity of a particle at any instantClassical MechanicsThe world is a clockThere are no fundamental barriers to an ultimate refinement of the apparatus to make such measurementsI.e. We possess the ability of achieving infinite accuracyQuantum MechanicsThe world is a roulette wheelIf is fundamentally impossible to make simultaneous measurements of position and velocity with infinite accuracyI.e. The measurement procedure itself limits the accuracy to which we can determine the position and velocity simultaneouslyUncertainty Principle: x * p h/(2)

  • Philosophical Implications of Quantum MechanicsWerner Heisenberg: Knowledge is fundamentally limited. The future is indeterminable.I believe that the existence of the classical path can be pregnantly formulated as follows: The path comes into existence only when we observe it. In the sharp formulation of the law of causality-- "if we know the present exactly, we can calculate the future"- it is not the conclusion that is wrong but the premise.

    Albert Einstein: Rejected Heisenber