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Physics 222Physics 222
D.S. DurfeeD.S. Durfee
The Physics Revolution of the The Physics Revolution of the 2020thth Century Century
Relativity – physics of the fastRelativity – physics of the fast Quantum Mechanics – law of the smallQuantum Mechanics – law of the small
– Atoms: clocks, discharge lamps, lasersAtoms: clocks, discharge lamps, lasers– Molecules: chemistryMolecules: chemistry– Nuclei: fission, fusionNuclei: fission, fusion– Solid State: semiconductorsSolid State: semiconductors– Degenerate Matter: superconductorsDegenerate Matter: superconductors– Coherence & Superposition: quantum Coherence & Superposition: quantum
computingcomputing
Puzzles at the Beginning of the Puzzles at the Beginning of the Twentieth CenturyTwentieth Century
Null result of the Michelson-Morley Null result of the Michelson-Morley ExperimentExperiment
Ultraviolet CatastropheUltraviolet Catastrophe Photoelectric EffectPhotoelectric Effect Maxwell’s Equations Spell the Demise of Maxwell’s Equations Spell the Demise of
Atoms!Atoms! Discrete atomic emission linesDiscrete atomic emission lines
Blackbody RadiationBlackbody Radiation
Maybe better called “Thermal Maybe better called “Thermal Radiation”Radiation”
Sun, hot burner plate, incandescent Sun, hot burner plate, incandescent lightslights
4ATeP
. and betweenfrequency a
has whichradiatedlight ofamount thei.e.
frequencyunit per radiated Energy )(
dv
du
Cosmic Microwave Cosmic Microwave BackgroundBackground
from http://aether.lbl.gov/www/projects/cobe/CMB_intensity.gif
COBE Map of Microwave COBE Map of Microwave BackgroundBackground
from http://aether.lbl.gov/www/projects/cobe/COBE_Home/cmb_fluctuations_big.gif
Blackbody RadiationBlackbody Radiation
Max Planck, Planck’s Max Planck, Planck’s constantconstant
Max Karl Ernst Ludwig Planck
Found an empirical Found an empirical formula which fit formula which fit the measured the measured blackbody curveblackbody curve
Looked for way to Looked for way to derive this empirical derive this empirical formulaformula
Photo from http://www-groups.dcs.st-and.ac.uk/~history/PictDisplay/Planck.html
Planck’s LawPlanck’s Law
1
8)(
/
5
kTXe
Xu
Max Planck, Planck’s Max Planck, Planck’s constantconstant
fhnE
Max Karl Ernst Ludwig Planck
Photo from http://www-groups.dcs.st-and.ac.uk/~history/PictDisplay/Planck.html
Are you pondering what I’m Are you pondering what I’m pondering?pondering?
Why do we only have problems with Why do we only have problems with the short wavelength (high frequency) the short wavelength (high frequency) part of the blackbody spectrum?part of the blackbody spectrum?
Is the energy in a light mode Is the energy in a light mode quantized because light can only exist quantized because light can only exist that way, or because the atoms that that way, or because the atoms that emit it can only emit quantized emit it can only emit quantized amounts of light?amounts of light?
Hints of Quantum Hints of Quantum MechanicsMechanics
Quantization of MatterQuantization of Matter– Very early idea, not really proven until ~1900Very early idea, not really proven until ~1900– Nature of mater not understood until much Nature of mater not understood until much
later (Rutherford, etc.)later (Rutherford, etc.) Quantization of Electric ChargeQuantization of Electric Charge
– Faraday’s law of electrolysis (1833)Faraday’s law of electrolysis (1833)– Zeeman, q/mZeeman, q/m– J.J. Thompson discovers the electronJ.J. Thompson discovers the electron– Millikan’s oil dropMillikan’s oil drop
Strange or Common Sense?Strange or Common Sense?
Does it bother you that mass is Does it bother you that mass is quantized?quantized?
Does it bother you that charge is Does it bother you that charge is quantized?quantized?
What are some differences in the What are some differences in the way that mass and charge are way that mass and charge are quantized?quantized?
Pay Attention...Pay Attention...
You need to know this to do the homework!
Statistics...Statistics...
represent?really )( doesWhat du
It represents the electro-magnetic energy in a box per unit volume per unit frequency...
How much electro-magnetic energy is there inside my oven?
Quick Writing AssignmentQuick Writing Assignment
In one minute, write a short, clear, In one minute, write a short, clear, and concise paragraph which and concise paragraph which explains how Max Planck came to the explains how Max Planck came to the conclusion that light was quantized.conclusion that light was quantized.
What did Einstein Win his What did Einstein Win his Nobel Prize for?Nobel Prize for?
photo from http://www.th.physik.uni-frankfurt.de/~jr/gif/phys/einst_7.jpg
Puzzles at the Beginning of the Puzzles at the Beginning of the Twentieth CenturyTwentieth Century
Null result of the Michelson-Morley Null result of the Michelson-Morley ExperimentExperiment
Ultraviolet CatastropheUltraviolet Catastrophe Photoelectric EffectPhotoelectric Effect Maxwell’s Equations Spell the Demise of Maxwell’s Equations Spell the Demise of
Atoms!Atoms! Discrete atomic emission linesDiscrete atomic emission lines
The Photoelectric EffectThe Photoelectric Effect
The Photoelectric EffectThe Photoelectric Effect
Photoelectric Effect IntuitionPhotoelectric Effect Intuition
What would a plot of current vs. light What would a plot of current vs. light intensity look like?intensity look like?
What would a plot of current vs. What would a plot of current vs. voltage look like?voltage look like?
What would a plot of threshold What would a plot of threshold voltage vs. light frequency look like?voltage vs. light frequency look like?
Wave Theory of Light and the Wave Theory of Light and the Photoelectric EffectPhotoelectric Effect
Existence of a threshold voltageExistence of a threshold voltage
Current increases with intensity Current increases with intensity
Threshold is a function of light Threshold is a function of light frequencyfrequency
No No intensity intensity thresholdthreshold
No time delayNo time delay
Fairly monochromatic electron Fairly monochromatic electron energiesenergies
Einstein’s Explanation... Einstein’s Explanation...
Planck was right!
photo from http://www.th.physik.uni-frankfurt.de/~jr/gif/phys/einst_7.jpg
What is the velocity of the What is the velocity of the electrons coming off the electrons coming off the
metal?metal?
2
2
1mv h
Quick Writing AssignmentQuick Writing Assignment
In one minute, write a short, clear, In one minute, write a short, clear, and concise paragraph which and concise paragraph which explains why the existence of a explains why the existence of a “threshold” voltage in the “threshold” voltage in the photoelectric effect suggests that photoelectric effect suggests that light is quantized.light is quantized.
Hmmm…Hmmm…
Does Planck’s work on blackbody Does Planck’s work on blackbody radiation prove that light is radiation prove that light is quantized?quantized?
Does the photoelectric effect prove Does the photoelectric effect prove that light is quantized?that light is quantized?
But don’t we already have proof that But don’t we already have proof that light is a wave?light is a wave?
Double Slit InterferenceDouble Slit Interference
Single Photon InterferenceSingle Photon Interference
X RaysX Rays
Discovered in 1895 by RoentgenDiscovered in 1895 by Roentgen Generated when electrons hit targetGenerated when electrons hit target Materials transparent to themMaterials transparent to them Not deflected by magnetic fieldNot deflected by magnetic field Roentgen saw no interference and little Roentgen saw no interference and little
diffraction!diffraction! Bragg diffraction – explained in 1912Bragg diffraction – explained in 1912 X rays are EM waves with wavelengths X rays are EM waves with wavelengths
from 0.01 to 0.10 nmfrom 0.01 to 0.10 nm
X-Ray TubeX-Ray Tube
-+
CathodeAnode
-FilamentTungsten
Target
Bragg DiffractionBragg Diffraction
d
θin=θout
θ
Bragg DiffractionBragg Diffraction
d
θ2d sin(θ) = m λ
Bragg DiffractionBragg Diffraction
What do X-Rays Have to Do What do X-Rays Have to Do with Quantum Mechanics?with Quantum Mechanics?
Compton EffectCompton Effect
Compton noticed that x-rays were Compton noticed that x-rays were “softer” after they had passed “softer” after they had passed through a substance.through a substance.
Compton showed that this could be Compton showed that this could be explained by treating scattering as explained by treating scattering as “collision” between a photon of light “collision” between a photon of light and a particle of matterand a particle of matter
Compton Scattering of a Compton Scattering of a “Photon”“Photon”
e-
Before
e-
After
Are you pondering whatI’m pondering?
Yes, but how will we ever get that many rubber bands?
No you buffoon! Why do youneed x-rays to see this?
Quick Writing AssignmentQuick Writing Assignment
In one minute, write a short, clear, In one minute, write a short, clear, and concise paragraph which and concise paragraph which explains why the Compton effect explains why the Compton effect suggests that light is quantized.suggests that light is quantized.
Puzzles at the Beginning of the Puzzles at the Beginning of the Twentieth CenturyTwentieth Century
Null result of the Michelson-Morley Null result of the Michelson-Morley ExperimentExperiment
Ultraviolet CatastropheUltraviolet Catastrophe Photoelectric EffectPhotoelectric Effect Maxwell’s Equations Spell the Demise of Maxwell’s Equations Spell the Demise of
Atoms!Atoms! Discrete atomic emission linesDiscrete atomic emission lines
Radiating AtomsRadiating Atoms