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