chapter 3 light at particles. blackbody radiation
TRANSCRIPT
Chapter 3
Light at Particles
Blackbody Radiation
Blackbody Radiation
• Light waves• Interference• Diffraction• Maxwell’s Equations• Ether?
Blackbody Radiation• Hot things radiate more energy (Stefan-Boltzmann
Law)– E = T4
– P = A T4
= emissivity (0-1, “how good of a blackbody”)• = 5.67 x 10-8 W/m2K4
• Hot things have a measureable spectrum• The spectrum shifts depending on temperature
– Wein’s Law (1893)max = b/T
– b = 0.002898 m K
Blackbody Radiationthought experiment
Radiation is absorbedthrough a hole and createsa long-lived standing wave.Eventually, a light waveescapes and can be detected.
A furnace at very high temperature
Blackbody Radiation
• Lord Rayleigh (John William Strutt) derived a classical expression based on standing waves
Lord Rayleigh’s Derivation
• Thermal Physics– Equipartition Theorem
Energy/wave = ½ kBT
• Ultraviolet Catastrophe
kBT
kBT
kBT
kBT
kBT
kBT
kBT...
.
.
.
Wilhelm Wien
• Spectrometers worked well at small wavelength (large frequency)
• Was able to derive a formula that worked in this range
• I f 3 e-af/T
– f : frequency– T : temperature– a : constant
Max Planck
• Quantized energies– E = 0, hf, 2hf, 3hf, … = nhf
•n: energy quantum number
Ludwig von Boltzmann
“… an act of desperation… a theoretical explanation had to be found at any cost, whatever the price…”
Distribution Comparison
Cosmic Background
• T = 2.727 K
• f = 160.2 GHz = 1.06 cm
Various EvaluationsWave Type Frequency (Hz) Wavelength T(K)
gamma 1021 0.3 pm 1.76 x 1010
X-Ray 1018 0.3 nm 1.76 x 107
UV 1015 300 nm 17,600
VIsible 6 x 1014 500 nm 11,000
IR 1014 3 µm 1760
microwave 1010 3cm 0.176
Photoelectric Effect• Wave description of light by Maxwell
– Light intensity should determine whether an electron is ejected
– Electric field vibrates the electron loose if there are enough waves (high intensity) to jiggle it loose.
Photoelectric Effect• James Clerk Maxwell
– EM waves traveling at c (1885)
• Heinrich Hertz– Sparks created from light hitting metal
electrodes (1886-87)
• Wilhelm Hallwachs• Clean charged metal surfaces (1888)
- - - - - - - - - - - - - - - - - - - - - - - -
Photoelectric Effect• J. J. Thomson
– Discovered that the particles ejectedwere electrons (1899)
– Cathode ray tube
• Philipp Lenard– Hertz assistant– Cathode tube (1902)
• Intensity• Wavelength
• Albert Einstein– Theory to describe photoelectric effect (1905)
Photoelectric Effect• Albert Einstein
– Theory to describe photoelectric effect (1905)– Photons are packets of kinetic energy– Nobel 1911
Photoelectric Effect• Robert Millikan
– Surface cleaning in-situ– Disagreed with Einstein’s theory– Experiments to verify Einstein’s eqn– Measured Planck’s constant, h, to within 0.5%
Metal Work Function ()
Cs 1.9 eV
K 2.2 eV
Na 2.3 eV
Mg 3.7 eV
Zn 4.3 eV
Cr 4.4 eV
W 4.5 eV
Conservation of energyKE = hf -
h = 6.626 x 10-34 J s
Photoelectric Effect• Einstein’s relationship gave a nice
linear way of determining Planck’sconstant
Metal Work Function ()
Cs 1.9 eV
K 2.2 eV
Na 2.3 eV
Mg 3.7 eV
Zn 4.3 eV
Cr 4.4 eV
W 4.5 eV
Conservation of energyKE = hc/ -
CsMgW
Photoelectric Effect• Einstein’s relationship gave a nice
linear way of determining Planck’sconstant
Metal Work Function ()
Cs 1.9 eV
K 2.2 eV
Na 2.3 eV
Mg 3.7 eV
Zn 4.3 eV
Cr 4.4 eV
W 4.5 eV
Conservation of energyKE = hf -
CsMgW
Multichannel Plate – Light Amplification by PE effect
-
+
lens
Phosphorescent screen
Incoming light
PE metal
Amplified electrons
photoelectron
X-Ray ProductionBremsstrahlung: “Braking Radiation”
X-Ray Production
Compton Effect – 1927 Nobel
Blackbody Radiation
Blackbody Radiation
Wien Distribution
Blackbody Radiation
Blackbody Radiation
Probability (a brief diversion)
Probability (a brief diversion)
Probability (a brief diversion)
P(2)
P(3)
P(4)
P(5)
P(6)
P(7)
Probability (a brief diversion)
P(12)
P(11)
P(10)
P(9)
P(8)
Probability (a brief diversion)
Planck Distribution
Planck Distribution
Planck Distribution
Solar Spectrum
2 4 6 8 10
51017
11018
1.51018
21018
Incorrect Solar Spectrum from only changing x-axis (=hc/)
Photoelectric Effect
Photoelectric Effect
Photoelectric Effect
Photoelectric Effect
Compton Effect
Compton Effect
Compton Effect
Compton Effect
Compton Effect w/ long
Limiting Cases
Pair Production
• Carl D. Anderson (1905 -1991)– Nobel Prize for the discovery of positrons
• 1936
– Discovered the muon in 1936– Worked for Robert Millikan
Pair Production
)(
)(
BveF
BveF
Pair Production
Pair Production
Pair Production
Pair Production
Particle vs. Wave
Particle vs. Wave
Particle vs. Wave
Diffraction & Interference
Diffraction & Interference
Double Slit Experiment
10 (a), 200 (b), 6000 (c), 40000 (d), 140000 (e).
Matter Waves
• 1924 doctoral thesis– Approved by Einstein
• 1929 Nobel
Particle vs. Wave
Compton Effect
Compton Effect
1 2 3 4 5 6
0.5
1
1.5
2