Chapter 3 Light at Particles
Blackbody Radiation
Light waves Interference Diffraction Maxwell’s Equations Ether?
Blackbody Radiation Hot things radiate more energy (Stefan- Boltzmann Law) –E = T 4 –P = A T 4 = emissivity (0-1, “how good of a blackbody”) = 5.67 x W/m 2 K 4 Hot things have a measureable spectrum The spectrum shifts depending on temperature –Wein’s Law (1893) max = b/T –b = m K
Blackbody Radiation thought experiment Radiation is absorbed through a hole and creates a long-lived standing wave. Eventually, a light wave escapes 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 k B T Ultraviolet Catastrophe kBTkBT kBTkBT kBTkBT kBTkBT kBTkBT kBTkBT kBTkBT
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 = K f = GHz = 1.06 cm
Various Evaluations Wave TypeFrequency (Hz)WavelengthT(K) gamma pm1.76 x X-Ray nm1.76 x 10 7 UV nm17,600 VIsible6 x nm11,000 IR µm1760 microwave10 3cm0.176
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 ( ) Wilhelm Hallwachs Clean charged metal surfaces (1888)
Photoelectric Effect J. J. Thomson –Discovered that the particles ejected were 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% MetalWork Function ( ) Cs1.9 eV K2.2 eV Na2.3 eV Mg3.7 eV Zn4.3 eV Cr4.4 eV W4.5 eV Conservation of energy KE = hf - h = x J s
Photoelectric Effect Einstein’s relationship gave a nice linear way of determining Planck’s constant MetalWork Function ( ) Cs1.9 eV K2.2 eV Na2.3 eV Mg3.7 eV Zn4.3 eV Cr4.4 eV W4.5 eV Conservation of energy KE = hc/ - Cs Mg W
Photoelectric Effect Einstein’s relationship gave a nice linear way of determining Planck’s constant MetalWork Function ( ) Cs1.9 eV K2.2 eV Na2.3 eV Mg3.7 eV Zn4.3 eV Cr4.4 eV W4.5 eV Conservation of energy KE = hf - Cs Mg W
Multichannel Plate – Light Amplification by PE effect - + lens Phosphorescent screen Incoming light PE metal Amplified electrons photoelectron
X-Ray Production Bremsstrahlung: “Braking Radiation”
X-Ray Production
Compton Effect – 1927 Nobel
Blackbody Radiation
Wien Distribution
Blackbody Radiation
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
Solar Spectrum Incorrect Solar Spectrum from only changing x-axis ( =hc/ )
Photoelectric Effect
Compton Effect
Compton Effect w/ long
Limiting Cases
Pair Production Carl D. Anderson ( ) –Nobel Prize for the discovery of positrons 1936 –Discovered the muon in 1936 –Worked for Robert Millikan
Pair Production
Particle vs. Wave
Diffraction & Interference
Double Slit Experiment 10 (a), 200 (b), 6000 (c), (d), (e).
Matter Waves 1924 doctoral thesis –Approved by Einstein 1929 Nobel
Particle vs. Wave
Compton Effect