MODERN PHYSICS REVOLUTION  Classical Newtonian Mechanics, Maxwell’s Electrodynamics, Statistical Mechanics proved hugely successful  BUT…at the beginning of the 20th century some experimental results did not agree with classical models

GOAL #1: Review important experimental results including:  Blackbody radiation  Atomic Spectra  Photo-electric effect  X-ray emissions  X-ray diffraction  Davisson-Germer 2-Slit Experiment  Rutherford Gold-foil experiment  Michelson-Morley  Compton effect  Pair production/Pair annihilation

 Heisenberg Uncertainty Principle  Schroedinger Wave Equation  Special relativity GOAL #2: Describe new concepts which led to a greater understanding of these results  Quantization of light energy  Wave-particle Duality  Quantized energy levels of Hydrogen

Blackbody Radiation (cont.)  The peak of a blackbody's spectrum is determined by its temperature  The hotter the object the “bluer” the light.  The Sun (6000 K) emits most of its radiation at visible wavelengths ( nm).  Room temperature objects (300 K) emit most of their radiation at infrared wavelengths.

Stefan-Boltzman Law  The total energy emitted by each square meter of a blackbody’s surface depends only on the temperature.  If you double the temperature of a blackbody the amount of energy radiated goes up 16 times!

Wien’s Displacement Law  The wavelength corresponding to the maximum intensity of radiation is inversely proportional to the temperature

Blackbody Spectrum CLASSICAL MODEL

UV Catastrophe  Classically, energy should be shared equally between all of the possible energy modes (equipartition theorem)  Allowed modes are EM standing waves.  Classical prediction provided good agreement with experimental data at longer wavelengths  But, more allowed modes at short wavelengths led to prediction that the intensity of UV light would “blow up”

Planck’s Hypothesis (1900)  Planck sought a mathematical “band-aid” to patch up the classical model so it would agree with experiment  Proposed that the radiation energy associated with each frequency had to come in “chunks”

 Classically, energy levels of radiation are continuous.  Planck proposed that energy is “quantized”  At large wavelengths (small frequencies) quanta are so tiny we don’t notice them.  At small wavelengths (high frequencies) the quanta are large…“chunkiness” of the energy is more noticeable.  But…in 1900 no one thought this made any sense. It was just a trick to make theory agree with experiment!

Slide 15