Quantization of Light It seemed to be a wave...

Black-body Radiation Does not preferentially absorb or emit any particular frequency Light in thermal equilibrium with surroundings Experimentally realized as “Cavity” radiation

Black-Body Spectrum Classically, emission at short wavelengths would be infinite Planck correctly modeled spectrum by assuming E = hf Sharp drop in high-f “photons”

Photoelectric Effect Light absorbed by a metal can eject electrons Energy of photoelectrons did not depend on light intensity P/A (Instead, that determined the current) Light frequency determined photoelectron energy

Photoelectric Effect Explained by Einstein E = hf – f slope = h E f –f Verified by Millikan

Photoelectric Effect Work function f of metal property of the metal potential energy of bound electron light carries and delivers energy in hf packets (photons) Light can act as discrete particles, not just continuous waves

X rays “Braking radiation” from stopped electrons Electromagnetic radiation X – + V hfmax = Ke = Ve

X rays are Light Uncharged Diffract from crystals

Compton Effect X-rays lose energy scattering from electrons hf1 hf2 X-rays lose energy scattering from electrons Interpreted as elastic scattering conserving momentum

Photon Momentum E2 = (mc2)2 + (pc)2 (hf)2 = (pc)2 hf = pc p = hf/c p = h(c/l)/c p = h/l