1. Waves, Light & Quanta Tim Freegarde Web Gallery of Art; National Gallery, London

2. 2 Light and optics RAYS straight propagation paths least time (Fermat’s principle) reflection, refraction, lenses, telescopes, microscopes WAVES Huygens’ description of propagation, reflection, refraction polarization, colour (wavelength, frequency) diffraction, interference, beats, interferometers directrix focus PHOTONS Planck, Compton, Einstein Maxwell’s electromagnetism, Einstein’s relativity

3. 3 Radiation pressure © Malcolm Ellis Comet Hale-Bopp, 1997 intensity (energy per unit time per unit area) pressure (momentum per unit time per unit area) light carries both energy and momentum (Maxwell’s electromagnetism) sun sunshine torque (angular momentum per unit time per unit area)

4. 4 Radiation pressure © Malcolm Ellis Comet Hale-Bopp, 1997 intensity (energy per unit time per unit area) pressure (momentum per unit time per unit area) light carries both energy and momentum (Maxwell’s electromagnetism) torque (angular momentum per unit time per unit area) www.a3bs.com R A Beth, Phys Rev 50 115 (1936) Crookes radiometer sun sunshine

5. 5 Blackbody radiation RAYLEIGH-JEANS DISTRIBUTION consider modes of given volume of space assume equipartition: average energy kT per mode mode density hence wavelength intensity Rayleigh- Jeans

6. 6 Blackbody radiation intensity Rayleigh- Jeans observed spectrum BLACK BODY perfect absorber, hence ‘ideal’ emitter no spectral features beyond Planck curve wavelength ULTRAVIOLET CATASTROPHE classical thermodynamics predicts monotonic increase with frequency quantization of radiation field supplies required correction

7. 7 Blackbody radiation intensity Rayleigh- Jeans observed spectrum BLACK BODY perfect absorber, hence ‘ideal’ emitter no spectral features beyond Planck curve wavelength ULTRAVIOLET CATASTROPHE classical thermodynamics predicts monotonic increase with frequency quantization of radiation field supplies required correction PLANCK’S DERIVATION hence modify equipartition: energy quantized in units of

8. 8 Photoelectric effect WORK FUNCTION threshold for photocurrent no current above threshold wavelength regardless of intensity A optical frequency photocurrent increasing intensity applied voltage BIAS VOLTAGE applied voltage changes threshold threshold voltage proportional to optical frequency Planck’s constant optical frequency work function electron charge voltage

9. 9 Light and optics RAYS straight propagation paths least time (Fermat’s principle) reflection, refraction, lenses, telescopes, microscopes WAVES Huygens’ description of propagation, reflection, refraction polarization, colour (wavelength, frequency) diffraction, interference, beats, interferometers directrix focus PHOTONS Maxwell’s electromagnetism, Einstein’s relativity energy quantized in units of ( h = Planck’s constant) momentum quantized in units of angular momentum quantized in units of

10. 10 Compton scattering A H Compton, Phys Rev 22 409 (1923) GRAPHITE TARGET 0.711 Å X-RAYS wavelength shift angle 04590135 photon momentum

11. 11 Davisson-Germer experiment C Davisson & L H Germer, Phys Rev 30 705 (1927) NICKEL TARGET ELECTRON DIFFRACTION electrons behave like waves electron wavelength

12. 12 Diffracting molecules S Gerlich et al, Nature Physics 3 711 (2007) MOLECULE DIFFRACTION molecules behave like waves molecule wavelength

13. 13 Quantum theory PHOTONS energy quantized in units of ( h = Planck’s constant) momentum quantized in units of angular momentum quantized in units of blackbody radiation photoelectric effect Compton scattering PARTICLES frequency determined by energy de Broglie wavelength determined by momentum electron diffraction angular momentum quantized in units of atomic theory

14. 14 Bohr model of the hydrogen atom + BOHR MODEL quantized angular momentum quantized energy levels circular orbits de Broglie wavelength Hydrogen energy level measurements and calculations agree to 15 figures

15. 15 Bohr model of the hydrogen atom allowed energies Rydberg constant energy 0 n = 1 n = 3 n =  emission wavelengths n = 2

16. 16 Atomic line spectra allowed energies Rydberg constant emission wavelengths energy 0 n = 1 n = 3 n =  n = 2

17. 17 Atomic line spectra energy 0 n = 1 n = 3 n =  n = 2 Lyman Balmer Paschen universe-review.ca scope.pari.edu

18. 18 Hydrogenic atoms allowed energies Rydberg constant energy 0 n = 1 n = 3 n =  emission wavelengths n = 2

19. 19 Franck-Hertz experiment accelerate electrons through atomic vapour periodic modulation of measured current inelastic collisions when electron energy equals atomic transition energy singlettriplet Hg G Rapior et al., Am J Phys 74 423 (2006) J Franck & G Hertz, Verh. Dtsch. Phys. Ges. 16 457 (1914)