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

2 Michelson interferometer interference by division of amplitude beamsplitterdetector source δxδx

3 Michelson interferometer interference by division of amplitude beamsplitterdetector source δxδxδxδx sodium doublet optique-ingenieur.org chemistry.oregonstate.edu FTIR: Fourier transform infrared

4 Diffraction grating x

5 Grating spectrometer scope.pari.edu

6 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

7 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)

8 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) R A Beth, Phys Rev (1936) Crookes radiometer sun sunshine

9 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

10 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

11 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

12 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

13 Compton scattering A H Compton, Phys Rev (1923) GRAPHITE TARGET Å X-RAYS wavelength shift angle photon momentum

14 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