27.7 Wave-particle Duality Experiments show light has wave properties or particle properties.

1. 27.7 Wave-particle Duality • Experiments show light has wave properties or particle properties. • Diffraction • Interference • Photoelectric effect • Compton scattering • Einstein → Eparticle = hfwave APHY201

2. 27.8 Wave Nature of Matter • Do particles have wave properties? • De Broglie wavelength of a particle • Not noticed for macroscopic objects but important for microscopic objects • Diffraction of electrons from a crystal • Shown for other subatomic particles as well APHY201

3. X-ray diffraction 1912 Electron diffraction 1927 APHY201

4. 27.10 Early Models of the Atom • Plum-pudding model and Rutherford’s experiment APHY201

5. 27.10 Early Models of the Atom • Rutherford’s “planetary model” - the atom is mostly empty space with a small positive nucleus. APHY201

6. 27.11 Atomic Spectra APHY201

7. 27.11 Atomic Spectra • Each element has its own distinct set of spectral “lines” – like human fingerprints or UPC labels • Helium was discovered in the Sun (1868) before on the Earth (1882) • The solar spectrum has lines that were not known on the Earth at that time • Applications: astronomy, chemistry, forensics, … APHY201

8. 27.11 Atomic Spectra APHY201

9. 27.12 The Bohr Model • Why do atoms only emit and absorb at certain wavelengths? Why are atoms stable? • Assumptions of Bohr’s theory • 1. The e- moves in circular orbits around the nucleus • 2. Only certain orbits are stable • 3. Radiation is emitted when the e- “jumps” from a higher energy state to a lower one. • 4. The size of the orbits is determined by the e- orbital angular momentum. APHY201

10. 27.12 The Bohr Model APHY201

11. 27.13 The de Broglie Hypothesis APHY201