Dr Martin Hendry University of Glasgow

1. Light in Lumps or ripples ? Reach for the Stars Dr Martin Hendry University of Glasgow

2. Isaac Newton 1686

3. White light Prism Particle theory of light

5. Refraction of light

6. Refraction of light Particles move faster in more “optically dense” medium

7. Reflection of light i r Incident angle (i) = Reflected angle (r)

8. Rival theory due to Christian Huygens Light waves propagate through the luminiferous ether Wave theory could explain equally well reflection and refraction

9. Diffraction could, in principle, distinguish the models Barrier Light Wave Intensity

10. Particle theory dominated until early 1800s: Experiments by Thomas Young and Augustin Fresnel changed all that!

11. Diffraction of light Outgoing Circular Waves Barrier Direction of waves

12. Interference of light Direction of waves

13. Interference of light Direction of waves

15. Maxwell’s theory of light Early 1900s: accelerated electron radiates

16. How do atoms persist?

17. Black-body radiation

19. Wilhelm Wien Ultraviolet Catastrophe Intensity Wavelength

20. The UV Catastrophe could be avoided if light energy was quantised in packets, or photons of energy E = hf Max Planck

21. Black-body radiation Quantised assumption keeps the black-body brightness finite

22. Albert Einstein, 1905

23. The Photoelectric Effect Incoming light, produces electric current Metal plate Meter B: measures speed of the ejected electrons Meter A: measures current of ejected electrons

24. The Photoelectric Effect Incoming light, produces electric current Metal plate Meter B: measures speed of the ejected electrons Meter A: measures current of ejected electrons

25. The Photoelectric Effect Incoming light, produces electric current Metal plate Meter B: measures speed of the ejected electrons Meter A: measures current of ejected electrons

26. The Photoelectric Effect Incoming light, produces electric current Metal plate Meter B: measures speed of the ejected electrons Meter A: measures current of ejected electrons

27. The Photoelectric Effect …. Incoming light, produces electric current Metal plate Meter B: measures speed of the ejected electrons Meter A: measures current of ejected electrons

28. The Photoelectric Effect …. No effect for blue light Incoming light, produces electric current Metal plate Meter B: measures speed of the ejected electrons Meter A: measures current of ejected electrons

29. The Photoelectric Effect …. Incoming light, produces electric current Metal plate Meter B: measures speed of the ejected electrons Meter A: measures current of ejected electrons

30. The Photoelectric Effect Effect seen for UV light Incoming light, produces electric current Metal plate Meter B: measures speed of the ejected electrons Meter A: measures current of ejected electrons

31. 1909 It is my opinion that the next phase in the development of theoretical physics will bring us a theory of light that can be interpreted as a kind of fusion of the wave and the emission theory

32. 1909 It is my opinion that the next phase in the development of theoretical physics will bring us a theory of light that can be interpreted as a kind of fusion of the wave and the emission theory 1911 I insist on the provisional character of this concept, which does not seem reconcilable with the experimentally verified consequences of the wave theory

33. 1909 It is my opinion that the next phase in the development of theoretical physics will bring us a theory of light that can be interpreted as a kind of fusion of the wave and the emission theory 1911 I insist on the provisional character of this concept, which does not seem reconcilable with the experimentally verified consequences of the wave theory 1924 There are therefore now two theories of light, both indispensable…without any logical connection

34. The Bohr atom, 1913

35. Absorption e - e -

36. Emission e - e -

39. If light waves also behave like particles, why shouldn’t electrons also behave like waves? Pilot Waves Davisson & Germer; Thomson & Reid, 1937 Louis de Broglie, 1923

40. Making Quantum Mechanics Work Werner Heisenberg Erwin Schrodinger Neils Bohr : Max Born Paul Dirac Wolfgang Pauli John von Neumann

42. All physical systems and events are inherently probabilistic, expressed by the Wave Function Only when the quantum system is observed, the wave function collapses Copenhagen Interpretation

43. Heisenberg Uncertainty Principle The precision of measurements in a quantum system is limited in principle

44. Heisenberg Uncertainty Principle DpDx ~ h The precision of measurements in a quantum system is limited in principle

45. Heisenberg Uncertainty Principle DpDx ~ h The precision of measurements in a quantum system is limited in principle Position and momentum are complementary properties: the action of measurement determines which of the two properties the quantum system possesses

46. Schrodinger’s Cat : Radioactive source Poison Gas

47. Schrodinger’s Cat : Radioactive source Poison Gas

48. Schrodinger’s Cat : Radioactive source Poison Gas R.I.P.

49. Schrodinger’s Cat : Radioactive source + Poison Gas R.I.P.

50. versus Complementarity asserts that it is not just meaningless to talk about knowing simultaneously exact values of position and momentum; these quantities simply do not exist simultaneously.