Quantum Theory of Light

1. Quantum Theory of Light A TimeLine

2. Light as an EM Wave

3. Light as an EM Wave (Maxwell 1865-1873) • Quantum theory did not begin with an attempt to explain the behaviour of light.

4. Light as an EM Wave (Maxwell 1865-1873) • Quantum theory did not begin with an attempt to explain the behaviour of light. • Scientists had already accepted Maxwell’s description of light!

5. Light as an EM Wave (Maxwell 1865-1873) • Maxwell had proposed that light was an electromagnetic disturbance created by extremely high frequency oscillators.

6. Light as an EM Wave (Maxwell 1865-1873) • Maxwell had proposed that light was an electromagnetic disturbance created by extremely high frequency oscillators.

7. Light as an EM Wave (Maxwell 1865-1873) • It was assumed from this theory that these oscillators (resonators) were able to emit light of frequency equal to their own.

8. Light as an EM Wave (Maxwell 1865-1873) • Hertz was successful in indirectly proving (was no way frequencies higher than 109 Hz) the theory by showing that it described the properties of light.

9. Light as an EM Wave (Maxwell 1865-1873) • Hertz was successful in indirectly proving (was no way frequencies higher than 109 Hz) the theory by showing that it described the properties of light. • I.e. Reflection, interference etc.

10. Light as an EM Wave (Maxwell 1865-1873) • Hertz was successful in indirectly proving (was no way frequencies higher than 109 Hz) the theory by showing that it described the properties of light. • I.e. Reflection, interference etc. • (Hertz unknowingly discovered the photoelectric effect during his experimental verifications)

11. Light as an EM Wave (Maxwell 1865-1873) • The success of the theory lead to its application to the blackbody radiation problem.

12. Light as an EM Wave (Maxwell 1865-1873) • The success of the theory lead to its application to the blackbody radiation problem. • However all attempts failed. (see Wien, Rayleigh-Jeans Law)

13. Planck (1900) • The discovery by Planck was the beginning of quantum theory. • Interpolating Wein’s law and the relationship at low frequency he showed that

14. Planck (1900) • From his work he concluded that the energy was quantised, where the frequency f can only be an integral multiple of hf.

15. Planck (1900) • That is the energy which an resonator can lose is nhf, where n = 1,2,3… .

16. Planck (1900) • That is the energy which an resonator can lose is nhf, where n = 1,2,3… . • The idea of quantised energy levels (states) was the significant development.

17. Planck (1900) • That is the energy which an resonator can lose is nhf, where n = 1,2,3… . • The idea of quantised energy levels (states) was the significant development. • This assumption contradicted what was accepted classically.

18. Planck (1900) • Using this assumption he was able to correctly produce a theory which agreed with experimental results.

19. Planck (Summary) • In deriving his formula he made two assumptions: • The energy of an oscillator of frequency f can only be nhf. • During an emission or absorption the change in energy is hf.

20. Planck (Summary) • Despite the significance, it was left to Einstein to develop these ideas to the next step.

21. Photoelectric Effect(1905)

22. Photoelectric Effect (1905) • When a metallic surface is illuminated by light can cause electrons to be emitted from the surface.

23. Photoelectric Effect (1905) • The number of electrons ejected from the metal surface per second depends on the intensity of the light.

24. Photoelectric Effect (1905) • The number of electrons ejected from the metal surface per second depends on the intensity of the light. -expected

25. Photoelectric Effect (1905) • The number of electrons ejected from the metal surface per second depends on the intensity of the light. • The kinetic energy of the electrons did not depend on intensity.

26. Photoelectric Effect (1905) • The number of electrons ejected from the metal surface per second depends on the intensity of the light. • The kinetic energy of the electrons did not depend on intensity. • Kinetic energy of the electrons depends on the wavelength of the light.

27. Photoelectric Effect (1905) • The number of electrons ejected from the metal surface per second depends on the intensity of the light. • The kinetic energy of the electrons did not depend on intensity. • Kinetic energy of the electrons depends on the wavelength of the light. And there was a cut of wavelength where no electrons are emitted.

28. Einstein - Photoelectric Effect (1905) • Noted that although Maxwell’s classical theory described the interaction of light over a long period, a new description was needed for the individual interactions of light and matter.

29. Photoelectric Effect (1905) • He extended Planck’s ideas to include that light was also quantized.

30. Photoelectric Effect (1905) • He extended Planck’s ideas to include that light was also quantized. • Light and matter interactions occur in discrete packets called photons. • The energy of a photon is

31. Compton Effect (1922)

32. Compton Effect (1922) • The Compton effect shows that photons behave like particles with momentum

33. Compton Effect (1922) • The Compton effect shows that photons behave like particles with momentum • Classical theory predicted that incident radiation of frequency f0 would cause electrons to be acceleration in the direction of the radiation

34. Compton Effect (1922) • The Compton effect shows that photons behave like particles with momentum • Classical theory predicted that incident radiation of frequency f0 would cause electrons to be acceleration in the direction of the radiation and that the freq of the scattered radiation depend on the intensity and time of exposure.

35. Compton Effect (1922) • Instead Compton showed that only on the scattering angle.

36. Compton Effect (1922) • Instead Compton showed that only on the scattering angle. • This result disagrees with classical theory

37. Compton Effect (1922) • Instead Compton showed that only on the scattering angle. • This result disagrees with classical theory and is only explained if the photons act as particles.

38. Wave-Particle Duality

39. Wave-Particle Duality • Various experiments have been shown which highlight either the wave nature or particle nature of light.

40. Wave-Particle Duality • Various experiments have been shown which highlight either the wave nature or particle nature of light. • Is light simultaneous a wave and a particle?

41. Wave-Particle Duality • Various experiments have been shown which highlight either the wave nature or particle nature of light. • Is light simultaneous a wave and a particle? • Many questions on the nature of light arise from issues in classical mechanics.

42. Wave-Particle Duality • Classically the two have mutually properties. • Both views are required to describe the behaviour of light.

43. Wave-Particle Duality • Classically the two have mutually properties. • Both views are required to describe the behaviour of light. • Neither model can exclusively describe radiation adequately.

44. Wave-Particle Duality • Classically the two have mutually properties. • Both views are required to describe the behaviour of light. • Neither model can exclusively describe radiation adequately. • Therefore for now both must be used.

45. Bohr Atom Particle Nature (1913)

46. Bohr Atom • The Bohr model was developed from the work of Rutherford to describe a stable atomic model.

47. Bohr Atom • The Bohr model was developed from the work of Rutherford to describe a stable atomic model. • He provided to first successful theory of atomic line spectra.

48. Bohr Atom • For his model he postulated that classical radiation theory did not hold at the atomic level.

49. Bohr Atom • For his model he postulated that classical radiation theory did not hold at the atomic level. • He also used the work Planck and Einstein for the idea of quantised energy levels and quantisation of light.

50. Bohr Atom • From these ideas he proposed that electrons generally remained in stable, stationary states. • However when an electron moves between states specific frequency radiation is emitted.