1. 1 Quantum Mechanics Experiments 1

2. Photoelectric effect

8. Lenard 1902: Studied energy of the photoelectrons with intensity of light. He could increase the intensity thousand fold. 1.Noticed a well defined minimum voltage V stop to stop the current in the circuit. V stop was independent of the intensity of light.

9. Current vs Voltage Cut-off voltage Different intensities: I b > I a

10. 2. Increasing the intensity of light would increase the current 3. He performed the experiment with various coloured lights and found the maximum energy of the electrons did depend on the frequency of light. Qualitatively he obtained more the frequency more the energy.

11. Objections with wave theory 1. Kinetic energy (K) of the photo-electron should increase with intensity of the beam. But K max was found to be independent of the intensity of the falling light.

12. 2. Effect should occur for any frequency of light provided only that light is intense enough to eject the electron. But a cut-off frequency was observed below which photoelectrons were not ejected (no matter how intense was beam).

13. 3. Energy in the classical theory is uniformly distributed over the wave front. If light is feeble, there should be a time lag between the light striking the plate and ejection of photoelectrons. Ejection is instant, t < 10 sec - 9

14. Einstein equation

15. Light is wave : Interference, Diffraction, polarisation Light is a stream of photons/wave packets (particles) So wave behaves like particle

16. Compton effect Collision between photon and electron

17. Arthur Holly Compton (1892-1962) Compton Effect in 1920

18. Partial transfer of photon energy

19. m = m(v) is the relativistic mass

22. Conservation of momentum along initial photon direction Conservation of momentum along perpendicular to initial photon direction Square and add the above two expressions

23. Energy conservation

24. Compton wavelength Compton shift = 2.4 pm ~

25. Compton Experiment

29. Franck and Hertz experiment (1914) (James Franck and Gustav Hertz) Confirmation of discrete energy levels in atom

30. VoVo

31. 100 200 300 I in mA Accelerating Voltage in Volts 5 1015 Peaks at 4.9 V and its multiples

32. Ground state 1 st excited state 2 nd excited state 4.9 eV 6.7 eV Continuum E=0 E= -10.4eV

33. Particles behaving as waves Electron diffraction Davisson –Germer (USA) and Thompson (UK) (1927) Experiments

35. Diffraction?

36. Apply Bragg’s law From X-ray diffraction

37. KE=54 eV is non relativistic de Broglie wavelength of electron

39. “In a situation where the wave aspect of a system is revealed, its particle aspect is concealed; and, in a situation where the particle aspect is revealed, its wave aspect is concealed. Revealing both simultaneously is impossible; the wave and particle aspects are complementary.” Bohr’s Complementarity Principle