WEBSITE NOTES: Quantum Theory and Nuclear Physics. 1. The photoelectric effect involves electrons acquiring energy by absorbing particles of light (photons). These energetic electrons are then termed "photoelectrons". 2. The "200Year debate" (Light. What is it? Waves or particles?) was revived when it was observed that a metal plate emitted electrons when struck with light from a spark discharge. 3. The photoelectric effect contradicts classical physics, but can be explained assuming that energy is quantized, or occurs in discrete bundles or units. 4. In the visible range, the frequency of light determines its color. Red light is at the lower end of the frequency scale, and violet is at the upper. 5. The energy of a photon, or light quantum, depends on the frequency of the light, E = hf . The frequency, f, is related to the wavelength, λ , through the wave equation for light, c = fλ . 6. Max Planck (18581947), determined the value of h, which came to be known as Planck's Constant, h = 6.6x10^{34} Js. 7. Heinrich Hertz (18571894) experimented with electromagnetic waves and confirmed the work of the theoretical physicist, James Maxwell (18311879) who postulated their existence. The unit of frequency is the Hertz (Hz). 8. The minimum energy required for an electron to escape from a metal depends on the threshold frequency of the metal. This is known as the work function which is W_{o} = hf_{o }, with f_{o}, this time, being threshold frequency. 9. The maximum kinetic energy of photoelectrons is then found by using the equation KE_{max} = E  W_{o} = hf  hf_{o} . Another way to determine is by using a "stopping potential", KE_{max} = qV_{o} . The unit that we use for this energy is the electronVolt, or eV, with 1 eV = 1.6x10^{19} J. 10. Light now has a dual nature, wave and particle, but each particle has no mass. We call these massless particles, photons. 11. The wave equation for light still applies, c = f·λ which is the velocity of a wave equals the product of its frequency and wavelength, with, as we should already know, c = 3.0x10^{8} m/s , the speed of light. 12. Ernest Rutherford (18711937) performed the famous scattering experiment (GoldFoil) revealing that all of atom's positive charge and almost all of the mass is at the center, or nucleus. Most of the atom is empty space. 13. Robert Millikan (18681953) determined the charge on the electron with his famous "OilDrop" Experiment. He found that charge, q, is quantized, or only occurs in multiples of the elementary unit of charge, 1.6x10^{19} C. His equation was qE = mg. 14. Sir Joseph J. Thomson (18561940) found the masstocharge ratio for the electron by experimenting with the CRT. His equations were Bqv = mv^{2}/r and qE = Bqv , which produced v = E/B and m/q = Br/v . 15. Since each gas has a unique emission and absorption spectrum, the Danish Physicist, Niels Bohr (18851962) proposed a QuantumMechanical Atomic Model instead of Rutherford's Planetary Model. 16. He proposed that electrons can move from one energy level to another by absorbing or emitting photons. His equations were r_{n} = 5.2x10^{11} m x n^{2} , and E_{n} = 13.6 eV x 1/n^{2} , with n = 1,2,3,... , the energy level. As stated above, the electronvolt (eV) is the energy unit for electrons, with, 1 eV = 1.6x10^{19} J . 17. Count Louis Victor de Broglie (18921987) postulated that, "if waves can have particle properties, why can't particles have wave properties?" His equation was λ = h/mv for the wavelength of a matter particle. 18. Albert Einstein (18791955) explained this, as well as the photoelectric effect, to get the Nobel Prize in 1921 for his work in 1905. This is why, 100 years later, 2005 was the "World Year of Physics". His famous equation is E = mc^{2} . 19. Werner Heisenberg (19011976) determined that it is not possible to know the exact position and momentum of the electron, the Uncertainty Principle. 20. Arthur Compton (18921962) bombarded a graphite block with Xrays demonstrating the momentum of photons (The Compton Effect ). The equation is mv = p = h/λ . 21. James Chadwick (18911974) an original member of Rutherford's research team proved the existence of neutrons in 1932. 22. Light Amplification by Stimulated Emission of Radiation (LASER), which was explained by Einstein in 1917, was invented in 1960. Laser light is very directional, powerful, monochromatic, and coherent, making it very useful. 23. Early atomic models were: (i) single indivisible particle, (ii) "plum pudding" model, (iii) planetary model. Today we have model (iv) the planetaryquantum model. 24. Henri Becquerel (18521908) accidentally found that all compounds containing uranium emitted rays that penetrate and fog photographic plates, after examining a mysterious rock. 25. Ernest Rutherford (18711937) identified alpha, beta, and gamma radiation and used alpha particles to bombard gold foil. He found that most of an atom is empty space but contains a massive positively charged nucleus. 26. The Curies, Pierre and Marie, were the first to discover other radioactive elements, for example, Polonium and Radium. 27. Atoms having the same number of protons but different amounts of neutrons are called isotopes. 28. The nucleus of an atom contains most of the mass, consists of protons and neutrons, and number of protons is the atomic number. 29. The nucleus can be characterized by a mass number, A, an atomic number, Z, and a neutron number, N, with A = Z + N. 30. The change, transmutation, in an atomic nucleus can be natural or artificial. Enrico Fermi (19011954) successfully produced artificially radioactive elements in the laboratory. 31. Radioactive decay produces three kinds of particles: alpha, helium nuclei; beta, highspeed electrons; and gamma ray photons. 32. Bombardment of nuclei by protons, neutrons, alpha particles, electrons, gamma rays, or other nuclei can produce a nuclear reaction. 33. Linear accelerators, synchrotrons, and supercolliders produce highenergy protons and electrons which can collide with each other or an atomic nucleus. 34. Particle detectors include photographic plates, the GeigerMuller tube, scintillation screens, and the cloud chamber. 35. Alpha can be stopped by thick paper, beta by thick aluminum foil, and a few centimeters of lead will stop gamma. 36. During positron decay a proton changes into a neutron with the emission of a positron and a neutrino. 37. When matter and antimatter combine, all matter is converted into energy, or lighter matterantimatter particle pairs. By pair production, energy is converted into a matterantimatter particle pair. 38. The weak interaction operates in beta decay while the strong force binds the nucleus together. During beta decay a neutron changes into a proton and the nucleus emits a beta particle and a massless antineutrino. 39. The binding energy is the energy equivalent of the mass defect. The assembled nucleus has less mass than its constituent parts due to massto energy conversion, E = mc^{2} . 40. Nuclear reactors use the energy released in fission as heat to boil water, which produces steam, that turns turbine blades to run a generator. 41. The binding energy of the nucleus is the difference in energy between its nucleons when bound and its nucleons when unbound. Energymass equivalent can be computed using 1 amu = 931 MeV. 42. The halflife is the time required for half the original nuclei of a radioactive substance to undergo radioactive decay. 43. The decay constant, lambda, indicates the rate of radioactive decay. Half life can be calculated by dividing .693 by the decay constant, λ, lambda. 44. Nuclear reactions involve a change in the nucleus and can be given by equations. In equations for nuclear reactions, subscripts and superscripts must agree on both sides. 45. In a nuclear equation the sums of the subscripts (atomic number or particle charge) on both sides of the equation are equal and the sums of the superscripts (mass number) on both sides of the equation are equal. 46. In fission, heavier nuclei split to form lighter nuclei and energy is released. In fusion, lighter nuclei combine to form heavier nuclei with more binding energy. 47. And still, we need these steps to solve any problem in Physics: (i) read the problem and identify the given variables (ii) determine what you are asked to solve for (iii) find the correct equation to use (iv) use Algebra, Trigonometry, and/or Calculus to isolate the unknown. (v) substitutein your known values and solve. View Nuclear and Atomic Slides.

