Lecture 23: Quantum Physics - Wave-Particle Duality and Uncertainty Principle, Study notes of Physics

Download Lecture 23: Quantum Physics - Wave-Particle Duality and Uncertainty Principle and more Study notes Physics in PDF only on Docsity! 1 4/16/08 U. W. Physics 104, Spring 2008 1 Lecture 23: Quantum Physics •  Electron diffraction •  Compton Effect •  Uncertainty Principle 4/16/08 U. W. Physics 104, Spring 2008 2 Main point from the last lecture Light is massless particle and a wave at the same time Electron is a massive particle and a wave at the same time Both interpretations are valid simultaneously because of quantum mechanics 4/16/08 U. W. Physics 104, Spring 2008 3 Electrons are waves The act of “looking” affects the outcome of the experiment 4/16/08 U. W. Physics 104, Spring 2008 4 Schrodinger’s Cat •  Place cat in box with some poison. If we don’t look at the cat it will be both dead and alive! Poison Wave nature of particles •  It is a wave of “probability amplitude distributions” •  The fact that particles “could have” gone through either slits in Young’s interference setup (if you did’t look) leads to interference • Quantum Mechanics is: a consistent set of rules on how to account for these probabilities in agreement with obsevations 4/16/08 U. W. Physics 104, Spring 2008 5 4/16/08 U. W. Physics 104, Spring 2008 6 So far only for photons, but De B postulated that it holds for any object with momentum- an electron, a nucleus, an atom, a baseball,…... Should be able to see interference and diffraction for material particles!! De Broglie Waves 2 4/16/08 U. W. Physics 104, Spring 2008 7 Photon with energy E=1eV: What about an electron with 1eV (non-relativistic) kinetic energy? Find λ = 1.23 nm for a 1eV electron B ig d iffe re n c e ! Wavelength of Electrons Solve for 4/16/08 U. W. Physics 104, Spring 2008 8 X-Ray Diffraction (Bragg’s Law) Note: Angle is measured with respect to the horizontal 4/16/08 U. W. Physics 104, Spring 2008 9 Structure of Biological Molecules X-Ray diffraction from DNA Rosalind Franklin’s picture 4/16/08 U. W. Physics 104, Spring 2008 10 A stone is dropped from the top of a building. What happens to the de Broglie wavelength of the stone as it falls? 1. It decreases 2. It stays the same 3. It increases Speed, v, KE=mv2/2, and momentum, p=mv, increase. De Broglie Question 4/16/08 U. W. Physics 104, Spring 2008 11 De Broglie Question Compare the wavelength of a bowling ball with the wavelength of a golf ball, if each have 10 Joules of kinetic energy. 1)  λbowling > λgolf 2)  λbowling = λgolf 3)  λbowling < λgolf 4/16/08 U. W. Physics 104, Spring 2008 12 Compton Scattering