Early Quantum Theory-College Physics B-Lecture 15 Slides-Physics, Slides of Physics

Download Early Quantum Theory-College Physics B-Lecture 15 Slides-Physics and more Slides Physics in PDF only on Docsity! PHY 2054C – College Physics B Fall 2004 Electricity, Magnetism, Light Optics and Modern Physics Dr. Ingo Wiedenhöver Dr. A. Volya Today: 1) Early Quantum Theory Early Particle Physics Today we are re-enacting the early discoveries of Quantum Physics. Unlike Einstein's Theory of Relativity, the development of Quantum Mechanics was driven by experimental observations on particles and atoms. Where Einstein taught us the structure of space and time, Quantum Mechanics is describing the structure of atoms and sub-atomic particles. Light: Wave or Particle ? 1) If light is a wave, the power transmitted is prop. to its amplitude2. Bright light – large amplitude – large power Dim light – small amplitude – small power It can deliver any energy value, large or small to a particle it hits. A Light: Wave or Particle ? 2) If light consists of particles (photons), the energy it transmits comes in portions. The power transmitted is that kinetic energy of one photon times the number of photons per second. If light knocks an electron out of an atom, the electron receives the KE of one photon (minus the work required to knock it out) γ γ γ electron 1) 2) γ γ electron Experiment:Photo-Electric Effect Light which strikes metal surfaces knocks out electrons. Any electron requires a certain amount of energy in order to leave the metal surface, W 0 . For light of different frequency f, we measure the KE of electrons. It shows that with h = 6.63 10-34 J s (Planck's constant ) The electron KE is independent of light intensity ! KE f KEmax=hf −W 0 Compton Effect  Scattering of light by electrons  Behaves like an elastic collision between photons and electrons (energy of photons changes with their outgoing direction).  Wave theory would predict that the energy of the outgoing light does not depend on the direction. θλ λ’ φ λ’ = λ + h/moc * (1 - cosθ) The Compton effect shows that light has momentum, is a particle ! Wave-Particle Duality  Light is a wave. Light is a particle. Now what ? It is both. You have to live with it.  “normal” particles can behave like waves, too !  The principle of complementarity  Sometimes light (matter) behaves like a wave.  Sometimes light (matter) behaves like a particle.  Which concept applies depends on the experiment !  This is Heisenberg's uncertainty principle at work ! Double-Slit Experiment The screen shows the impact of individual electrons The double-slit interference pattern determines the probability to find a particle.