Lecture-10 - Chem111 Fall 2005 9 Paradox 2 The Photoelectric Effect Photons = quantization of light wave-particle duality Æ waves can behave like

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Unformatted text preview: Chem111 Fall, 2005 9 Paradox 2: The Photoelectric Effect Photons = quantization of light wave-particle duality Æ waves can behave like particles in 1905 Albert Einstein latched onto Max Planck's idea of quantized properties and applied it to electromagnetic radiation and the photoelectric effect Photoelectric effect e- metal surface electromagnetic radiation beam Experimental observation : Lenard (~1900) showed that the photoelectron effect has the following characteristics: It is observed that • electrons are only emitted when ν > ν • increase in intensity Æ increase in #of e- emitted, but no Δ T e • increase in frequency Æ increase in T e of emitted e- . Lecture 10: • Breakdown of CM & 4 Paradoxes (cont) • Bohr Atom • Wave Mechanics – de Broglie material absorbs enough energy to let an electron fly off with kinetic energy T m v e e = 1 2 2 T e ν of light 0 ν = threshold frequency • ν is a characteristic of the material Chem111 Fall, 2005 10 CM predicts that the energy in a wave is proportional to intensity and is independent of frequency Æ expect T e to increase with an increase in intensity Æ expect T e to be independent of ν Æ CM predicts no threshold ν Einstein suggested that • light is quantized (photon) • light has both wave-like and particle-like properties Energy of photon E = h ν • energy of a photon is associated with its frequency • intensity of a beam of photons is associated with the number of photons • energy associated with a beam of light is the sum of the energies of the individual photons • in the photoelectric effect, the light beam interacts with surface as a collection of photons • one photon can remove one e- , if the photon has enough energy Einstein's explanation of photoelectric effect 1. When h ν < h ν Æ photons interacting with the surface do not have sufficient energy to dislodge an e-, regardless of the beam intensity 2. When h ν > h ν Æ photons interacting with the surface do have sufficient energy to dislodge an e- • more photons = higher intensity Æ more e- dislodged • more energy per photon = increased ν Æ e- with higher KE T max = ½ m e v 2 = h ν- Φ Note: the work function is the minimum energy required to dislodge an electron from a metal surface. It is basically the attractive energy between a (negatively charged) electron and the (positively charged) nuclei in the metal lattice like energy that molecules and atoms can emit energy of photon work function Φ = h ν = energy required to dislodge the e- Chem111 Fall, 2005 11 Note : Einstein's conclusions caused problems in the chemistry & physics community. How can light behave like a wave in some situations (diffraction patterns , interference) and like a particle in others (photoelectric effect)?...
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This note was uploaded on 02/08/2010 for the course CHEM 111 taught by Professor Kenney during the Spring '08 term at Case Western.

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Lecture-10 - Chem111 Fall 2005 9 Paradox 2 The Photoelectric Effect Photons = quantization of light wave-particle duality Æ waves can behave like

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