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Photoelectric Equation:

Incident light. Cathode. Anode. A. V. +. -. Stopping potential:. V o = 0.800 V. Example 4: The threshold frequency for a given surface is 1.09 x 10 15 Hz . What is the stopping potential for incident light whose photon energy is 8.48 x 10 -19 J ? . Photoelectric Equation:.

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Photoelectric Equation:

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  1. Incident light Cathode Anode A V + - Stopping potential: Vo= 0.800 V Example 4:The threshold frequency for a given surface is 1.09 x 1015 Hz. What is the stopping potential for incident light whose photon energy is 8.48 x 10-19 J? Photoelectric Equation: W = (6.63 x 10-34 Js)(1.09 x 1015 Hz) =7.20 x 10-19 J

  2. Total Energy, E Total Relativistic Energy Recall that the formula for the relativistic total energy was given by: For a particle with zero momentum p= 0: E =moc2 A light photon has mo= 0, but it does have momentum p: E =pc

  3. Wavelength of a photon: de Broglie Wavelength: Waves and Particles We know that light behaves as both a wave and a particle. The rest mass of a photon is zero, and its wavelength can be found from momentum. All objects, not just EM waves, have wavelengths which can be found from their momentum

  4. Momentum from K: Finding Momentum from K.E. In working with particles of momentum p = mv, it is often necessary to find the momentum from the given kinetic energy K. Recall the formulas: K = ½mv2 ; p = mv Multiply first Equation by m: mK =½m2v2= ½p2

  5. - e- 90 eV Example 5:What is the de Broglie wavelength of a 90-eV electron? (me = 9.1 x 10-31 kg.) Next, we find momentum from the kinetic energy: p = 5.12x 10-24 kg m/s l = 0.122 nm

  6. Planck’s Equation: E = hf (h = 6.626 x 10-34 J s) Photon 1 eV = 1.60 x 10-19 J The Electron-volt: E = hf 1 MeV = 1.6 x 10-13 J 1 keV = 1.6 x 10-16 J Summary Apparently, light consists of tiny bundles of energy called photons, each having a well-defined quantum of energy.

  7. Incident light Cathode Anode A C Ammeter Threshold wavelength lo A + - Summary (Cont.) If lis innm, the energy ineV is found from: Wavelength in nm; Energy in eV

  8. Stopping potential V Slope y fo x Frequency Summary (Cont.) Planck’s Experiment: Incident light Cathode Anode V A + - Potentiometer Kmax = eVo

  9. Wavelength of a photon: de Broglie Wavelength: Summary (Cont.) Quantum physics works for waves or particles: For a particle with zero momentump = 0: E =moc2 A light photon has mo = 0, but it does have momentum p: E =pc

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