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The Photoelectric Effect

The Photoelectric Effect. 27-3. 1905: Einstein said that if the energy of a vibrational oscillator is quantized (Planck), then the light that it emits would be quantized as well (conservation of energy) Each light packet would be E = h f This suggested packets of light or photons

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The Photoelectric Effect

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  1. The Photoelectric Effect 27-3

  2. 1905: Einstein said that if the energy of a vibrational oscillator is quantized (Planck), then the light that it emits would be quantized as well (conservation of energy) • Each light packet would be • E = h f • This suggested packets of light or photons • Einstein proposed an experiment to test this based on the Photoelectric effect Origins

  3. Apparatus

  4. Electron emission in photoelectric effect is consistent with wave theory • EM wave exerts a force on the electrons and knock some loose • A variable voltage source is placed across the gap and adjusted until the current reaches zero • This is the stopping voltage (Vo) • The is equal to the maximum KE of the photons • KEmax = e Vo Wave Explanations

  5. Wave theory says that as you increase the intensity (amplitude) of the light, • the more energy the electrons will have • the greater the Vo • Frequency should have *no* effect on the KEmax of the ejected photons, only the intensity • This was NOT the case. Wave Explanations

  6. Photon theory says that an electron is held to the metal plate by a Work Function – Wo • A photon (light quanta) that strikes the electron transfers its energy to the electron and the photon ceases to exist! • If the energy of the photon (E=hf) is less than Wo , then nothing happens. • If an electron is ejected then the KEmaxis the energy of the photon minus the Wo • KEmax = hf- Wo • hf = KEmax + Wo Photon Explanations

  7. An increase in the intensity of light only increases the number of photons, it does not increase the KE • An increase in the frequency of the light will increase the energy of the photon and thus the ejected electron • If the light frequency is below the cutoff frequency, fo, then NO electrons will be ejected no matter the intensity of the light Photon Explanations

  8. 1921 Nobel Prize in Physics • Photoelectric Effect • 1905 • Published Photoelectric Effect • Published Special Relativity • 1912 • Published General Relativity • Died in 1955 at 76 after refusing surgery. Big AL!

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