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Modern Physics

Modern Physics. Light as a collection of particles: photons Particle-wave duality Electron wave Special relativity A new mechanics for atom-sized objects (Quantum mechanics). Photoelectric Effects.

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Modern Physics

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  1. Modern Physics • Light as a collection of particles: photons • Particle-wave duality • Electron wave • Special relativity • A new mechanics for atom-sized objects (Quantum mechanics)

  2. Photoelectric Effects • When light shines on a clean metal surface, electrons can escape from the metal  photoelectric effect. • Normally, the electrons are trapped in the metal like being trapped in a well. The light gives sufficient energy to the electrons so that they can “jump out”. • At the beginning of the 20th century, a number of “bizarre” properties of the photoelectric effect were found.

  3. Bizarre Properties • For low frequency light (e.g. red), there are no photoelectrons, no matter how long the surface is exposed to the light. For a particular metal, the photoelectron are produced only when the frequency is above a certain critical value. • When the frequency is higher than the critical value , the electrons come out instantly, no matter how low the intensity of the light is.

  4. The energy of the photoelectrons has to do with the frequency of the light. The higher the frequency, the higher the electron energy. • The intensity of the light affects only the intensity of the electron current, i.e. the number of electrons produced. • None of these facts can be explained by the wave theory of light!

  5. Einstein’s Photon • In 1905, Einstein gave an explanation of the photoelectric effect • Light is made of many tiny particles called photons. • Each photon has energy hf where h is Planck’s constant and f is the frequency. Photons also have momentum. • A photoelectron is produced when a photon is absorbed by an electron.

  6. The chance of an electron absorbing two photons is extremely small. So when the photon has an energy that is too low, electrons do not get sufficient energy to jump out. • The final electron energy depends on its binding energy to the metal and the photon energy. • The intensity of the light is directly related to the total number of photons.

  7. Particle-Wave Duality • Is light made of waves or particles? How do we reconcile the double slit experiment with the photoelectric effect. • One can send one photon at a time to do the double slit experiment. At first, the photon goes through one of the slits and hits the screen randomly. But repeating the same experiment many times, a pattern appears on the screen!

  8. Thus the light is made of many small particles (photons), but the behavior of these particles do not follow Newton’s mechanics, rather they move in space like a wave. Particle-Wave Duality! • The interference of the light is the interference of the moving behavior of the photons. • The energy in light comes in discrete units of hf.

  9. Electron Wave • Electrons, classically considered as particles, also have the properties of wave. Postulated by de Broglie (1924) Verified by Davisson and Germer (1925, Bell Lab)  Quantum Mechanics

  10. Electron interferometer

  11. Neutron diffraction Electron diffraction

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