Quantum

• Photoelectric effect


  Emission of photoelectrons from a metal surface when light shines on it
• Photoelectric effect


  • No photoelectrons emitted if radiation is below threshold frequency
  • Photoelectrons emitted with a variety of kinetic energies, maximum energy increases with frequency
  • Intensity of radiation affects number of photoelectrons, not their kinetic energy
• Max Planck


  Proposed that electromagnetic waves can only be released in discrete packets (quanta)
• Einstein's photons


  Electromagnetic waves and the energy they carry are wave-packets called photons
• The energy of light causes electrons to be lost from a zinc plate by the photoelectric effect
• Work function


  Energy needed for an electron to leave the metal surface
• Electron volt (eV)


  Kinetic energy gained by an electron accelerated through a potential difference of 1 volt
• Energy levels in atoms


  1. Ground state

  2. Promotion to higher energy level

  3. Deexcitation causes electron to cascade down energy levels
• Ionisation energy is the energy needed to remove an electron from an atom
• Photon emission in fluorescent tubes


  1. High voltage accelerates electrons that ionise mercury atoms

  2. Free electrons collide with mercury electrons, exciting them and releasing UV radiation

  3. Phosphor coating absorbs UV and emits visible light as electrons cascade down energy levels
• Light emission spectra


  • Each line corresponds to a particular wavelength of light
  • White light is continuous so all wavelengths merge together
• Line absorption spectra


  • Photons of the correct wavelengths are absorbed by electrons to excite them, these wavelengths are then missing from the spectrum
• Comparing emission and absorption spectra


  The lines in the same places show the energy differences of electron transitions are the same, the photons causing these have the same energy and wavelength
• Wave-particle duality


  Light can exhibit properties of both waves and particles (photons)
• Electron diffraction demonstrates the wave-like nature of electrons