Quantum

• Photons


  Discrete energy quanta ('packets') of electromagnetic radiation
• Electromagnetic radiation


  • Travels through space as a continuous wave
  • Interacts with matter as discrete energy quanta (photons)
• Energy of a photon (E)

  Directly proportional to the frequency (f) of the electromagnetic radiation

  E = hf = hc/λ, where h is the Planck constant
• Electronvolt (eV)


  More appropriate unit of energy for photons than joules

  1 eV = energy transferred when an electron travels through a potential difference of 1 volt
• Using LEDs to estimate the Planck constant


  Set up potential divider to vary voltage through LED

  Determine threshold potential difference to turn on LED

  Equate energy of electron in LED to energy of photon produced

  Use equation eV = hc/λ to determine Planck constant
• Photoelectric effect


  When electromagnetic radiation is shone on a metal, electrons are released from the surface
• Observations of photoelectric effect

  • Visible light does not remove electrons, even at high intensity

  UV light removes electrons instantly, even at low intensity
• Work function (φ)


  Minimum energy required to free an electron from the surface of a metal
• Einstein's photoelectric equation


  hf = φ + KEmax, where KEmax is the maximum kinetic energy of the released electron
• Increasing intensity of radiation above threshold frequency increases rate of electron emission, but does not increase kinetic energy of electrons</b>
• Wave-particle duality


  Electromagnetic radiation and matter can exhibit both wave and particle properties
• de Broglie equation


  λ = h/p = h/mv, where λ is the wavelength associated with a particle, p is its momentum, and m is its mass
• Evidence for wave-particle duality

  • Diffraction of electrons by thin graphite

  Diffraction is a wave property, but electrons are classified as particles