Particle nature of light

• Explain why excited atoms only emit certain frequencies of radiation.

  • Electrons exist in discrete energy levels
  • After excitation the electrons move to a lower energy level
  • so photons are emitted
  • Frequency of photons depend on the difference in energy between the energy levels
  • Since certain energies are possible, only certain frequencies are possible
• Photon

  Discrete packet of energy
• Work function

  minimum energy required to emit a photoelectron from a surface of a metal
• Why photocell works with Ultraviolet but not visible light

  • Ultra violet (less than 400nm) has a higher frequency than visible light

  small wavelength, high frequency

  • Visible light has a frequency lower than the threshold frequency or Ultra violet light has a greater frequency than the threshold frequency
• Increase in intensity without changing frequency

  Increase in photons per second

  increase in electrons emitted

  So current increases
• De brogiles wavelength

  λ = h/mv = h/p
• Energy of a photon

  E = hf
• hf = φ + KEmax
• Threshold frequency

  Minimum frequency of light required to emit a photoelectron
• No electrons are emitted when the frequency of the radiation is below a certain value

  • One photon interacts with one electron
  • Photon energy is equal to hf
  • Photon energy is less than the work function of the metal
• Explain how helium in the Sun's atmosphere caused this set of dark lines

  • Photon absorbs an electron
  • Electron moves to a higher energy level / Electron excited
  • where photon energy = difference in energy levels
  • Only certain changes possible
  • between discrete energy levels
• Suggest why the energy levels all have a negative value

  • Free electrons have zero energy state
  • Electrons need to gain energy to move to a higher energy level
• Photon model

  • One photon interacts with one electron
  • Energy of photon is equal to hf
  • Electron is emitted instantly if energy of photon is greater than the work function
  • Any photon energy over and above work function is gained by the electron as KE
• Wave model

  • It would be expected that the energy of the electron would build up and eventally be emitted
  • The energy of the electrons would depend on the intensity of the wave and not the frequency
• Find momentum when frequency of photon is known

  λ = h / p

  c = fλ

  p = hf/c
• Increase in intensity of light

  • No. of photons per second increase
  • since one photon interacts with one electron
  • No. of electrons per second increase
• Constant intensity

  • No. of photons per second is the same
  • No. of electrons per second remains the same
• Intensity of light effects current but doesn't effect kinetic energy max
• Explain why monochromatic light is important in diffraction experiments

  • Emits a small range of wavelengths
  • So small variation at each diffraction angle
  • Produces sharper interfernce patterns
• When superradiance occurs the atoms all absorb the same amount of energy.

  Explain how this results in all atoms emitting radiation of a particular frequency

  • Atoms (acting as photon) contain discrete energy levels
  • Atom loses energy and falls energy levels emitting a photon
  • Photon energy is equal to difference in energy levels
  • Energy of photon is proportional to frequency E = hf
  • so emitted frequency correspons todifference in energy levels of a particular atom
• Compare and contrast the photoelectric effect with the effect of radiation incident on an LDR

  • Electron absorbs a photon
  • Photon needs a minimum amount of energy
  • so light must be above a certain frequency
  • An increase in intensity increases the no. of photons released per second
  • In photoelectric effect electrons are emitted from the surface
  • In an LDR the electrons remain inside