Radiation and Quantum Phenomena

• Are EM waves transverse or longitudinal?

  Transverse
• What phenomenon can be used to show that light behaves as a particle?

  The photoelectric effect.
• Describe the photoelectric effect.

  When light above a particular threshold frequency is shone on a metal, electrons are released - these electrons are "photoelectrons".
• What is threshold frequency?

  The minimum frequency of light required for a photoelectron to be emitted.
• What equation determines the energy of a photon?

  E = hf
• Why does a photon need to have a minimum frequency in order to liberate and electron?

  The energy of the photon is determined by its frequency, the photon's energy must be greater than the work function in order for an electron to be emitted. 
• If a photon has a frequency higher than the threshold frequency, what would occur?

  The electron would be liberated and the remaining energy is the kinetic energy of the electron.
• If light is incident on a metal and photoelectric emission does NOT occur, what is the effect of increasing light intensity?

  Higher intensity would mean more photons incident on the metal each second. Each photon would still have the same amount of energy as before therefore it still cannot liberate an electron.
• What is the photoelectric equation?

  hf = phi + Ek(max)
• Determine the work function.

  The energy required by an electron to overcome the metallic bond holding it in the metal.
• What is an electron volt?

  The kinetic energy of an electron that has been accelerated from rest through a potential difference of 1V.
• How do you convert electron volts to joules (and vice versa)?

  eV * 1.6*10^-19 = J

  J / 1.6*10^-19 = eV
• How does a flourescent tube work?

  High voltage applied across gas vapour accelerates fast moving free electrons which collide with the mercury atoms.

  Mercury electrons are excited and then return to the ground state, releasing a UV photon.

  The tube's coating absorbs the UV photons and its electrons are excited, they cascade down the energy levels and emit visible light photons.
• What can be used as evidence for the discrete energy levels in atoms?

  Line emission and absorbtion spectra as the lines appear at discrete points which show where a light photon of specific frequency and wavelength has been absorbed or emitted, this shows electrons can only absorb an exact amount of energy to be excited to the next discrete energy level.
• What is wave particle duality?

  All particles have both particle and wave properties, waves can have particle properties e.g. light acts as a particle in the photoelectric effect and as a wave when it is diffracted.
• What is the equation for de Broglie wavelength?


  wavelength = h / mv

  where mv is momentum.