Diffraction and Nature of Light

• What is diffraction


  Diffraction is the spreading out of a wave as it passes through a gap
• What must be met for maximum diffraction to occur


  The size of the gap must be of the same magnitude as the wavelength of the wave
• What happens when the gap is smaller than the wave

  The wave will be reflected instead
• Diffraction grating equation


  n (lander) = dsin0
• What does electron diffraction provide evidence for


  The wave nature of electrons, it suggests particles can demonstrate wave like properties
• Diffraction pattern produced by electrons

  Concentric circles of bright and dark fringes from a central bright point
• Whats the name given to the wavelength of a particle


  De Broglie wavelength
• What two factors does de broglie wavelength depend on


  • Mass
  • Velocity
• De broglie wavelength equation


  (lander) = h / mv
• Whats the process of pulse - echo technique


  • A wave pulse is emitted
  • It is transmitted and reflected at the boundry between the two media
  • The returning wave is detected
  • The speed and time taken are used to calculate the distance to the object
• What model does the photoelectric effect provide evidence for


  Particle model
• Outline the photoelectric effect


  • Light is shone on a metal plate
  • If the light has a high enough frequency electrons are emitted from the metal surface
  • If the frequency is too low no electrons are emitted
• How do you calculate energy of a photon


  E = hf
• Explain how a photon can liberate an electron


  One photon interacts with one electron and will transfer all its energy to it. If this energy is greater than the work function of the metal the elctron will have sufficient enough energy to be released
• What is threshold frequency


  The minimum frequency of light that a photon requires to liberate an electron from its surface
• Why are photoelectrons emitted with a range of kinetic energies


  The electrons are at different depths in the metal so require different amounts of energy to be liberated. The excess energy from a photon once an electron has been liberated is the kinetic energy of the electron
• What happens when electrons transition between energy levels


  • If electrons move to higher energy levels, radiation must be absorbed
  • If electrons move to lower energy levels, radiation is emitted
• Why can only certain frequencies of radiation be absorbed by an atom to cause an electron to transition


  The electrons can only exist in discrete energy levels, so the energy of the photon absorbed must be the exact amount of energy required to cover the difference between the two discrete energy levels
• Evidence light is a wave

  • Reflection
  • Refraction
  • Diffraction
  • Interference
  • Polarisation
• Evidence light is a particle


  • Reflection
  • Refraction
  • Photoelectric effect
• Atomic absorption spectrum


  • An electron absorbs a photon
  • The electron is excited and moves to a higher energy level
  • Each photon has a specific energy, E = hf, only discrete energy changes are possible
  • The photon energy must be equal to the difference in energy levels
• Atomic emission spectrum


  • When an atom is in an excited state an electron can move to a lower energy level
  • As it does the electron emits a photon of electromagnetic radiation
  • Only specofoc energy changes are possible
  • The frequency of this photon depends on the difference in discrete energy levels
• De Broglie equation


  λ = h/mv

  The de Broglie wavelength indicates the length scale at which wave-like properties are important for that particle.
• Work function


  Minimum amount of energy needed by an electron to escape from a surface of a metal
• Wave particle duality


  • The concept that all matter and energy exhibit both wave like and particle like properties.
  • Interference and diffraction show wave behaviour
  • The photoelectric effect shows particle behaviour
  • Electron diffraction shows the wave nature of electrons
• Interference patterns


  • Get sharper when you diffract through more slits
  • The spread of the diffraction pattern depends on the relative sizes of the wavelength and the slit width
  • Shining white light through a diffraction grating produces a spectra
• Huygens construction


  • Every point on a wavefront may be considered to be a point source of secondary wavelets that spread out in the forward direction at the speed of the wave
  • The new wavefront is the surface that is tangential to all of these secondary wavelets