physics

• Diffraction is a phenomenon which only a wave theory can explain satisfactorily
• Diffraction of light


  1. If a set of waves strike a barrier which they cannot penetrate, they will reflect or be absorbed

  2. If there is a small gap in the barrier, the little part of each wave which 'sneaks through' now behaves very curiously

  3. Once through the barrier the energy is radiated as if it is coming from a 'point source' of waves

  4. A new set of waves radiate outwards in a circular pattern

  
• You can see diffraction occur if you watch water waves entering a harbour
• Wave interference after diffraction

  The 2 (or more) sets of spreading waves now meet each other and wave interference occurs according to the 'superposition' principle

  
• If light waves are diffracted, then projected onto a screen, or captured on photographic film, an interference pattern appears, usually in a line of light spots (where waves add together constructively) and dark zones (where waves are cancelling)

  
• The 'classical' physics of Newton's Laws cannot explain diffraction if light is a stream of particles
• Hygen's wave theory of light can easily explain diffraction behaviour because part of the theory is that every point on a wave front acts as a 'point source' of new waves

  
• Young's double slit experiment


  1. Allowed the interference of light to be observed

  2. Provided evidence that light travelled as a wave

  
• Newton argued that the properties of light could only be explained if it were made of particles (corpuscles), and follow the same laws of motion and gravitation that all bodies with mass do
• Newton could explain the law of reflection, refraction, and dispersion of light using his particle model
• Newton could explain polarisation by stating that light particles must have 'sides'

  
• Huygens' principle


  • Each point on a wave behaves as a point source for waves in the direction of propagation
  • The line tangent to these circular waves is the new position of the wave front a short time later

  
• Huygens' wave model could explain reflection, refraction, and diffraction of light

  
• In unpolarised light, the electric field oscillates in any direction, perpendicular to the magnetic field
• Polarisation means taking unpolarised light and restricting the electric component to oscillate in a single direction only

  
• As polarised light passes through one polariser only, its intensity is halved

  
• Wavelength of light


  Determined to be between 400 and 700nm

  
• With such small wavelengths, very little diffraction would occur
• Malus' Law


  1. I = Imaxcos θ

  2. For plane polarisation of light

  3. To evaluate the significance of polarisation in developing a model for light

  
• Young's proof was based geometrically, the Law of Malus was based on vector quantities, assuming that light is a transverse wave

  
• Unpolarised light


  Electric field oscillates in any direction, perpendicular to the magnetic field
• Polarisation


  Taking unpolarised light and restricting the electric component to oscillate in a single direction only

  
• Malus' Law


  • I = Imaxcos θ
  • θ is the angle between the direction of polarised light and polariser

  
• Unpolarised light with intensity 64Wm−2 is passed through a pair of polarisers


  • First is vertical, second is 45° to the vertical
  • Final intensity of the polarised light is 1/6

  
• Max Planck is considered to be the initial founder of the quantum theory of energy, discovering and publishing his work in the 1890s
• Planck discovered the quantum theory through attempting to mathematically explain black body radiation
• Black body


  An idealised physical body that absorbs all incident EMR (of all frequencies), and re emits all this EMR
• Planck's theory solved the UV catastrophe where experimental data and theories clashed when discussing black body radiation
• Planck's constant


  Relates the energy in one quantum of electromagnetic radiation to the frequency of that radiation

  
• Wien's Law


  • λmax = b/T
  • b = Wien's displacement constant = 0.898 × 10−3
  • T = temperature in kelvin
• Wien's Law was found to be in perfect agreement with the experimental data from actually measuring the wavelengths of radiation from hot objects

  
• Below a certain 'threshold' frequency of light, NO electrons were emitted at all, even if the intensity of the light was very high

  
• Above that threshold frequency, photoelectrons WERE emitted, even if the intensity was very low (different threshold frequency for each material)

  
• At any given frequency above the threshold, increasing the intensity of the light caused MORE photoelectrons to be emitted, but each one still had the same maximum kinetic energy

  
• Increasing the frequency of the light increased the max KE of the photoelectrons
• Einstein's photon model


  • Light can only exist in discrete quantities or packets called photons
  • Each photon has an energy determined by the frequency EMR
  • All or nothing - photons can interact with atoms in surface and be absorbed or reflected as whole photons

  
• Photoelectric effect


  • Photons that strike metal surface can be reflected or absorbed (reflected do not contribute)
  • An absorbed photon's energy is given to the electron
  • Electron can be excited to a higher energy level or ejected from the surface (photoelectron)
  • KE = hf - φ (work function)

  
• Einstein discovered that the speed of light in a vacuum is an absolute constant
• Time dilation, length contraction and mass dilation ensures that light in a vacuum must always travel at speed c relative to any observer and independent of the state of motion of the emitting body

  
• Diffraction


  A phenomenon which only a wave theory can explain satisfactorily