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  1. 1. diffraction
  2. 2. <ul><li>Definitions </li></ul><ul><li>Single slit diffraction </li></ul><ul><li>Multiple slits </li></ul><ul><li>Diffraction grating </li></ul><ul><li>Applications </li></ul>
  3. 3. Definition <ul><li>Diffraction = waves spread out as pass aperture </li></ul><ul><li>e.g. sound </li></ul><ul><li> water </li></ul><ul><li> radio waves </li></ul><ul><li>Amount diffraction depends on ratio of  to aperture width, a </li></ul>
  4. 4. Diffraction of light - single slit <ul><li> light 400 – 700 nm </li></ul><ul><li>V. small slit needed! </li></ul><ul><li>Huygen’s principle – predicts future position of wavefront </li></ul><ul><ul><li>each point on wavefront is a point sources of wavelets </li></ul></ul><ul><ul><li>Wavelets superpose + interfere to form future wavefronts </li></ul></ul>
  5. 6. <ul><li>Diffraction pattern = series of bright and dark fringes </li></ul><ul><li>Central maximum </li></ul><ul><li>Secondary maxima </li></ul>
  6. 7. <ul><li>Red filter used … </li></ul><ul><li>More diffraction if </li></ul><ul><ul><ul><li> increased </li></ul></ul></ul><ul><ul><ul><li>Aperture width decreased </li></ul></ul></ul><ul><li>What would pattern look like if blue filter used? </li></ul>
  7. 8. Diffraction - multiple slits <ul><li>Effects similar to Young's interference pattern </li></ul><ul><li>As number slits increases, maxima = brighter + sharper </li></ul><ul><li>Each slit acts as point source of 2 ° wavelets </li></ul><ul><li>individual diffraction patterns  interference </li></ul><ul><li>Superposition of single slit pattern on multiple patterns </li></ul>
  8. 10. Diffraction grating <ul><li>Transmission grating </li></ul><ul><li>Reflection grating </li></ul><ul><li>Use: produce spectra to measure  accurately </li></ul>2 nd order 1 st order Zero order 1 st order 2 nd order Laser
  9. 11. <ul><li>Coarse/fine grating </li></ul><ul><li>Increasing coarseness = increasing number of orders (and orders closer together) </li></ul>
  10. 12. Theory
  11. 13. <ul><li>Angle  is such that wave B in phase with wave A </li></ul><ul><li> path difference is </li></ul><ul><ul><ul><ul><li> for 1 st order principle maxima </li></ul></ul></ul></ul><ul><ul><ul><ul><li>2  for 2 nd order </li></ul></ul></ul></ul><ul><ul><ul><ul><li>3  for 3 rd order </li></ul></ul></ul></ul><ul><ul><ul><ul><li>n  for n th order </li></ul></ul></ul></ul>
  12. 14. <ul><li> ABC: </li></ul><ul><ul><ul><li>Sin  = AC = AC = n  </li></ul></ul></ul><ul><ul><ul><li> AB d d </li></ul></ul></ul><ul><ul><ul><li>Or n  = d Sin  </li></ul></ul></ul>
  13. 15. Spectrometer <ul><li>Collimator  parallel light  diffraction grating </li></ul><ul><li>Pattern observed </li></ul><ul><li> measured  find  </li></ul>
  14. 16. Applications <ul><li>Accurate method of measuring  </li></ul><ul><li>Can separate  s close together </li></ul><ul><li>Identify v. small quantities of material since different elements = different spectra </li></ul><ul><li>Analysis of light from stars, nebulae and interstellar gas  determine of structure </li></ul>