RP02 - Interference Effects

• What safety precautions should be taken when operating a laser?

  • Never look directly at the beam
  • Ensure no reflective surfaces are in the laser's vicininty
  • Display a warning notice so others know that a laser is in use
• Why should the screen, on which you use to display the interference patterns, have a matt finish?

  A matt screen should be used to reduce the likelihood of the laser beam reflecting from the screen into someone's eye and causing harm.
• Describe how you should measure the fringe spacing of an interference pattern


  When measuring the fringe spacing, you should measure across a large number of fringes then divide by the number of spaces measured across.
• Suggest a measuring instrument you could use to measure the slit separation

  A Vernier Calliper
• Suggest a measuring instrument that could be used to measure the distance from the slit to the screen

  A metre ruler
• How can the wavelength of the laser be determined from a graph of fringe width against the distance from the screen?

  • The gradient of the graph will be w/D
  • Multiplying the gradient by the slit separation will get you the wavelength
• What does it mean if two light sources are coherent?

  Coherent sources have the same wavelength and a constant phase difference
• What is monochromatic light?

  Monochromatic light is the light of a single wavelength or frequency
• What happens to light as it passes through a slit?

  • Light diffracts as it passes through a slit
  • The maximum diffraction occurs when the slit spacing matches the light's wavelength
• Why do bright fringes form on a screen when light is passed through a double slit?

  • The light diffracts as it passes through the slits and these diffracted waves meet and undergo superposition
  • At positions where the waves meet in phase, constructive interference occurs and bright fringes are formed
• Why do dark fringes form on a screen when light is passed through a double slit?

  • The light diffracts as it passes through the slits and these diffracted waves meet and undergo superposition
  • At positions where the waves meet in anti phase, destructive interference occurs and dark fringes are formed
• In a double slit experiment, why might the light be passed through a single slit before the double slit?

  • The single slit acts as a point source, and diffracts the light widely so it can pass through the double slits
  • This means that both slits are illuminated by the same source
  • They therefore have the same wavelength and a constant phase difference
• Describe the interference pattern produced if white light is passed through the double slits

  • A central bright white fringe will be produced
  • All other bright fringes would consist of a spectra of light, with blue light on the side closest to the centre and red on the far side of each fringe
• How is a diffraction grating's spacing calculated?

  The diffraction grating spacing is the reciprocal of the number of lines per metre
• How can the angle from the centre to a higher order line be calculated?


  • The distance from the diffraction grating to the screen (D) and the distance from the centre of the pattern to the higher order line (h) can be measured
  • The angle can then be calculated via trigonometry
• How does a diffraction pattern from a double slit experiment compare to one from a diffraction grating?

  • As the number of slits increases, the pattern becomes sharper and the brightness of the fringes increases.
  • This is because more rays are reinforcing the pattern
• Suggest a use for a diffraction grating

  • Diffraction gratings can be used instead of prisms to analyse spectra
  • e.g. X-Ray Crystallography