Young's Double Slit Experiment (CIE A Level Physics)

• Young's double-slit experiment produces a diffraction and an interference pattern using either:
  • The interference of two coherent wave sources 
  • A single wave source passing through a double slit

• In this typical set-up for Young's double slit experiment:
  • The laser light source is placed behind the single slit
  • So the light is diffracted, producing two light sources at slits A and B
  • The light from the double slits is then diffracted, producing a diffraction pattern made up of bright and dark fringes on a screen

Young's double slit experiment

Young’s double-slit experiment arrangement creates an interference pattern when light is passed through slits A and B

• Calculations can be made for the double slit interference of light using the equation:

• Where:
  • λ = wavelength of source (m)
  • a = distance between the centres of the slit (m)
  • x = fringe width (distance between successive bright fringes) (m)
  • D = distance between the slits and the screen (m)

Quantities for double slit interference equation

Double slit interference equation with a, x and D represented on a diagram

• In this experiment:
  • D is much bigger than any other dimension, normally several metres long
  • s is the separation between the two slits and is often the smallest dimension, normally in mm
  • w is the distance between the fringes on the screen, often in cm. This can be obtained by measuring the distance between the centre of each consecutive bright spot.

• The above equation shows that the separation between the fringes will increase if:
  • The wavelength, λ of the incident light increases
  • The distance, s between the screen and the slits increases
  • The separation, w between the slits decreases