Young's Double Slit Interference (AQA A Level Physics)

• Young’s double slit experiment demonstrates how light waves produced a diffraction pattern
• The experimental setup and results are shown below

Young's double slit experiment and the resulting diffraction pattern

 Young’s double-slit experiment arrangement. The screen showed fringes of light - this was a diffraction pattern

• A monochromatic source was used to ensure that the two rays were coherent
  • The resulting pattern on the screen showed an interference pattern
• This was in disagreement with corpuscular theory, which would have predicted only two bright regions
  • If you fired paintballs through two gaps at a wall, you would expect only two separate regions of the wall to have paint on them

Results predicted by Newton's corpuscular theory

Particle-like behaviour predicts only two bright regions and cannot account for an interference pattern that Young observed

• The only explanation for the interference pattern was that light diffracted through the thin slits, like a wave
  • This was evidence in support of Huygens' wave theory of light

A diagram showing the wave explanation of Young's interference pattern

Light behaving as a wave could adequately explain the observed interference pattern. Bright fringes showed where the two coherent sources constructively interfered and dark fringes showed destructive interference.

• This experiment was definite evidence for the wave theory of light as opposed to the corpuscular theory
  • Corpuscular theory, however, was not immediately rejected
• It took further experiments for the scientific community to consider wave theory as the accepted theory of light
  • The most important of these was Hippolyte Fizeau's experiment to determine the speed of light in water