• Earlier experiments that attempted to show the interference of light using two sources of light some distance apart were unsuccessful because the sources of light were out of phase and the wavelength of light is so small

Thomas Young’s successful experiment used diffraction through a double slit to create two sources of light from a single coherent source of light so that the sources were close together and in phase

• Young’s experiment produced a series of light and dark fringes on a screen placed in the path of the light, with a pattern that resembled the results of interference of water waves in a ripple tank

  

Bright vs Dark Fringes

Areas of constructive interference between the two diffracted rays created bright fringes, while areas of destructive interference between the two created dark fringes

• For a setup with slit separation $d$ and light of wavelength $\lambda$, the locations of bright fringes are given by $d \sin \theta = m \lambda$, for $m$ = 0, 1, 2, 3...
• The locations of dark fringes are given by $d \sin \theta = (n - \frac{1}{2}) \lambda$ for $n$ = 1, 2, 3

<aside> 📌 For a viewing screen at a distance L, the $m$th-order bright fringe is at location $x_m = \frac{mL\lambda}{d}$, and the $n$th-order dark fringe is at location $x_n = (n - \frac{1}{2})\frac{L\lambda}{d}$

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• When $\triangle x$ is given, no need for $n-\frac{1}{2}$ or $m$, use $\triangle x = \frac{L\lambda}{d}$
• Young’s diffraction experiment was evidence for the wave nature of light
  • The wave theory could explain all properties of light except its propagation through a vacuum