Young's Double Slits

[jmorris]

In 1801 Young demonstrated the interference (and diffraction) of light.
To do this coherent sources of light are needed. This was difficult to achieve at the time so one light source was used in front of 2 narrow slits.
Light is diffracted by the first slit, which acts as a point source of light emitting semi circular wavefronts. These wavefronts are diffracted by the each of the double slits, which act as two point sources of light which are coherent (constant phase difference, same frequency)
The two wavefronts overlap. If the waves arrive at the screen in phase, they constructively interfere producing a bright fringe. (the path difference between the waves =a whole number of wavelengths)
If the waves arrive out of phase by Pi radians or 180 degrees, they destructively interfere producing a dark fringe. (the path difference between the waves is a whole number +1/2 wavelength)
On the screen a pattern of evenly spaced dark and bright fringes appear. This is an interference pattern and demonstrated that light undergoes interference.

The demonstration of the diffraction and interference of light was evidence of wave theory.