3. Young’s Double Slit Experiment: This is an important historical experiment, because it provided convincing evidence that light was a wave. There was still a lot of controversy over light being a particle or a wave. Light from a single monochromatic source is incident on the barrier so the waves through the slit have the same frequency, same amplitude and same constant phase. Light incident on a pair of closely spaced slits generated a pattern of light and dark regions. This pattern is the result of wave interference. The slits act as point sources for spherical waves. (Huygen’s Principle) The two sets of identical waves now interact. This introduces varying degrees of constructive and destructive interference. The result is the light and dark fringe pattern, where the brightest part is complete constructive interference and the darkest part is complete destructive interference.