http://micro.magnet.fsu.edu/primer/lightandcolor/interferenceintro.html Thomas Young's Double Slit Experiment
Photograph of real fringe pattern for Young’s double slit
The two waves travel the same distance –Therefore, they arrive in phase S S'S'
The upper wave travels one wavelength farther –Therefore, the waves arrive in phase S S'S'
The upper wave travels one-half of a wavelength farther than the lower wave. This is destructive interference S S'S'
Young’s Double Slit Experiment provides a method for measuring wavelength of the light This experiment gave the wave model of light a great deal of credibility. Uses for Young’s Double Slit Experiment