Please explain Young's double hole experiment?

Young's Double Hole Experiment

Thomas young in 1801 reported his experiment on the interference of light. He made a pinhole in a cardboard and allowed sunlight to pass through. This light was the allowed to fall upon another cardboard having two pinholes side by side places symmetrically. The emergent light was received on a plane screen placed at some distance. At a given point on the screen, the waves from the two holes had different phases. These waves interfered to give a pattern of bright and dark areas. The variation of intensity on the screen demonstrated the interferences taking place between the light waves reaching the screen from the two pinholes.

The pattern of bright and dark areas is sharply defined only if light of a single wavelength is used. Young's original experiments were performed with white light and he deduced from the experiments that the wavelength of extreme red light was around1/36000 inch and that of the extreme violet was around1/60000 inch. These results are quite close to their accurate measurements done with modern instruments.