Two-wave interference: Young's slits
Light interference - Theory and applications

It was Thomas Young who, in 1801, discovered and correctly interpreted light interference. The experiment gained in luminosity by replacing the two holes with two slits perpendicular to the plane of the figure 1. Similarly, the source will be replaced by a thin slit parallel to the other two, illuminated with intense light. The S source is a narrow slit, which we'll discuss later in connection with spatial coherence. It illuminates the two slits F₁ and F₂ with two waves of the same amplitude and phase if S is on the bisector of F₁F₂ . Each slit diffracts light: the diffraction phenomenon can be interpreted in terms of interference between the infinite number of elementary sources that would have to be placed on slit F to replace source S. We won't go into this here. Suffice it to say that...