This applet shows you how an interference pattern is formed by the constructive and destructive inteference of waves from two point sources.

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Constructive interference occurs when 2 crests (red arcs) or 2 troughs (blue arcs) meet, resulting in a bright fringe.
Destructive interference occurs when a crest (red arc) and a trough (blue arc) meet, resulting in a dark fringe.

Distance between any 2 bright fringes x = λD/d
where λ = wavelight of light, D = distance from slits to screen, d = distance between the 2 slits.

Right-click here to download this page and the Java Class File.