Constructive/destructive interference
The following Java Applet shows two green waves of equal wavelength. The red wave is the new wave formed when the two green waves interfere. To move the green waves, click and drag the mouse anywhere around them.
The two white dots represent fixed points which the path difference is calculated from.
When the waves are a whole number of wavelengths apart constructive interference occurs, when the waves are exactly out of phase destructive interference occurs.