Interference of Waves

When two waves cross, their heights (amplitudes) add.  The following applet shows the addition of two red waves as a blue wave (bottom).  

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Java applet showing interference of two waves.Use the slider bar to shift the second red wave and see the effect on the blue (sum) wave.  The shift is called the phase of the wave and can range from 0 to 360 degrees.
If the two red waves are in phase (i.e. phase = 0o) then the addition is a bigger blue wave. This is called constructive interference.

If the red waves are out of phase (i.e. phase = 180o) then the addition results in no wave at all. This is called destructive interference as the wave vanishes.

Interference occurs in quantum physics whenever there is more than one possible path or process from a starting point to the final point.  The interference may be due to adding waves associated with a particle from two separate paths (as in the electron waves in the Introduction section).  It could also arise in more subtle situations such as when there are two different ways an electron can go from one orbit to another in an atom.