Phase opposition

When two waves begin exactly 1⁄2 cycle, or 180 degrees, apart, they are said to be in phase opposition. This is illustrated by the drawing of Fig. 1. In this situation, engineers sometimes also say that the waves are out of phase, although this expression is a little nebulous because it could be taken to mean some phase difference other than 180 degrees.
If two sine waves have the same amplitude and are in phase opposition, they will exactly cancel each other out. This is because the instantaneous amplitudes of the two waves are equal and opposite at every moment in time.
If two sine waves have different amplitudes and are in phase opposition, the peak value of the resultant, which will be a sine wave, is equal to the difference between the peak values of the two composite waves. The phase of the resultant will be the same as the phase of the stronger of the two composite waves.
The sine wave has the unique property that, if its phase is shifted by 180 degrees, the resultant wave is the same as turning the original wave “upside-down.” Not all waveforms have this property. Perfect square waves do, but some rectangular and sawtooth waves don’t, and irregular waveforms almost never do.

Fig. 1: Two sine waves in phase opposition.