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 the Figure. In this situation, engineers sometimes also say that the waves are out of phase, although the expression is a little nebulous as 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 cancel each other out. This is because instantaneous amplitudes of the 2 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, that will be a sine wave, is equal to the difference between the peak values of the two composite waves. The phase of resultant will be 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°, the resultant wave is the same as turning the original wave upside-down. Not all wave- forms have this property. Perfect square waves do, but some of the rectangular and sawtooth waves do not, and irregular waveforms almost never do.