<Dd> x (t − 1 4 T) = A ⋅ cos ⁡ (2 π f (t − 1 4 T) + φ) = A ⋅ cos ⁡ (2 π f t − π 2 + φ) (\ displaystyle (\ begin (aligned) x \ left (t - (\ tfrac (1) (4)) T \ right) & = A \ cdot \ cos \ left (2 \ pi f \ left (t - (\ tfrac (1) (4)) T \ right) + \ varphi \ right) \ \ & = A \ cdot \ cos \ left (2 \ pi ft - (\ tfrac (\ pi) (2)) + \ varphi \ right) \ end (aligned))) </Dd> <P> whose "phase" is now φ − π 2 (\ displaystyle \ scriptstyle \ varphi \, - \, (\ frac (\ pi) (2))). It has been shifted by π 2 (\ displaystyle \ scriptstyle (\ frac (\ pi) (2))) radians (the variable A (\ displaystyle A) here just represents the amplitude of the wave). </P> <P> Phase difference is the difference, expressed in degrees or time, between two waves having the same frequency and referenced to the same point in time . Two oscillators that have the same frequency and no phase difference are said to be in phase . Two oscillators that have the same frequency and different phases have a phase difference, and the oscillators are said to be out of phase with each other . </P> <P> The amount by which such oscillators are out of phase with each other can be expressed in degrees from 0 ° to 360 °, or in radians from 0 to 2π . If the phase difference is 180 degrees (π radians), then the two oscillators are said to be in antiphase . If two interacting waves meet at a point where they are in antiphase, then destructive interference will occur . It is common for waves of electromagnetic (light, RF), acoustic (sound) or other energy to become superposed in their transmission medium . When that happens, the phase difference determines whether they reinforce or weaken each other . Complete cancellation is possible for waves with equal amplitudes . </P>

What does it mean for waves to be in phase
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