If two oscillations reach their maximum displacement at the
same time, they are said to be in phase at that time
(if they also have exactly the same frequency, they will remain in
phase). 



If one oscillation is at its maximum displacement when
another is at its equilibrium position, the two
oscillations are said to have a phase difference of one quarter
of a time period (T/4). 



For many oscillations (a pendulum, a mass on a spring etc) a
graph of displacement, x against time, t is very similar to a graph
of sine of angle against angle, θ. 

For this reason, phase differences are usually expressed in
terms of angles rather than times. 



For example, the two oscillations shown below are T/4 or
π/2rads
(or 90°) out of phase with each other. 







Similarly, the next diagram shows two oscillations which are T/2
or πrads (or 180°) out of phase
with each other (in this case we also use the term “in antiphase”). 







As a continuous wave is a series of oscillations, having varying
phase relationships, the above ideas can also be applied to waves. 


