Imagine two pendulums swinging with a time period T for one complete cycle of 2 seconds.If one pendulum is 0.5 seconds behind the other one then the phase difference is a quarter of a cycle.Since one cycle is 2π radians, the phase difference will be (0.5/2.0) × 2π which is equal to ½π.The phase in the cycle is given by½π or (t/T) × 2π where T is the time period T for one cycle, t is the time from the start for which the pendulum has been swinging.phase = 2π(t/T) = 2πft = ωt 