Science Fair Project Encyclopedia The phase of a wave relates
the position of a feature, typically a peak or a trough of the
waveform, to that same feature in another part of the waveform
(or, which amounts to the same, on a second waveform). The phase
may be measured as a time, distance, a fraction of the
wavelength, or as an angle. A phase shift is simply a difference
or change in phase. It is an essentially abstract and arbitrary
notion with no absolute meaning. To get a grasp of it, consider
the two waves A and B in this diagram: It is apparent that the
positions of the peaks (X), troughs (Y) and zero-crossing points
(Z) of both waves all coincide. The phase difference of the
waves is thus zero, or, the waves are said to be in phase. If
the two in-phase waves A and B are added together (for instance,
if they are two light waves shining on the same spot), the
result will be a third wave of the same wavelength as A and B,
but with twice the amplitude. This is known as constructive
interference. Now consider