, if the two needles are going
up and down regularly) then the phase difference f at any point will not
change with time and we will have a stable interference pattern; i e , the
positions of maxima and minima will not change with time However, if
the two needles do not maintain a constant phase difference, then the
interference pattern will also change with time and, if the phase difference
changes very rapidly with time, the positions of maxima and minima will
also vary rapidly with time and we will see a “time-averaged” intensity
distribution