We observe that the dynamics of vortices in paraxial optics, including
vortex pair-creation and pair-annihilation, can be represented in a s
implified manner by the motion of a locus obtained from the complex op
tical field, for which the real and imaginary parts of the field are e
qual. When the locus lies within the transverse plane a dark line is p
resent in the optical field with a phase change of pi across. When the
locus intersects the transverse plane, a vortex exists at the interse
ction point. Thus the locus line appears like a three-dimensional vort
ex line. Examples are given ranging from emission in low order laser m
ode families to patterns calculated for plane mirror resonators.