We studied the dynamics of a chemical vortex whose core was disturbed by
visible light. The observed sharp change of the shape of vortex and
unfolding increase in the vortex wavelength proved to be tightly related
to a transformation of trigger-waves to phase-waves. Analytical
estimations and direct measurements show that the shape of the vortex is
well fitted by a logarithmic spiral. The results of experiments and of
computer simulations are in a good agreement.