We studied the dynamics of a chemical vortex whose core was disturbed by visible light. The observed sharp change of the shape of the vortex and the 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.