[en] We present a discussion of the physical significance of optical-potential ambiguities, motivated by recent successes in determining unambiguous potentials for certain ''transparent'' light-ion systems like ¹²C + ¹²C. These ambiguities turn out to be a direct consequence of the occurrence of nuclear rainbows, whose signature is a series of broad Airy maxima and minima in the farside component of the corresponding scattering amplitude. In most heavy-ion angular distributions measured so far, these maxima are badly obscured by superimposed higher-frequency Fraunhofer oscillations; consequently rainbows in general, and their role in discrete ambiguities in particular, have frequently gone unnoticed. A study of several recent cases shows the nearside component of the elastic scattering amplitude to be essentially independent of V_o, the depth of the real part pf the optical potential, while the entire farside Airy pattern shifts to larger angles with increasing V_o. The discrete potential ambiguities occur simply because appropriate increases in V_o can shift Airy minimum n to position n + 1,n + 2, etc, bringing the farside back in phase with itself and so restoring the forward section of the angular distribution. This occurs even when these minima are ''lost'' under Fraunhofer fringes which obscure the underlying rainbow phenomenon. This rainbow mechanism is found to be equivalent to the increment of π in low-l phase shifts; physically, it unambiguously signals deep interpenetration of the two nuclei during the scattering. (author)