  The electromagnetic form factor of a confined quark-antiquark pair is
calculated within the framework of point-form relativistic quantum mechanics.
The dynamics of theexchanged photon is explicitly taken into account by
treating theelectromagnetic scattering of an electron by a meson as a
relativistic two-channel problem for a Bakamjian-Thomas type mass operator.
This approach guarantees Poincare invariance. Using a Feshbach reduction the
coupled-channel problem can be converted into a one-channel problem for the
elastic electron-meson channel. By comparing the one-photon-exchange optical
potential at the constituent and hadronic levels, we are able to unambiguously
identify the electromagnetic meson form factor. Violations of
cluster-separability properties, which are inherent in the Bakamjian-Thomas
approach, become negligible for sufficiently large invariant mass of the
electron-meson system. In the limit of an infinitely large invariant mass, an
equivalence with form-factor calculations done in front-form relativistic
quantum mechanics is established analytically.
