  We investigate how the orbital angular momentum (OAM) of a paraxial light
beam is affected upon reflection at a planar interface. Theoretically, the
unavoidable angular spread of the (paraxial) beam leads to OAM sidebands which
are found to be already significant for modest beam spread (0.05). In analogy
to the polarization Fresnel coefficients we develop a theory based upon spatial
Fresnel coefficients; this allows straightforward prediction of the strength of
the sidebands. We confirm this by experiment.
