In non-central relativistic heavy-ion collisions a very strong magnetic field is formed. There are

several studies of the effects of this field, where B is calculated with the expressions of classical

electrodynamics. A quantum field may be approximated by a classical one when the number of

field quanta in each field mode is sufficiently high. This may happen if the field sources are intense

enough. In heavy-ion physics, the validity of the classical treatment was never investigated. In

previous work, we proposed a test of the quality of the classical approximation. We calculated

an observable quantity using the classical magnetic field and also using photons as input. We

focused on the process in which a nucleon is converted into a delta resonance, which then decays

into another nucleon and a pion, i.e., N → ∆ → N'+π. In ultraperipheral relativistic heavy-ion

collisions this conversion can be induced by the classical magnetic field of one of the ions acting

on the other ion. Alternatively, we can replace the classical magnetic field by a flux of equivalent

photons, which are absorbed by the target nucleons. We calculated the cross-sections in these

two independent ways and found that they differ from each other by ~10 % in the considered

collision energy range. This suggests that the two formalisms are equivalent and that the classical

approximation for the magnetic field is reasonable.