Optical transmission changes in ferrofluids when exposed to magnetic fields are difficult to predict: under similar conditions, exposure to a magnetic field may cause increasing, decreasing or even non monotonous optical transmission evolution in different samples. Absence or presence of coalescence has been conjectured as the key phenomenon that causes the different behaviors, but to our knowledge without any theoretical support. In the first part of this work, experimental data are provided in order to illustrate the different possible trends that may be observed. In the second part, a set of some thousands of particles is considered in different arrangements: distributed at random, grouped in single chains or with aggregates of several chains. The optical transmission of each arrangement is obtained by means of the CST software. The numerical results obtained provide a theoretical connection between the optical transmission experimental trends observed and the processes of chain formation and aggregation by coalescence, which facilitates a much deeper understanding of the phenomena observed.
