Features of the optical transmission by means of the thin ferrofluid (FF) layer in an external magnetic field are studied. The observed effect of optical-extinction trend inversion (OETI) occurs over time after the magnetic field switching on and switching off. The onset time of OETI depends on the magnitude of field amplitude and on the wavelength $\lambda$ of the probing optical radiation. The OETI existence is associated with magnetic-nanoparticles’ clusters’ transformation (formation or destruction) dynamics in FF under the magnetic field action. At the time moment when the size of magnetic-nanoparticles’ clusters formed from FF, $D$, becomes commensurable with the wavelength $\lambda$ ($D_{1}$ = $\alpha\lambda$, where $\alpha$ is a dimensionless coefficient), the optical-radiation scattering and absorption effect becomes maximal (OETI occurrence moment), and then begins to decrease. The discussion of experimental results is based on the proposed model for the magnetic-nanoparticles’ clusters’ formation and transformation in FF under the external magnetic-field action. The magnetic-nanoparticles’ chains’ clusters’ lateral aggregation is an important entry of this model. The practical importance of obtained results (for instance, in clusters’ density determination, new-type composite materials’ creation, etс.) is considered.