Extinction of polarized light in ferrofluids with different magnetic particle concentrations

► A ferrofluid with magnetite nanoparticles dispersed in transformer oil was investigated. ► Magnetic induced change in transmittance of light polarized at 54.74° was measured. ► A model for optical extinction in ferrofluids with particle agglomeration was developed. ► A polarization independent component of optical extinction coefficient was found, measurable at 54.74° polarization angle. ► Particle agglomeration and long-range particle correlations explain the experimental data.