Abstract
The transmission of light through ionic ferrofluids, in which the viscosity of the carrier liquid can be modulated, is measured under an applied magnetic field. The experimental results show that the transmission intensity would not vary monotonously with time when an external magnetic field is applied. The intensity can get to a stable value if the duration for which the magnetic field applied is long enough. The relaxation time of the variation of the light intensity with time is proportional to the viscosity of the carrier liquid and inversely proportional to the strength of the field. The character of the transmitted light is explained according to a model of microstructured transition relaxing in ferrofluids under an applied magnetic field.