Abstract: The effect of phase-independent response of a laser beam focused on a ferrofluid slab in sensing the 50 Hz magnetic field was reported.The refraction angles of a laser beam of 633nm wave- length focused on a slab of ferrofluid only respond to the magnitude of the magnetic field. Transient responses of the ferrofluid to switching magnetic field or light on/off were also measured to search the mechanism of the effect. A model of magnetic entropy conservation-based positive feedback in density redistributions of the magnetic particles in the ferrofluid is proposed for explaining this effect. This model also predicts a magneto-thermal instability in the ferrofluid.