We examined the optical anisotropy of a magnetic fluid film under magnetic fields parallel to the plane of the film by detecting its birefringence effect. It was found, that the transmission axis of the transmitted light was rotated significantly by an angle An, with respect to the axis of the linearly polarized incident light when an external magnetic field is applied. The angle Δη increases with an increase of the field strength and then becomes saturated for higher fields. By comparing the detected Δη's with the corresponding structures formed in the magnetic fluid film under various magnetic fields, the angle Δη was found to be proportional to the ratio of the chain area to the liquid area in the film.
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