In the paper, a new method of experimental study based on the induction of a so-called thermal lens (TL) by the focused high-level laser radiation has been put forward for magnetic fluids. This TL is a region with a spatially changing refractive index, which occurs due to the redistribution of the nanoparticle concentration in the material and the thermal expansion of the carrier fluid. This region was illuminated by the low-level light emission of an auxiliary laser. The optical response of the medium was recorded as diffraction patterns formed from each of these two beams. The shape of the diffraction pattern in the auxiliary beam was shown to depend on the angle at which it was directed to the TL. The diffraction spot sizes were found to depend on the strength of the applied external magnetic field.