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The dynamic photoelastic technique is employed to visualize and quantify the propagation properties of backward Lamb waves in a plate.Higher energy leakage of second-order symmetric backward wave mode S2b in contrast to third-order anti-symmetric backward mode A3b is shown by the dispersion curve of a plate immersed in water,and then verified by experiments.To avoid the considerable high leakage,the plate is placed in air,both group and phase velocities of modes S2b and A3b in the glass plate are experimentally measured.In comparison with the theoretical values,less than 5％ errors are found in experiments.