It is pointed out that the additional a-o diffraction is caused by reflected acoustic waves, and this phenomenon is explained from the following aspects: the distribution of reflected acoustic waves (longitudinal and transverse waves), their intensities, the diffraction angles of light, and the difference of a-o diffraction characteristies between acoustic longitudinal waves and transverse waves for the plane-polarized light. The results of experiments are in accordance with the analytical ones. The author believes that using these phenomena, an effective method for checking sound absorption can be provided.