The optical character of electrorheological (ER) and electromagnetorheological (EMR) fluids under the influence of an external field are investigated. It is found that the transmission of ER fluids could be adjusted through selecting the proper electric field and suspension concentration. The IR spectral transmissions of ER fluids increase with an applied electric field. The maximum transmittance for low-concentration fluid changed from 4% to 23%. It is also observed that the window of transmission does not change with fluid consistency and external electric field. We find that the scattering patterns of ER fluids consist of the many bright and dark bands as a scattering grating. The widths of bright bands increase with increasing electric field. The scattering pattern of EMR fluids under electric and magnetic fields superimposed in the perpendicular is also observed. These phenomena provide new mechanics for the design of tunable optical devices and will have direct industrial application.