The phase gratings have been recorded in silver halide emulsion (Slavich PFG-01) by utilizing a two-beam laser interferometric technique and Kodak DA-163 as a developer. Such gratings are observed to conform to both the surface relief and refractive index modulations. In the present work, we have studied the contribution of refractive index modulations to diffraction efficiency of the phase gratings for a broad range of exposure time (30-180 s) of recording. It has been observed that contribution of index modulation to diffraction efficiency of the gratings increases with increasing time of exposure in the range of 30-90 s and then saturates. Contribution of relief modulation to diffraction efficiency is also discussed.