Symmetric diblock copolymers are known to formlamellar structures in the bulk of an organic thin film. Polymer/polymer and polymer/substrate interfaces play a critical role in this application. Here, we report the investigation of multiple buried interfaces by using a novel technique resonant soft X-ray reflectivity which benefits from enhanced contrast between different polymers near the carbon K-edge. This allows us to obtain a precise interface structure. We also present an alternative method to determine optical constants of polymers by fitting X-ray reflectivity of polymers with known structural parameters at specific soft X-ray energies. This approach is compared with the way of obtaining beta by NEXAFS and calculating delta via the Kramers-Kronig relationship. Finally, by using the determined index of refraction, the precise structure of a multilayer formed by a diblock copolymer is obtained by successfully fitting the resonant soft X-ray reflectivity profile.