Engineered colloidal suspensions of nano-sized particles in base fluids (nanofluids) have shown promise in heat transfer research as a potential for future heat transfer fluid. However, the conflicting reports on the thermal properties of these fluids necessitate a detailed look into the physical mechanisms behind this enhancement. As heat transfer is a surface phenomenon, near-wall fluid flow and temperature measurements could provide considerable insight in this regard. Optical methods are commonly used to probe the flow at micro/nano scales. As the accuracy of these measurement techniques depend on knowledge of the optical properties of the fluid under consideration, a detailed understanding of these properties is required. For example, evanescent wave-based nanoparticle image velocimetry (nPIV) can be an effective tool in exploring near-wall nanofluid velocity/temperature field. In this method, an evanescent wave illumination is used to measure near-wall velocity fields with an out-of-plane resolution of the order of O(100 nm). In this study, optical properties of SiO2-water nanofluids at various particle concentrations varying from 0 to 6% by weight are investigated. Measurements of refractive indices and the optical transmittance of nanofluids, which are directly related to the depth of penetration in nPIV measurements are carried out. The effect and the correction required in the nPIV measurements due to the modification in optical properties of nanofluids are discussed. The results obtained demonstrate that addition of nanoparticles exhibit a change in optical properties that could affect optical-based measurement in nanofluids.