Particle size distribution (PSD) is essential information for those in the fields of engineering and environmental geosciences, sedimentology, and pedology. The PSD can be determined by conventional sieve, hydrometer, and pipette methods. However, since the 1980s, the PSD of soils has also been determined via the laser diffraction method, which is more reliable and faster and which can be used repeatedly. With this method there are two optical theories commonly used for the determination of PSD: the Mie and the Fraunhofer theories. The main difference between these theories is that the Mie theory requires the refractive index (RI) and absorption coefficient (AC) values of the particles and the RI value of the suspending medium, whereas the Fraunhofer theory does not. Although the Mie theory is more appropriate for soil samples, especially for the determination of clay-size fraction, there are not yet agreed-upon values of RI and AC for soils samples that can be used by researchers and practitioners. In this study, acceptable values for RI and AC were investigated extensively by analyzing 35 natural soil samples that were mainly of sedimentary origin. Based on the results obtained, the values of 1.55 and 0.1 for RI and AC, respectively, are suggested for laser diffraction analysis of naturally occurring sedimentary soil samples with respect to geotechnical practices. Furthermore, in order to demonstrate the pitfalls of using the Fraunhofer theory for the analysis of soil samples, especially for clay-size particles, the PSDs and clay-size fractions obtained from the Mie and Fraunhofer theories were also compared by setting different RI and AC values for the Mie theory.