Laser diffraction spectrometry is an optical technique that uses information from the scattered light distribution to determine particle size distribution. When laser light diffraction spectrometry is used with small transparent particles, secondary scattering effects are present. As a result, the particle size distribution determined via the Fraunhofer approximation predicts the existence of a fine particle fraction that in reality does not exist. Application of the Mie theory eliminates the false prediction of a fine particle fraction, but only if the material's refractive index and absorption index are exactly known. This phenomenon was confirmed theoretically for transparent particles by simulating the scattered light distribution and experimentally by comparing results with those obtained using non-optical methods.