The spectral shape of the light absorption by marine particles with power-law size distribution was analyzed. The anomalous diffraction approximation (ADA) in its conventional form for optically “soft” homogenous particles was used to model the absorption cross section. Based on this theory, particular formulas describing the spectral shape of absorption of an assemblage of particles were derived. Exemplary calculations were performed for particles of selected shapes (spheres, thin cylinders and disks). The results suggest that relative absorption spectra of marine particles in many cases can be approximated by a simple formula, i.e. a power function of the ratio of their imaginary part of the refractive index and the light wavelength. As long as the ADA is considered, the value of the exponent varies between 0 and 1 and depends on the particle size distribution slope, the range of particles' sizes, their shapes, and the magnitude of the refractive index. These factors also influence the accuracy of this approximation.