Magic clusters are the smallest ordered quantum dots; hence, they represent promising materials for optoelectronics. However, their optical properties still have not been studied. Therefore, the purpose of this work is investigation of optical properties of magic clusters formed both in reactive (Cr/Si(111)) and non-reactive (In/Si(111)) systems. The 'adsorbate-silicon' system named 'reactive', if reactions resulting in silicide(s) formation take place in it. Using differential reflectance spectroscopy, optical functions of Indium and Chromium magic clusters were obtained. Having analyzed obtained spectra, we conclude that Cr magic clusters are CrSi islands and Si surface covered by In magic clusters is a semiconductor. A new method of obtaining saturation coverage of studied surface structures was proposed. [DOI: 10.1380/ejssnt.2006.650]