The optics of small particles are useful in the interpretation of observational phenomena related to extinction, scattering and emission of radiation by dust grains in space. This review presents three components of dust modelling: Optical constants; Light scattering theories and models. The author aims to show how the general laws of the optics of dust particleswork and to highlight the information about cosmic dust. Part II will be dedicated to the consideration of scattered radiation, dust absorption and emission, radiation pressure and dust properties.