The invention relates to an optical system for transporting natural light into a building, formed by a hollow guide (3) having a peripheral layer formed by two types of prismatic plastic sheets performing guiding and extraction functions, without additional geometric elements. The light guiding sheet (30) is such that the edges of the prisms face outwards and are parallel to the longitudinal axis of the guide, and the sheet (40) that performs the extraction functions is such that the edges of the prisms face outwards and are parallel to the longitudinal axis of the guide. The amount of light extracted in a determined position of the guide can be regulated by means of the increase or decrease in the area of the extraction sheet.