Duga je kompleksna svjetlosna pojava koja za potpun opis zahtjeva primjenu i geometrijske i valne optike. U ovom radu obrađeni su samo efekti koji se mogu tumačiti geometrijskom optikom. To su kutovi pod kojim se vidi duga određenog reda, redoslijed boja po visini, te veći intenzitet svjetlosti pri dnu nego pri vrhu duge. Ako dugu promatramo kao liniju koje dijeli nebo na prostor unutar i izvan nje protumačena je skokovita promjena intenziteta svjetlosti pri prijelazu preko nje.

Rainbows are complex light-based phenomena, the description of which requires both geometrical and wave optics. This paper revolves around the features that can be described using geometrical optics only. These features are: the angle at which rainbows are visible, the order of colours and the increase of light intensity at the bottom, as opposed to the top of the rainbow. If we are to consider the rainbow to be a curve separating the sky into a „inside“ and „outside“ regions, we can also successfully explain the difference of light intensity which occurs when crossing from one region into another.