A sensor for monitoring a medium has an electromagnetic radiation source, mirrors, and a detector for electromagnetic radiation arranged inside a cup-shaped housing part. The cup-shaped part has flat, angularly arranged wall areas defining a cavity in which, outside of the housing, the medium is contained. The flat wall areas are transparent for electromagnetic radiation and arranged in the beam path from the electromagnetic radiation source to the detector so that the electromagnetic radiation passes through first wall, medium in the cavity, and second wall. The first and second wall areas and the medium form a refracting prism. A cover closes off the cup-shaped part so that electromagnetic radiation source, detector, and mirrors are enclosed in the housing. A data processing system is connected to electromagnetic radiation source and detector so that electromagnetic radiation of different wavelengths is refracted in the prism and the resulting spectra are detected and evaluated.