A refractive index based detector based on so called back scattering interferometry (BSI) has been described in the literature as a unique optical method for measuring biomolecular binding interactions in solution. In this paper, we take a detailed look at the optical principle underlying this technique to understand fully the constituents and behaviour of the fringe patterns generated. The simulated results are compared and validated with experimental measurements. Hereby, we show that BSI does not operate as a resonant cavity as often stated in the literature. Recently, we have questioned the claims made that BSI in general can be used to measure molecular bindings. Here we explore this topic further in three cases using fluorescence spectroscopy as a reference method. Finally, we explore whether refractive index sensing can be used to measure the enzymatic phosphorylation of glucose to glucose-6-phosphate.