This paper describes how parameters of investigated substances and the fiber-optic Fabry-Pérot sensing interferometer affect the spectrum of the optical radiation at the output of the sensor. First, the modeling of the operation of the sensing interferometer was conducted. Most important parameters and effects that were taken into account are: dependences of the refractive indices of the core and the cladding, as well the mode field diameter of a single mode fiber on the wavelength, changes in the parameters of an optical beam inside the interferometer caused by refractive index and the absorption of the medium inside the cavity, including the intensity of the beam, the beam diameter, the beam divergence, the curvature of the wavefront, and the phase shift caused by the Gouy effect. Impact of these parameters and effects on the spectrum of the optical radiation at the output of the sensor was subsequently investigated. Following, spectra from selected Fabry-Pérot optical sensors, applied to measurement of refractive index, were presented. Measurement results were compared with the spectra obtained by modeling.