The focus of this work is to study the refractive index in thin films of blue light-emitting poly(9,9-dioctylfluorene) (PFO) that contain extended rigid chains (β-phase). Using a post-deposition exposure to toluene vapour, β-phase chains were induced in glassy PFO films that had been spin coated from a range of solution concentrations to provide thicknesses from 50-200nm. With the aid of absorption spectroscopy, a semi-empirical calculation of the change in refractive index due to the presence of β-phase chains is undertaken. Over the spectral region where optical gain occurs, the refractive index of β-phase PFO is found to be appreciably larger than for as-spin-coated glassy PFO. Further development and explicit evaluation of the Kramers-Kronig relations lead to analytic and closed form expressions describing the change in index Δn which are used to highlight the key contributions to index change. We find that the spectral response of Δn is largely governed by the emergence of a characteristic absorption band associated with the presence of β-phase chains. The changes in refractive index are experimentally studied by tracking the spectral shifts of optical resonances observed from films spin coated directly onto sub-wavelength 1D grating structures. These measurements are supported and compared with rigorous calculations of the spectra, which incorporate the full optical descriptions for glassy PFO and β-phase PFO. The excellent agreement confirms the accuracy of our semi-empirical extraction approach. Similar polymer-coated grating structures are examined under lasing conditions and can also be used to support the expected spectral response and the decline of Δn for longer wavelengths. © IOP Publishing Ltd.