For a single quantum well (SQW), interference of light from the well and the top surface plays a crucial role in determining the electroreflectance. The lineshape for the N=1 interband transition is a linear combination of the first differential of the real and imaginary parts of the QW dielectric susceptibility. This mixture is determined by the phase change for light making the round trip from the top surface, to the wall and back to the top. The authors change this phase factor experimentally (on a GaAs/AlGaAs SQW) by varying the angle of incidence and observe lineshape evolution in good agreement with predictions. Least-squares fitting to the spectra, using a QW dielectric susceptibility comprised of contributions due to light- and heavy-hole excitons, suggests Lorentzian broadening dominates at low temperature and Gaussian at higher temperatures. These conclusions are reached by not only fitting individual spectra, but additionally by examining which broadening mechanism gives the better description of the lineshape evolution with angle of incidence. They observed an unexpected difference between spectra taken with TE and TM polarised light and suggest that this may be because of a thin film of intermediate reflective index on top of the sample, rather than any property of the QW or the cladding, This thin film may ultimately limit the accuracy to which electroreflectance spectra can be modelled.