The present disclosure relates to methods and apparatus for characterising an interaction between a stimulus and a liquid thin film. An aspect of the disclosure provides a spectroscopic ellipsometry apparatus for characterising an interaction between a stimulus and a liquid thin film, the apparatus comprising: a stimulator, configured to provide a stimulus to a liquid thin film disposed on a volume of liquid to generate a wave in the liquid thin film, the liquid thin film and the volume of liquid having an interface therebetween, light beam optics for illuminating an area of the liquid thin film with a light beam, the light beam having a first polarisation, and a light collector coupled to a detector for receiving the light beam after reflection by the liquid thin film, the light beam having a second polarisation after reflection by the liquid thin film; a wave measurement module coupled to the light collector and configured to provide surface wave data to characterise a Lucassen wave in the liquid thin film based on the second polarisation.