An ultra-thin crystalline silicon solar cell with an active silicon layer of 200 nm has been fabricated and fully characterised (I-V characteristic, spectral response) and optically (Variable Angle Spectroscopic Ellipsometry). Interference effects were observed in the spectral response of the cell due to multiple reflections from the layers within the cell. A mathematical model was developed to account for the different reflections and transmission within the cell which reproduced excellently the essential features of the experimental spectral response.