Simplified Theory of Reflectometric Thickness Measurement of Structured Soap and Related Films

Abstract

The intensity of reflected light is now used in the exploration of the structure of soap films, and of the forces responsible for it. An accurate interpretation must take into account the complicated sandwich structure of these films whose surfaces are formed by monomolecular layers upon an aqueous core. A direct simple approach assuming only that the surface layers are very thin compared to the wavelength of light shows that each layer of thickness d and refractive index n is optically equivalent to an increase of the thickness of the core having a refractive index m by a thickness d′=d(n²−1)/(m²−1), in agreement both with a physical picture of the phenomenon and with results obtained by more complicated calculations. This approach also permits immediate generalization to a structure formed by any number of very thin layers and also to closely related films of lipids in water which are used as model biological membranes.