An interferometric measurement of the wall thickness of a cylindrical glass tube with application to a draining liquid film

A method is described whereby a laser beam irradiates the wall of a hollow glass cylinder, and reflections from the inner and outer curved surfaces of the wall interfere to produce a fringe pattern which is analysed to give the wall thickness. The method is dependent on the fact that a curved surface presents an incident laser beam with a continuously varying angle of incidence. A comparison is made with experiment. The method is also applied to a film of water draining down the interior of a vertical cylindrical tube.