A simple apparatus employing a semiconductor diode laser and PIN photodiode has been used to investigate thermal capillary waves on liquid surfaces. These waves act as a weak, time‐varying diffraction grating for the incident laser light; the diffracted light can be heterodyned with the light directly reflected from the liquid surface to extract fluid properties (surface tension and viscosity). In this paper we present a discussion of the phenomenon of surface waves and describe the construction of an apparatus to observe them. Results of measurements with this apparatus for the surface tension and viscosity of water with and without oil films and of benzyl alcohol at different temperatures demonstrate the effectiveness of the technique as well as environmental effects on liquid properties. This problem provides rich ground for a study of wave and thermal phenomena as well as an introduction to a variety of experimental techniques.

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