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When parallel-polarized light is reflected at an angle Â¿ by the interface between two transparent media with refractive indices n and N, the interface reflectance is switched from 1 to 0 by varying the refractive index ratio N/n from sin Â¿ to tan Â¿. This is the basis of a light-reflection liquid-level sensor that uses a prismatic window in the side wall of the vessel whose liquid level is to be monitored. Such a sensor is described and its design procedure is developed. Remote liquid-level telemetering is possible by using fiber optics to lead the light to and from this particular sensor.