An interesting optical focusing effect occurred in the early heating phases of a simple model of a lava lamp that was constructed to demonstrate convection effects. During this early heating phase, the interface between the two immiscible liquids was found to form a surface of rotation with a conic cross section that acted as a mirror to produce an excellent image of the filament of the bulb within the lower liquid. The relevant features of the lamp construction are discussed briefly, and photographs of this focusing effect are shown. A simple analysis is presented that transforms the photographed cross section of the liquid interface into the true cross section by removing the effect of the cylindrical lens formed by the fluid-filled bottle, and the resulting cross section is then fitted to the shape of an ellipse. The possible cause for the shape of this liquid interface is discussed and compared and contrasted with the somewhat analogous situation of a stretched circular membrane that is subjected to different gas pressures on either side of the membrane.
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