Refraction Gauge Glass:

A third category of gauge glass is the refraction type that was show in the figure.  This category is especially meaningful in areas of decreasing lighting; lights are commonly attached to the gauge glass. Operation is based on the principle in which the refraction, or bending of light, will be different as light passes by several media. Light is refracted, or bent, to a greater extent in water than in steam.  For the portion of the chamber which holds steam, the light rays travel relatively straight and the red lens is illuminated. For the portion of the chamber which contains water, the light rays are bent, causing the green lens to be illuminated.  The portion of the gauge holding water appears green; the portion of the gauge from which level upward appears red.

Figure: Refraction Gauge Glass (overhead view)