Temperature 172-182°F Dimensions 18 feet diameter. Blue Funnel Spring is a small, blue concentric pool located in the center of West Thumb Geyser Basin. When one walks past this spring, its vent appears to move and reposition. This phenomenon is not unique to Yellowstone's thermal features, but it is easily observed in Blue Funnel Spring. It is an optical illusion caused by refraction. It results when light traveling through the air strikes the surface of water at an oblique angle. One side of the wave front enters the water before the other and is retarded—since light travels more slowly in water than in air—while the other side continues to move at its original speed until it too reaches the water surface. As a result, the light ray bends in the denser water and is refracted, giving the illusion that an object has a different location than it actually has.
