Deck prisms: http://en.wikipedia.org/wiki/Deck_prism http://en.wikipedia.org/wiki/Deck_prism The main difference here is that deck prisms are flat on top, whic

Deck prisms: http://en.wikipedia.org/wiki/Deck_prism http://en.wikipedia.org/wiki/Deck_prism The main difference here is that deck prisms are flat on top, which limits the amount of light entering the prism. The water in the bottles is filled to the top, creating a dome shaped concentrator. Since water has a refractive index close to glass (~1.3 vs ~1.5) the bottom of the bottle has a similar effect to the deck prism, except with slightly more light.