Liter of Light Project

Liter of Light is a global open source movement with the aim to provide an ecologically sustainable and free-of-cost source of interior light to rooms in simple dwellings with a thin roof. The device is simple: it is a transparent 1.5-2l plastic bottle, as typically used for carbonated drinks, filled with water plus a little bleach to inhibit algal growth, fitted through the roof of a house. During daytime the water inside the bottle refracts sunlight, delivering about as much light as a 40- to 60-watt incandescent bulb to the interior. A properly installed solar bottle can last up to 5 years.

https://www.youtube.com/watch?feature=player_embedded&v=o-Fpsw_yYPg