The litre of light as its coined is a solution I wrote about previously to do with letting light into homes that are in dark areas. Often slums have over lapping roofs and other structures that prevent natural light penetration into homes. Adding this plastic bottle with a mix of water and bleach allows light into the home as well as keeps the roof sealed from rain. Which may not seem much unless you live in one of these homes that lacks natural light. Your recycling old plastic bottles, reducing electricity cost and improving peoples general well being with things that are readily available and often discarded.
At the same time why restrict it to just the poor? I know many places where this would come in handy such as enclosed garages,out buildings and other structures that lack windows to allow natural light in. In a more environmentally friendly world we should be seeing more of these sort of things appear at the same time in a recession it makes more sense to the average person to utilize anything that can save a few pesos,pounds,euros or dollars.