Brilliant and cheap solution to getting light into tropical buildings that have dark rooms in the day time. A 2 litre pop bottle is caulked into a hole in the metal roofing. The bottle is filled with water and has some bleach added to keep the water clear. For free, enough light comes in to match a 50W conventional bulb. The inventor, Alfredo Moser, is a Brazilian engineer. This generates jobs using junk and is catching on in the Philippines and Brazil.
The above clip is from Brazil and features the inventor.
The 3 minute clip linked here is from the Philippines and shows a neighborhood handyman who is making a living with these solar lights.