From Haiti to the Philippines, many poor people live in windowless corrugated-iron-roofed shacks. Even with an opening cut out of the corrugate iron for a door, these dwellings are dark in the day. Electricity, if available, is an expense sometimes bettered saved for food.
Liter of Light has come up with a simple, nearly free solution: Fill plastic soda bottles with water (add some bleach to keep algae from growing) and insert them into custom-cut holes in the roof. The water in the clear plastic bottles refracts the sun's rays, creating the equivalent of about 55 watts of light in a dark room.