Low-tech solar light bulbs are helping impoverished families in developing countries see the light. Literally. Discarded plastic soda bottles are filled with water and wedged into corrugated metal rooftops. Tropical sunlight is diffused through the water and provides 55 watts of light without electricity. The result means poor families no longer have to spend precious funds to light their window-less homes in daylight hours, a tremendous benefit to families who often survive on less than $2 a day.