When people build shantytowns and impromptu housing (as millions do around the world), it’s hard to get sufficient light inside because of lack of electricity. So, people live in the dark or have to go outside to see anything. Solution: plastic bottle light bulbs.
Liter of Light takes empty pop bottles, fills them with water and a little bleach (to prevent mold), and installs them in the roofs. The result is similar to solar light tubes and it allows people to have light in their homes throughout the day, without electricity.
Good.is explains how it works: “plastic bottles refract the sun’s rays, scattering about 55 watts of light across a would-be pitch black room.”
(The video on Liter of Light is even better!)