A liter of light - DIY

We have lightbulbs made from glowing metal filaments, fluorescent gas, and LED diodes. And now we have one made of water. There is also a virtually unlimited supply since the "bulb" is composed of nothing more than one-liter plastic bottle, water, and bleach. The simple technology can be installed in less than an hour, lasts for five years, and is equivalent to a 60-watt bulb.

It works simply: The water diffracts the light, letting it spread throughout the house instead of focusing on one point. The bleach keeps the water clear and microbe-free.

Adapted by students from the Massachusetts Institute of Technology focused on "appropriate technologies," the solar bottle bulb is illuminating poor settlements across the Philippines, where the organization Isang Litrong Liwanag ("A Liter of Light") has already installed 10,000 of them.

The physics of the concept are straightforward: the bottles are placed in roofs – half outside, half inside – and their lower portions refract light like 60-Watt light bulb but without the need for a power source. A few drops of bleach serve to keep the water clear, clean and germ-free for years to come.

In total, one of these do-it-yourself lights takes maybe an hour to install, cutting an appropriate hole, inserting a bleached-water-filled bottle, and resealing around the resulting gap. Even where clean water is rare, a little can generally be spared for a half-decade of lighting.

Millions of poor homes in Manila--and far more around the world--are left in the dark because metal roofs block all light and there are no connections to the electrical grid in cramped informal settlements. This simple bottle bulb, installed through a sealed hole cut in the metal roofs, provides a surprising amount of light by deflecting sunlight into gloomy interiors.