A Liter of Light is a project aimed at providing poor homes in developing (and not so developing) countries almost no-cost daytime light.
It is carried on mainly in the Philippines by MyShelter foundation (with government funding for the materials), and it's based on the work done by students at the MIT under the concept of appropiate technologies, which was in turn based on what Brazilian engineer Alfredo Moser did in 2002 to be able to keep working in his workshop during power shortages.
The concept behind the project is so straightforward you won't believe it: transparent plastic (water or soda) bottles filled with purified or distilled water and bleach. That's it. Oh, well, and some holes in the roof.
The preparation and installation of these so-called solar bulbs consists of three main steps, explained at a zillion places, including the project's homepage:
Making a circular hole in a square piece of galvanized iron sheet matching the shape of the roof, making half the bottle go through it, and sealing the borders so as to avoid leaks.
Cleaning the bottle and filling it with distilled/purified water and bleach, and sealing its cap.
Cutting a hole in the roof and putting the sheet with the bottle over it, so that half the bottle is inside the house and the other half remains outside. Then fastening it with rivets.
(This is a quick and dirty run-down, if you are planning to actually do it refer to the real instructions, which were written by people who - I hope - know what they are doing).
"And why not just cut holes in the roof and cover them with some transparent sheet of something?" You may be wondering, like I did at the beginning. The answer is by doing this you get a greater spread of light across the room, similar to that obtained by using a lightbulb.
It blinds me with its science
As simple as it is, the project relies heavily on physics and (bio)chemistry.
The innocent water-filled bottle behaving much like a 50-60W incandescent lightbulb is made possible by two optical phenomenons: refraction (described by Snell's Law) and total internal reflection.
Refraction happens when a ray of light passes from a medium with a certain refractive index to another medium with a different index (air and water in this case), and its angle of incidence is different than 90º and 0º.
Total internal reflection occurs when light tries to pass from a medium with a higher refractive index to a medium with a lower one, and its incidence angle is greater than a certain angle called critical angle.
These two phenomenons combined make the light inside the bottle feel utterly confused, and it ends up going out in all directions.
As for the chemistry involved, NaClO, the active ingredient in bleach, prevents the growth of mold in the internal surface of the bottle, hence keeping it clear.
NaClO dissociates in water forming sodium cations (Na+), hypochlorite ions (ClO-) and hypochlorous acid (HClO). Hypoclorite ions and hypoclorous acid are thought to be capable of destroying the membrane and cellular proteins, and to interfere with the cellular metabolism of mold.
What's cool about it
Even if it only works during the day, it can prove to make a huge difference in lighting conditions of otherwise unilluminated rooms.
It reduces the probability of fires caused by defective electrical wiring, which are way too common in precarious homes.
It saves some money in the power bill (which can be a lot to families that take advantage of this technology) and has a negligible carbon footprint.
Each solar bottle costs nearly nothing to make, between $2 and $5, and has a lifespan of about 5 years.
It creates jobs, which despite being perhaps temporary, may help many people (like Solar Demi, the guy that appears in the project's official ad) in poor countries.
What's not cool about it
As these bottles don't store energy, they are only effective during the day.
The roof has to be made of metal, and you have to be willing to cut a hole in it.
If you happen to arrive from a party in the morning and want to get some sleep, you won't be able to. Unless you devise a way to cover the damn thing without much effort.
It looks like a permanent temporary solution for poor people to use in their single-room fragile and almost subhuman homes while the rest of us carry on with our comfortable and wasteful way of life.
Liter of Light Homepage
Solar Bulb Out of One Liter Water and Bleach
Q: How does bleach kill mold?
Solar Bottle SuperHero @ Physics Central
Total internal reflection