Some of the best inventions aren't high tech, but technology adapted to the needs and resources of poor people in developing nations. When on Earth has a list of 10 of them, including these self-adjusting glasses.
The system is called Child ViSion. A pair will fit a child between the ages of 12 and 18 and withstand a lot of punishment. More importantly, they don't require an optometrist. The user can adjust them by twisting the rods on the side, which move the lenses. Once the user can see well, s/he removes the adjusters until it's time to tweak them again.
Read about 9 other great inventions to help the world's poorest at When on Earth.
* Solar ovens take FOREVER to warm-up food. You have to leave your food out in the sun ALL DAY LONG like a weak & inconvenient crock-pot. They have been given away to the poor for many years, and they DON'T USE THEM at all.
* Water filtration only requires letting it sink through 1-2 meters of sand. It costs nearly nothing and is infinitely renewable. An expensive manufactured filter that needs replacement parts is a gimmick you sell to rich hikers, because the poor could never afford them, and they would despise the effort required (imagine filtering several gallons every day, when you've got tons of other chores to do).
* Mosquito netting pretty well solves the bug problem, and doesn't need frequent & expensive replacements.
* Generic glasses are available for under $10 in drug stores here in the US, and would be cheaper to an NGO ordered direct from China. You just go in and try them on until you find the lenses that works for you. Your eyes change only little over the years (and glasses wear-out quicker than that), so adjusting them is a very expensive gimmick that few need.
* Hand-pumps are pretty cheap and last forever with minimal maintenance and work well for young and old. A solar-powered monstrosity is just an albatross sucking funds from more useful projects.
* An incubator for premature children is provided free by nature... Skin-on-skin contact with the mother's body will properly maintain & regulate the infant's body temperature.
* I could fashion a water cart from some wooden wheels pretty easily, or just roll a barrel with tight lid. Neither works very well over soft sand, mud and other rough terrain, which is why water gets carried, regardless.
* Hexayurts are hippie tents for Burning Man. Requires very expensive Styrofoam boards and strong tapes, are a severe fire hazard, and aren't very strong. Meanwhile, locally produced clay/adobe houses are dirt cheap, have plenty of thermal mass, aren't flammable, and can be repaired with local dirt.
* Pretty sure anybody can make sandals with local materials (wood or leather soles) for cheap. More a matter of convincing folks the benefits are worth the effort.
* Like potato clocks, a salt water lamp works by acid eating away copper & zinc rods (or a few other metals). They're low power, require a steady supply of expensive metals, and just not worth it. You're really just giving them one disposable lantern battery... Meanwhile, a solar light with good battery can cost as little as $2 and last for 6 years of regular use.