Solar Bottle SuperHero

Solar Demi is a man who travels around installing solar bottles in order to bring light into homes in some of the poorest neighborhoods in the Philippines.

It's amazing to me that there are places where even minimal lighting is a luxury. I'm glad to see that basic physics principles are bringing a little sunlight into the dark days of some impoverished Filipinos. (Of course, if you feel like sending a donation to the cause, you can help spread the light and the love a little further.)

I've seen a lot of speculation about how, and even whether, solar bottles can light a room so well. The fact is, they do work, and they rely on some pretty nifty physics to do it.

To begin with, cutting a hole in your roof will let some sunlight in. Even a small hole in a darkened room can make a big difference. There are, however, some problems with a simple hole.

Besides letting in the rain and insects, you only get as much light as falls directly through the hole. As Solar Demi knows, you can do much better than that with some grassroots engineering. Putting a water-filled container in the hole collects more light due to two fundamental optical effects: Snell's Law and total internal reflection.


When light moving through the air runs into a denser material like water, it changes direction. That is, the light beam kinks where it encounters the water surface so that it's traveling more directly down into water. That's very helpful if you're trying to get more light to go down through the hole.

Despite the help that Snell provides, some of the light will still be on a path to the opposite side of the cylinder. A portion of the light is trapped in the cylinder because of simple reflection, which can happen anytime light passes from one transparent material to another. However, during much of the day, the light will strike the cylinder walls in such a way that lots of light will be reflected down toward the hole. This is called total internal reflection. Light reflected this way will bounce back and forth as it travels down through the water column, much like signals passing through an optical cable.
A laser beam trapped in a plastic illustrates the effect well.

So, without a solar bottle, this is all the sun light you can expect to collect.

After Solar Demi gets through with your roof, this is how much light you'll gather.

Once the light is in the room, the bottle offers another major benefit. If you only had a hole, or even a small glass window, then sunlight passes straight through, and illuminates a spot on the floor or wall, like this . . .


Because of multiple reflections and different paths caused by Snell's Law refraction and internal reflection, light spreads out as it passes through the water column. That's what made the bottles appear to glow in the video of Solar Demi doing his stuff.


In the first of these two sketches, sunlight passes directly through hole. In the second sketch, I've added a few of the paths the light can follow as a result of the water column. There are actually an enormous number of possible paths the light can take, which results in an apparent overall glow, much like a light bulb.

All in all, it's a simple, clever, cheap, and environmentally sound lighting solution. In fact, I'm so impressed that I think I'll go donate right now.