The water is far from clean there and the bleach prevents the sunlight from encouraging bacterial/fungal growth. The bleach lasts as long as the bottle does.
On sailing ships of yore it was a prism usually solid glass, sometimes slightly tinged green as old glass was in those days.
Today (and for years) modern houses use what's known as Solar Tubes. A mini-skylight that can be very bright. I had several in my house and one thing I learned the hard way: don't put the solar tube over a ceiling fan's bade space or you'll get a constant and very annoying blanking of the light as each fan blade passes under it.
P.S. It is only as good as the sunlight that hits it. no sun, no light. There is no chemical reaction just rays passing through the bottle. The bottle's neck shape helps to collect more sunlight that's why it's placed neck up in the ceiling.
Last edited by C2C; 10/08/11 05:14 PM.