Disclaimer: first time on Physics stack-exchange, I hope I'll be asking the question as per policy of this site, please help me edit if my wording seems off topic.

I've seen a project to light up shacks with a soda bottle filled with bleached water, inset halfway through the sealing.

bottle light

Now I wonder:

  1. how does light exactly propagate through bleached water?
  2. would it also work with longer tubes laid horizontally across a dark room? (see diagram below)

water light in room

  • $\begingroup$ Don't worry- you did fine with your first question! $\endgroup$ – Floris Feb 8 '15 at 11:08

The technology is described in this Wikipedia article. The bottle is acting as a light pipe. It doesn't create any light itself, it just transfers sunlight outside the building into the building.

Here in the UK light pipes aren't much used, probably because electric light is cheap and natural light tends to be rather variable. And of course light pipes are absolutely no use when the Sun is down. In parts of the world where sunlight is more reliable and cost is more of an issue I can see how useful the idea would be.

Re your question about using a longer tube: I suspect you're thinking of something along the lines of a fluorescent tube, but this isn't how the light pipe works. You could probably adapt the pipe to make it leak so light came out of the side as well as then end, but the light would be rather dim as it's only transferring the light that hits the end of the tube that sticks out of the roof.

  • $\begingroup$ Thanks. So is there a way to increase the amount of light hit at the end of the tube? How can I predict the amount of light propagated this way or lost in the process? I'll take a read at the article. Thanks for pointing at it. $\endgroup$ – Benjamin Feb 8 '15 at 7:18
  • $\begingroup$ @Benjamin: the light hitting the end of the tube is just sunlight. You could make the end of the tube bigger, so it captures more sunlight, or I suppose you could use some arrangement of mirrors to concentrate sunlight on the end of the tube. $\endgroup$ – John Rennie Feb 8 '15 at 7:19
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    $\begingroup$ @Benjamin Further to this answer, I think the bleach in the water is extraneous to the essential physics here. I think the bleach is simply used in this DIY project because you are meaning to leave the light pipe, and its water, installed for a long period of time. The bleach keeps the water clear indefinitely: it kills off algae, protists and bacteria that would otherwise grow and make the water cloudy and mucky. It wouldn't work very well as a light pipe then. $\endgroup$ – WetSavannaAnimal Feb 8 '15 at 7:39
  • $\begingroup$ @WetSavannaAnimalakaRodVance Thanks. I thought for a while that the bleach made the water translucent rather than transparent, which would help diffusing light. $\endgroup$ – Benjamin Feb 8 '15 at 8:07
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    $\begingroup$ To make the light "leak out" you could sand the surface of the bottle - this improves the probability of transmission. If you are clever you change the amount of surface roughness as a function of distance in order to get uniform illumination. I believe some version of such tricks are used in the back illumination of LCD displays (which are sometimes lit "from the side"). But as John pointed out this is a "low tech " solution that just works for people who would otherwise have nothing. I love those ideas! $\endgroup$ – Floris Feb 8 '15 at 11:07

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