Why can't I creat fire with a plastic bottle of water?

[tomwalker154]

Hello guys;
Making fire with a manifying glass is super easy, everyone knows that but a few days ago I watched a video on Youtube showing how to make fire with just a plastic bottle. It's so cool! They said that all I need is a bottle containing pure water inside and a piece of black-color paper (Black color can absorb heat easily, everyone knows that). That's it! Then choose a airy area covered with sunlight at noon, let the sunlight go through the bottle (with no covers), reach to the black-color paper, BOOM! smoke comes out and then FIRE. Ok that's how it works on theory! But in fact, I can't focus sunlight in a point, "weak" sunlight goes out site to every direction. After 20 minutes, standing under the sunlight of the noon, I rather was burnt than setting fire to the paper. I start doubting about this. After review some comments on that video, I see some can do that normally.
So what's the thing I am doing wrong? Anyone did this before?
(I live in a tropical area and I can ensure about the sunlight density at noon :D)

[Yuki Akuma]

Could you maybe provide a link to this video?

[tomwalker154]

Here it is:


https://www.youtube.com/watch?v=QwQJ-3pZfwc

I've tried with glass bottle. It didn't work as well.:smallmad:

[NecroRebel]

I wouldn't expect all bottles to be shaped correctly to focus light very well. If they're shaped wrong, the water might end up diffusing rather than gathering light, making it useless for setting fires. Maybe try a different bottle?

I see no reason why this wouldn't work (as far as physics knowledge goes). You might, however, need to hold the bottle at a particular angle relative to the paper and the sun in order to get the lensing right.

[Cazero]

The obvious parameters that would make it fail here are bottle shape (yes, you need the exact same model, making a lens is accurate geometry), bottle material (a different kind of plastic might be enough to make it fail), bottle color (some plastic bottles are kinda blue colored and this might filter/redirect enough light to make your lens too weak for ignition), and using non-pure water (impurities change the properties of the lens).

[snowblizz]

Environmental factors also impact. Mythbusters did test something similar and differences in humidity and I think wind conditions made it very very hard for them to replicate the test. It was about a bowl of water focusing or reflecting the sun to cause a fire.

Quite simply, conditions have to be juuust right. A magnifying glass is a much much MUCH more powerful focuser of (sun)light, so it does it much better.

[halfeye]

Water?

Water is the material with the highest themal capacity of any relatively common material, there are higher, but they are rare/labs only.

Take out the water and you probably have a chance.

Burning plastics are fairly foul, are you sure this is a good idea?

[Knaight]

I suspect your problem might be a matter of the index of refraction of your plastic bottle. The idea is to use a bottle-water system as a single lens, and that's made much easier if the plastic and water refract similarly.


Water?

Water is the material with the highest themal capacity of any relatively common material, there are higher, but they are rare/labs only.

Take out the water and you probably have a chance.

Burning plastics are fairly foul, are you sure this is a good idea?

Thermal capacity isn't particularly relevant here - you're not trying to heat the water, you're using the water as a lens to burn the paper. Similarly, the plastic is also not being burnt but is part of the lens apparatus.

[Togath]

But for heat to hit the paper, would it not first need to make it through the water and plastic?

[5a Violista]

But for heat to hit the paper, would it not first need to make it through the water and plastic?

Yes, kind-of, but not in the way you're thinking. Radiation energy is traveling through the lens (water and the plastic: I'll call it lens from now on because that's what it is), but not really in the form of heat. It's really traveling through the lens as light, rather than thermal energy. This happens largely because the water and lens are transparent to energy in the form of light.

Then, the lens focuses the light into a single spot. The energy was spread out over the entire width of the lens before reaching it, but then the lens focuses it into a single spot. That spot, with the energy concentrated that way, heats up from the radiation (and not through heat flowing through the water or something).

Here's (https://www.reference.com/science/magnifying-glass-start-fire-d1d8340290188333#) somewhere you can read if you want to know in more detail.