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A bit hypothetical, but ruling out being able to use a solar still, or the SODIS method, could you make water drinkable by using the suns heat through a magnifying glass?

Paper self ignites at 451° F , water boils at 212° F, but that is at one concentrated point when paper burns using a magnifying glass. Would it be possible to boil a small about of water using the same method? For sake of it lets say you have a magnifying glass that is 3.5" across, and a plethora of different small containers available glass or metal.

I realize you could then start a fire, and boil water in a metal container but i am just curious if it is possible to heat the water enough to be drinkable without placing it on a fire using this method. As fires cause smoke lets say you are in a less than desirable place where the smoke from a fire could give away your location when you didn't want it to.

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  • newatlas.com/solar-kettle/27594 and amazon.com/Portable-Water-Heater-SunRocket-Sunlight/…. Never used them, reviews are mixed.
    – bishop
    Commented Aug 22, 2017 at 1:01
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    Solar disinfection seems to use a similar principle, letting sunlight kill the bacteria through heat and/or uv rays. There are some products around as well, using the simple magnification of the clear plastic in front and a reflective back which is supposed to bring the water to a safe state. gearjunkie.com/sol-water-purification-bladder. And, offhand, knowing how hot it can get when light gets into a closed system (ie, through car windows), I would expect heat would be a part of such a system even if not the whole of the process.
    – Megha
    Commented Aug 22, 2017 at 2:57
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    If you're using a magnifying glass, make sure you use one with diameter significantly larger than the diameter of the container. That's because a magnifying glass concentrates the light. In other words, it takes all the light which should've hit the entire shadow of the lens, and sends it to a single point. Heat dissipates quickly in water, especially if it is allowed to move freely in the container. Therefore, if the shadow of the magnifying glass is no larger than the container, then the same amount of sunlight hits the container regardless of whether you use the magnifying glass.
    – Arthur
    Commented Aug 22, 2017 at 5:46
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    There was an episode of the Discovery show "Naked and Afraid" where the female contestant took a magnifying glass with her as her single survival item, and proceeded to attempt this. If I remember rightly, she had to leave the show with heatstroke because she had been sat in the sun too long trying to decontaminate some water.
    – Craig H
    Commented Aug 23, 2017 at 7:03
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    Would not be my first choice of sole survival items... lol
    – Nate W
    Commented Aug 23, 2017 at 14:49

3 Answers 3

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Let's do some back-of-the-envelope calculations.

The specific heat of water is the amount of energy needed to raise the temperature of a particular amount of water by 1 degree C. This is 4.186 joules/gram °C (reference).

To raise the temperature of 1 L of water (1000 g) by 80 degrees C (to boiling from room temperature), would be 4.186 * 1000 * 80 = 334,800 joules.

The amount of sunlight falling on a square metre of the earth's surface is at maximum about 300 watts/m² (reference). A watt is a joule per second, so this means you could get 334,800 joules from a square metre of sunlight in 334,800 / 300 = 1116 seconds, or about 19 minutes.

Of course, a magnifying glass with an area of a square metre would be rather heavy and unwieldy to carry. A largeish one of 9 cm (3.54 inch) diameter would be about 64 cm², or 0.0064 square metres. Using this magnifying glass would boil our 1 litre of water in about 157 times as much time, or about 175,000 seconds, or just over 48 hours.

As you can see, the amount of energy you can collect directly from the sun is insufficient to boil any useful amount of water in a reasonable time. Even if you only wanted a small drink of 100 mL, it would still take all afternoon.

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    Thanks for the explanation and science in numbers behind it, i figured it was unlikely but i enjoyed hearing the math behind why!
    – Nate W
    Commented Aug 21, 2017 at 22:49
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    Note that one could get (and carry, folded up) a large parabola of foil a lot easier than a large magnifying glass. The same area calculations apply, except it is not the actual area of the foil that matters, but the apparent area of the parabola as seen from the direction of the sun.
    – Arthur
    Commented Aug 22, 2017 at 5:48
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    @whatsisname: evaporation and condensation in the solar still will demineralize (salt) water, and separate the water from solid contaminants. But the evaporation can and does take place at temperatures considerably below boiling (or T considered necessary to kill of microbial contamination), so I'd not autmatically consider the water safe for drinking from a microbiological point of view. Boiling water achieves "the opposite": the whole container is heated to kill off microbes, but salt and other contaminants stay in.
    – cbeleites
    Commented Aug 22, 2017 at 10:30
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    300 W/m² is rather low: the reference averages including night en.wikipedia.org/wiki/Solar_constant gives 0,5 - 1 kW/m². Also, the lens would be positioned parallel to the incident sun light, so latitude doesn't matter as much as for the power on earth surface. Electric water kettles draw in the range of 1 - 3 kW, so 1-3 m² efficient collection (and insulation) would get you to the same time as boiling the water in an electric kettle. However, both collection and insulation efficiency would mean a fully grown en.wikipedia.org/wiki/Solar_cooker which is not light backpacking...
    – cbeleites
    Commented Aug 22, 2017 at 10:49
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    The math is technically correct, but the numbers aren't... Otherwise this wouldn't be a thing amazon.com/Portable-Water-Heater-SunRocket-Sunlight/…
    – fgysin
    Commented Aug 22, 2017 at 11:47
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The discussion is indeed theoretical because it seems impractical - however possible - to me. First of all, there is a big difference between paper (solid) and water (liquid). In the liquid, the absorbed light energy is distributed by molecular movement rather quickly and you will not necessarily get such a hot spot as on a piece of paper. The fraction of absorbed light will of course anyway heat the water.

Also, there is a difference if you use a magnifying glass or a mirror. A magnifying glass works in transmission without changing the mean direction of the light much. Therefore, it will only shine significantly more light onto the water - and with this cause an additional overall heating effect - if its surface is bigger than the projection of the container surface into the direction of the incoming light. A mirror is different indepentent of its size because it works in reflectance an can redirect a lot of light onto the water which would not have hit the water otherwise.

However, as paparazzo already indicated in a comment, water is transparent in the UV and most of the visible spectrum, so a big part of the solar energy is not absorbed by water at all. This source claims that 50% of the solar light energy is infrared light which is readily absorbed by water. They say that almost all infrared light is absorbed within the first 10 cm, so your container would have to be that deep for maximum energy absorption, and with this maximum heating rate, caused by light absorption of water. As we can safely assume that we are in the domain of linear optics, absorption follows Beer´s Law which states that light intensity drops exponentially with penetration depth, so having a little less than 10 cm container depth is probably not a big deal.

What is equally important like light absorption by water is light absorption by the container. If you have a black pot it will additionally absorb most of the visible (and probably UV) solar light and this will then heat up your water much quicker. This is also how solar showers (note that they do not use any focussing optics), cookers etc. work. Appropriate estimations of heating rates under such full absorption scenarios are already given in the answer by Greg Hewgill and corresponding comments.

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Maybe you could have a very thin glass tube attached to a bigger will of water a bit like a watering can but with a thin spout then use a magnifying glass to boil water in the very top of the tube and condense the steam The well of water would top the tube up So all the time you are boiling a teaspoon ish amount of water I have seen how instantly a magnifying glass can burn my skin Just an idea to stop having to heat up all the water in one go

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    It takes the same amount of energy boil 1 L of water, if do it small amounts at a time or if you do it in large amounts.
    – James Jenkins
    Commented Jul 5, 2019 at 8:10

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