Can You Purify Water With a Magnifying Glass?

Perhaps you’ve been watching too much reality TV lately, and you noticed someone try this stunt, or maybe you grew up with the sort of kids who liked to french fry ants on the pavement. Hopefully, neither of these was the case. Regardless of how you learned that a magnifying glass can be used to concentrate light, it does seem like a useful trick to know, but it seems a bit small for such a big job. I decided to do some digging to see if my pocket magnifier was enough to keep me in potable water if things ever get so rough I haven’t got a match left.

Can you purify water with a magnifying glass? This is a trick question. The answer is twofold. First, no. If you are thinking of pointing a magnifier at a cup (or coconut) full of water to get boiling action and thus save the day, you are probably going to die. You would need a huge lens for this, and not a portable magnifier. Secondly, yes, there is a way to use your magnifying glass to create conditions in which you can purify water. It’s not by directly pointing the beam at the water. You need to use it to build a fire.

What Does a Magnifying Glass Do to Light

A magnifying glass is a lens. The ability to magnify is part of the nature of the shaped glass. Lenses let light through them, and in this case, because of the curve, it bends the light. That bent light gets redirected into a tighter line than when it hit the lens. This concentrates the light energy on a focal point.

Alternately, when you are on the other side of that beam it makes things below it appear larger because the light is being bent the other direction and expanded. Light is constantly passing through both ways.

There are plenty of uses for the lens in survival situations. The boiled water with a pocket lens trick just isn’t one of them. You can see splinters and use it to clean and stitch wounds more accurately for example. In lieu of a lighter, you could use it to melt a plastic bag closed to keep out insects by moving your beam along the plastic.

Why Doesn’t the Magnifying Glass Boil my Water

The concept of heating water with a concentrated light beam seems perfectly sensible on the surface. The trouble is, water is a great conductor of heat. The pinpoint where you are actually physically heating water may get very hot. Unfortunately, the rest of the water works like a heat sink and sucks all the energy right out of the reaction. The surface area of your water is probably enough to let the heat go into your environment too quickly to be useful.

In short, you could boil a drop of water at a time. If that seems a bit tedious and time-consuming, you’re right. The amount of energy and time in boiling water by-the-drop is insane. Not to mention, with such a small amount, the evaporation would make this effectively useless anyhow.

You could boil water with a much larger lens. The problem is that a magnifying glass is portable. The pocket-sized version of a lens doesn’t collect enough light to do the job. Unfortunately, a larger lens is heavier and can’t be carried around like the common magnifying glass.

How do I  Know if the Water Needs Purifying?

There are different cues to let you know your water has gone off. Not everything is going to be obvious, but typically you can trust your senses. They are your first line of defense, and if they warn you something is off, then it probably is. If you’ve heard it once, you’ve probably heard it a million times, trust your gut. Having a “feeling” isn’t magical nonsense. It’s your instincts trying to tell you something important.

When something visible gets into your water it’s easy enough to tell it’s contaminated. The way your water looks is a great clue. If there’s obvious dirt, or discoloration it’s not pure. Even just cloudiness is an indication that something is up.

Likewise, a bad smell or funky flavor are signs that your water is less than stellar. Not every problem is deadly, but it’s best not to take chances. It’s harder to detect microbes and pathogens, but when your nose or tastebuds tell you to stop that’s your first warning. Whatever makes you hesitate, if you have to wonder, don’t risk it.

Though it seems odd, water can actually be too clean. Ultra-purified water has its uses, but most of them involve medical production and microchips. This sort of water has been demineralized and can remove necessary minerals from your body. You should avoid water that has no mineral content at all. You shouldn’t worry too much about causing this accidentally however, it’s just something to be aware of.

For those who have a questionable water source available, you might just want to consider purchasing a compact lightweight portable filter that will easily fit in a book bag that can clean up to 1000 gallons. It will turn stagnant muddy water into drinkable purified water. Click here to get the latest pricing from Amazon.

How can I DIY Clean Water in a Pinch

The easiest ways to clean water should be combined. Use one method to get what you see, and a different way to kill what you can’t. For example, heat kills most things you can’t see, but it won’t remove any particulate matter. Combine heating with filtration to get clean and debris free water. Likewise, evaporation is great for getting the particles out, but it doesn’t always remove unseen contaminants, so using a UV purifier or iodine tab can help. Think of water cleaning like zombie killing, always double-tap if you can.

  • Bleach– Kills bacteria. Stir and let sit 30 minutes. Make sure you use the right amount to avoid poisoning yourself.
  • Iodine– Kills bacteria and viruses. This method also takes about half an hour and requires the right proportions. Fortunately it’s a little less dangerous than bleach, but still, avoid overdoing it.
  • Heat/Boiling– Destroys anything living at sufficient temperatures. It takes between 1 and 3 minutes depending on altitude. I’d give it five at a rolling boil just to be certain.
  • Evaporation/Distilling– Removes large contaminants and salt. This method won’t kill germs or remove things like oil or gasoline. Basic evaporation requires less heat than boiling. You’ll need a way to trap and heat your water and run the droplets into a clean container. Generally, this takes most of a day.
  • Filtration– Depending on the sophistication of your filter this can remove everything from large debris, like a sieve, to serious contaminants that require activated charcoal. Time varies.
  • Solar/UV– This kills bacteria and viruses if done right. Time varies, and you do need special equipment. (Not a magnifying glass)

How Can I Make Fire With A Magnifying Glass

Ok, so you’ve given up on the idea of using that tiny little magnifier to directly shoot a laser beam of magic sunshine into your water to clean it instantly. What you can do is point that same beam at some paper or kindling to start a fire. You will have to find a way to make a container for your water, but it’s relatively easy to make a fire this way.

If at first, you don’t succeed, give it time. It does take a minute or two to get fires started this way. You can adjust the distance your magnifier is held from the flammable material to help focus in on a spot to burn. Work on adjusting your beam before you need this trick to survive.

Final Thoughts

It is infuriating to see someone, especially on TV, put out a ridiculous false idea. Boiling water with a beam like this might be possible, but you’d need special conditions and/or a much larger lens. Math and science are against you here. Realistically, spreading dangerous disinformation about survival should be criminal.

If you are in a survival situation and you happen to have nothing but your pocket magnifier with you, please don’t waste your time and energy. You have enough problems in this scenario. Pointing that tiny spark into water is the opposite of what you should do. Instead, build yourself a fire. This will assure you of clean water, warmth for your body and a hot meal if you’ve anything to cook.

This is why I suggest getting a quality highly rated portable and compact water filter called a lifestraw. Click here to get the latest pricing and information from Amazon.

Additional Questions

  1. How can I improvise without an actual magnifying glass? It’s fairly simple to put together a water lens that has the same effect. You need flexible clear plastic, and water, plus a frame to hold the plastic in place. Saran wrap or any clear plastic sheeting will do. Set up a frame to stretch the plastic over. This needs to be horizontal, not vertical like a picture. Once you have your plastic secured, add water so the plastic makes an improvised bowl. Sunlight can now be focused and used to start a fire.
  2. Can I cauterize a wound with my magnifying glass? Yes, in theory. It would be a lot more difficult than using another more instant method that cauterizes all at once. You’d need to burn along the wound. In the book Naturalis Historia written in 79 AD, there is a record of someone using a magnifying lens to cauterize wounds. The technique has been around for centuries. Like the water, you might be better off building a fire to do this 
  3. Can I point a magnifying glass at a solar cell to get more power? No. Collecting and using the suns light is a brilliant strategy, but that’s not how it works.  The Photovoltaic cells are a complex bit of scientific wizardry. The short version of the explanation is that they use the photons to knock electrons out of atoms. The magnifying glass doesn’t make more sun, it just takes what there is and turns it into a fine stream. You can burn a hole in your solar cell this way, but it won’t do much else.