Practical Tips: Clear Bottles with Water can cause Fire

Do you know that, by using clear water bottles filled with water can make fire for your campfire while stuck in the Rainforest? Do you know that leaving the same bottle in the car can cause fire too ? What’s the mystery behind the warning ?

Physics 101 : Properties of Light

In school books we learn about how light will behave when it pass through a transparent body like the magnifying glass, and how it can use to light a paper on fire.

Light as an electromagnetic radiation waves that carries energy across transparent medium and even vacuum. That’s how we can feel heat when exposed under direct sun light or a strong beam of artificial light.

Light can be in the visible or invisible spectrum. Most of the naturally occurring lights are at the invisible spectrum (including infra-red, ultra-violet, x-rays, gamma rays etc) as our visual spectrum is very limited at 400-700nanometer range.

When a light meets another medium (let it be air, water, semi-transparent body, solids etc …), which up to certain degree, it can be absorbed, reflected or refracted.

The refraction of the Light beam when a light cross different substances.

When lights are entering a semi-transparent or transparent medium at a direct 90 degree perpendicular angle, it will go through directly without changing its angle both entry and exit angle.

It will be interesting when light is entering the same medium not at a direct angle. The direction of light will change when moving across the medium, causing a change between entry and exit angle.

Carefully attenuation of light direction can be very useful, for example magnifying glass, spectacle lens, prism and even the one we have – the lens in our eyes. Without which we cannot attain a focus of beam of light.


The Light, The Water and The Bottle

Ever wondering what this three entities is important in today’s concerned topic ?

Bending of light or refraction is the keywords in this phenomena. When a light passing through different medium of different Refractive index, a Refraction of light occurs.

When light passing through a empty transparent bottle (glass or plastics), most of the light is dispersed and unable to focus well, due to its thin flat bottle walls (yes, the wall of bottles are thin and flat if you cut open). Refraction also doesn’t goes much in normal atmospheric air in the bottle.

What happens if you add water inside the clear transparent bottle ?

The comparison between light going through a bottle with air and the bottle with water.

Air has a refractive index of about 1.0003, and water has a refractive index of about 1.3330. This is a very important statement. It means water can refract or bend light much better than air. The higher the refractive index, the more “bending” of light will occurs.

That’s why you can have a very high corrective index with a very thin glass in a spectacles which are using high index lens. It is also noticeable when a drop of clear water on a mobile screen, you can see a magnified view of the underlying RGB (Red Green Blue) light diodes.


In a regular smooth rounded bottle, the water in the bottle are mold round according to the bottle. Instead of thin flat bottle wall, the water is basically forms a transparent cylinder or parabolic medium (as seen from the side cut section), which enhance the properties of focusing light.

Same scenario can be found comparing a pencil in a glass cup and a pencil in a glass cup with water. The one with water will cause the pencil to be appeared bend.

Hence the main culprit is the water in the bottles and not the bottle itself that can form a more focused beam of light. So in fact, it doesn’t care you use glass bottles or plastic bottles, once filled with water it can be a magnifying glass.

So are all bottles with water a good thing or a bad thing ? let’s find out.

Starting a campfire with a bottle with water

When you are traveling in a rainforest, you might encounter that you have wet your matches or lighter and unable to start a fire. You will be in luck if you have a sunny day and a bottle (glass or plastic).

It would be rather simple to fill your bottles with water and find some good light source and shine it across to a pile of combustible dry leaves or paper. At the correct angle and distance, it forms a focused point of light which concentrates the energy of the light on one spot and increase the temperature of that spot.

Walah, after some time the hot spot start to smoke and soon a fire.

Many other variants of life hacks in the internet that use water to start fire. All using the same principle by using the refraction properties of water (the most common found transparent medium on earth) that is enclosed in a curved container.

Warnings about unattended bottle with water

Unattended bottle with water wont cause much harm by itself, but it can be dangerous under right condition. “Anything that can go wrong will go wrong” as indicated by Murphy’s Law.

A strong light source, a medium to focus light, a correct distance to a flammable medium, a favorable environmental condition – Thats all it needs for it to go wrong.

Water bottle with water can cause fire in vehicle under hot summer sun !

In a hot sun, the interior of a enclosed vehicle act like a oven and can reach 40 to 60 degree Celsius. It is a demonstrable facts that clear water bottle with water can cause burnt marks and even start fire on black leather seats or dark colored seats. To solve this problem, you don’t left unattended water bottles with water on places that can have direct contact with sunlight.

Solutions: To place water bottles below the back seat or in the drawer compartment or even better option – don’t place them in vehicles in the first place.

Anyway, under hot temperature in the vehicle, plastic bottle tends to release chemicals into the drinking water, causing it to taste “plasticky” which may cause infertility and cancer in some research.

Water bottle with water abandoned at the forest floor can cause forest fire !

It is unwise to throw bottles in the forest. It is not only concern about the pollution it may bring to the native forest, or animals that can eat plastics and died of suffocation or chocking. The worried concern is causing forest fires. Although it don’t have a direct documented incident, but it still stay a risk to get note of.

Don’t place water bottles with water beside a open window !

There had been cases documented fires started at a vicinity of the open window during hot sunny day. Sunlight may hit the water filled bottles and cause a focused beams on nearby objects like papers or curtains.

Small things that we are not aware of is the next big thing that can rise disaster. Never underestimate what a clear bottle with water can do.