Mirror for Making Fire Using Sunlight.




by tonytran2015 (Melbourne, Australia)

Figures: Reverse sequences of lighting a cigarette by the system at 5seconds and 1second.

This post shows how to set up a mirror and use it for quickly starting fire using moderate sunlight or for heating tiny components very cleanly using strong sunlight. The aim is to set up a low cost, robust, reliable system from common household items. The system serves as a useful fire lighting device composed of only two useful common devices (a mirror and a magnifying glass) in any prolonged emergency such as after a disaster.

The tiltable mirror allows alignment of its axis towards the Sun and its flat mirror combines with the magnifying glass to produce a tiny image of the Sun with all energy going through the magnifying glass converging on that tiny image.

The radiative heat flux from the Sun is concentrated by a factor C given by

C = Pi*d*d/(Pi*f*a*f*a) = (d/f)*(d/f)*(1/a)*(1/a)

where d and f are respectively the diameter and focal length of the lenses and a is the angular diameter of the Sun (a = 0.5*3.14/180radian = 0.0087 radian).

The geometric concentration of factor of this system is

C = (1/1.5)*(1/1.5)*(115)*(115) = 5800.

Allowing for some transmission loss, the concentration is downgraded but is still quite high and the heated area is a not too small 2mm diameter circle. So fire making should be easy with it.

The mirror and the magnifying glass cost 3USD and 3USD respectively. Do NOT use the concave sides of common make up mirrors as they are not accurately made and cannot focus sunlight into sharp pictures.

WARNING: The system is very potent and should be locked away from sunlight or dismantled after use to avoid house fire caused by its misplacement.

Step 1: Required Materials.

Required materials:

1. A tiltable make-up mirror,

2. A quality, 100mm diameter glass magnifying.

3. Some 2m of clear sticky tape.

The magnifying glass used here has a focal length of 300mm and is colourless.

Step 2: Assembling the Mirror.

1. Tape the sharp metal edge of the metal frame of the glass magnifying glass to prevent scratching the make-up mirror.

2. Use sticky tape to tape the magnifier handle onto the FLAT side of the tiltable mirror.

3. Use sticky tape to tape the diametrically opposite metallic side of the magnifying glass to the mirror.

4. Use more sticky tape to keep the magnifier strongly attached to the mirror.

Note that the concave side of most make-up mirrors is not precise enough for this type of set up.

Step 3: Aligning the Mirror and Finding the Hot Spot.

1. Let the mirror axis points towards the Sun.

2. Move a thin, narrow strip of paper across and along the axis of the magnifier to find the focal point where sunlight converges to a smallest size circle with extreme brightness.

Step 4: Making a Fire.

Use a dry, fresh cigarette as tinder/fuel.

Place the tip of the cigarette at the focal point (where concentrated sunlight is brightest) and let it point towards the mirror. Depending on its brand, the cigarette may light up more easily at its inside filling or at its paper enveloping side.

In the experiment in the pictures, smouldering began in less than 1 second and after 5 seconds the tip of the cigarette was already burning with a red glow. The test was carried out in winter (2016 Jan 5th, 14:30hr) in Saigon (latitude 10 degrees N) and the Sun elevation was about 35 degrees, just breaking out of the clouds in a cloudy day. The cigarette in use was from a freshly opened package of (Vietnamese) Craven brand.


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[1]. tonytran2015, Lighting cigarettes and making fire with sunlight, Instructables , https://www.instructables.com/id/Lighting-cigarette..., posted on Nov 17, 2015.



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    14 Discussions


    3 years ago

    Your focal length will be fixed for getting the most intense focus. You should create a little bracket to attach to the back of the mirror and come around the bottom (or top, I don't judge). The length of the bracket would be the same length as the focal point. That way you would just set your flammable object on the end of the bracket and *woosh*! Does that make sense?

    2 replies

    Reply 3 years ago

    Thank you for your suggestions. They are quite valid and would be useful to anyone wanting to keep the materials for longer time at the focal point.

    I suppose that with the bracket(s) the system would look like a radar disk.


    Reply 3 years ago

    "I suppose that with the bracket(s) the system would look like a radar disk."

    Exactly what I was thinking of when I saw your 'ible.


    Reply 3 years ago

    You are right. Heat flux density is power divided by area of the image at the focal plane. It increases by four when the diameter of the image of the Sun is halved while the total heat flux does not increase. The increase in heat flux density comes from the shortening of the focal length. There is also an undesirable accompanied loss as light reflects off the mirror and then goes through the lens a second time.

    The advantage here is the low cost for a precision system that can focus light into a sharp, crisp circular disc. Any prescribed concave mirror satisfying the requirements would cost a lot. Most make-up concave mirrors cannot satisfy the requirements.

    Just4Fun Media

    3 years ago

    Interesting build. I have never used a magnifying glass and a mirror together. How warm was it outside when you conducted your read?

    Have a great day! :-)

    1 reply
    tonytran2015Just4Fun Media

    Reply 3 years ago

    Thank you for your comments.

    My test data are: Overcast sky, with the Sun just breaking out from the clouds, temperature of 31 degrees Celsius, humidity of 71%, wind speed of 5km/hr and altitude of 5m above see level. It would be interesting if you can conduct the test in other conditions and supply your test data.

    You are correct ! However precision concave mirrors are too expensive for most people. The system of a mirror and a magnifying glass is equivalent to a concave mirror but is much cheaper.