You may already have in your possessionsome excellent lenses for making fires. Knowing about them will help you make fire quickly from sunlight when facing an emergency. All lenses described in this blog can make a cigarette smoulder in less than 30 seconds and can light up that cigarette afterwards.
1. Concentration of radiating heat flux by a lens.
The radiating heat flux from the Sun is concentrated by a factor C given by
C = Pi*d*d/(Pi*f*a*f*a) =
C = (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 angular diameter of the Sun cannot be changed. It is obvious that we have to increase the ratio (d/f), which is called the aperture number of the lenses, to increase the heat flux concentration. The deciding factor for success…
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