Fresnel lens are made of a thin plastic, flat on one side and ridged on the other.  The side that is ridged is made up of rings like the rings of a tree. Each ring is slightly thinner than the next and focuses the light toward the center.  So each rings angle is different and faces towards the center concentrating all light on to one focal point.  They are very flexible and light in weight.  If wanting to start a fire with a fresnel lens it just takes a few seconds to focus and have one burning.  Be cautious not to focus on someone's skin or eyes.  Fresnel lens come in all different sizes and can be found online or even in bookstores.  They are used for people to read with but are efficient for starting fires and for survival fire starters.