Fresnel lens (pronounced fre-nel) are a very interesting variety of lens. They are a type of lens that enables the construction of large lens with short focal length while maintaining a low weight and volume in comparison to the traditional lens types. These lens are found in video projectors and are also a favourite science toy. They pack so much power in such a small and light package that a good quality fresnel lens of manageable size can easily be used to focus the sunlight so as to instantly vaporize/burn the many things that surround us. The careless handling of a fresnel lens on a bright sunny day can result in burns on the part of the body that the lens may accidently focus the beam on! Read this for some details on using a fresnel lens to melt asphalt and metal coins.

I had purchased an A4 size Fresnel lens earlier this year as part of my experimentation with building a projector on my own. I took out a left over lens today and had some "burning" fun with this (powerful) lens...