The technical bit!
A Fresnel magnifying lens is able to produce a magnified image because it contains a series of concentric grooves that are integral to the surface of the thin acrylic sheet that makes up the lens. In a normal magnifying glass, the piece of glass acts as a single lens, bending parallel rays of light to a common focal length. In the fresnel lens, the concentric grooves all act as tiny lenses to bend parallel light with the same effect.
The deeper the grooves in the fresnel lens, the greater its ability to magnify. But, image quality is only retained at up to x5 magnification, so fresnel lens magnifiers tend to be either approx. x3 or x4 magnification. At these levels of magnification, clarity is excellent and this magnification is ideal for reading small or difficult to read print.
This makes fresnel lens magnifiers ideal for close work - genealogy, reading handwritten documents, reading contracts, terms and conditions, maps, bus and train timetables, reading books, doing crosswords, reading food labels and drug warning labels.