A magnifying glass "bends light rays to make things look bigger than they are," according to About.com. This fools the eye and brain, causing the light to look as if it comes from a different object. As light rays pass through the convex lens, they spread, making the object appear to be larger than it actually is.
The magnification provided by a lens depends on its placement between one's eye and the object being viewed. Optimal magnification occurs when the user places the lens as close to the eye as possible and then moves the eye and glass together in order to focus. However, most users place the lens closer to the object than the eye.
The first evidence of man using a magnifying glass dates to Alhazen's "Book of Optics" from 1021, according to Reference.com. Such lenses are useful in eyeglasses, as shown in Italy during the mid-13th century.
Convex lenses are thicker at the center than at the edges. When making large convex lenses, the thickness becomes problematic. Fresnel magnifiers use concentric rings to make a large magnifier that remains thin in the center. While difficult to note in a visual inspection of these magnifiers, each concentric ring is slightly thinner than the next. Lighthouses often depend on Fresnel lenses to focus the beam of light that they produce to warn ships of danger.