Simple though it may seem, corrective eyeglasses are a very complicated invention. The actual inventing of eye glasses evolved throughout a very extensive time period but the closest date to what could most nearly be classified as the invention of eyeglasses would be sometime in 1303 when physician Bernard of Gordon noted that the use of spectacles could be used in correcting long-sightedness and then later in the century it is speculated that Roger Bacon probably actually invented this device (Broton). From there on researchers began experimenting with curvatures and widths of lenses to eventually aid the visually impaired.
Eyeglasses have a variety of structures to that are used for aiding and correcting defective sight. Generally they consist of a pair of lenses in a frame to hold them in the correct position in front of the eyes. Lenses are in most cases made of ground glass. A lens works by creating an image through the refraction of light. Each lens surface is made as a part of a spherical surface. To each surface there is what is called a center of curvature. This is the point in he middle of the sphere at which every point on the surface is equally spaced around. There are two centers of curvature in a lens and the line joining the two centers passes through a point called the optical center. The light is refracted, or bent, through the entire lens except through this point. Where the line joining the two centers passes through is called the principal axis. Any line that passes at an angle to the principal axis is called a secondary axis. Any light that is refracted through the lens is bent toward the thickest portion of the lens. There are basically two basic types of lenses, concave and convex. Concave lenses are thicker at the edges then at the center while convex lenses are thinner at the edges then at the center. These are used separately or in coordination with each other to correct different eye problems (Lens, In Optics, Physics).