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).

In glasses used to correct nearsightedness, at least one surface uses concave glass; this can also be considered a minus lens or a diverging lens. These lenses bend parallel rays away from one another. This spreads the light way from the center of the lens and moves the focal point forward. Either a double concave lens which has both surfaces curving inward, a plano-concave with one surface flat and the other one is concave, or the convexo-concave which has one convex surface and one concave surface may be used (Lens).

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