Corrective Lenses can either be spherical or cylindrical. The purpose of an eyeglass is to alter the focal length and point of focus in the eye. The focusing ability of an eyeglass is determined by the difference in curvature on its front and back surface, its thickness and its index of refraction. The precise lens can then be chosen to correct any vision condition.

Spherical Lenses

• Diverging (Concave)

  A diverging lens diverges rays of light which are travelling parallel to its principal axis. These lenses are thinner across their middle and thicker at their upper and lower edges. Diverging rays can be traced backwards until they intersect in a point, as seen below for a double concave lens. This is the focal point, F.

  
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• Converging (Convex)

  A converging lens converges rays of light which are travelling parallel to its principal axis. These lenses are thicker across their middle and thinner at their upper and lower edges. Converging rays can be traced until they intersect in a point, as seen below for a double convex lens. This is the focal point, F.

  
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Cylindrical lenses are used most often to correct astigmatism.

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