Myopia, commonly known as near-sightedness or short-sightedness, occurs when the light entering the eye is refracted too much.  This causes the image to form in front of the retina.  To correct this type of disorder, a diverging lens can be placed in front of the eye to alter the light-ray paths so that they converge on the surface of the retina and restore clear vision.

Image by slightly_different from Pixabay

 

The diverging lens is identified as a negative prescription because it has a negative, virtual focal length.  The amount of refraction caused by the lens is identified numerically using a unit called diopters, which is the reciprocal of the lens's focal length.  For example, a −2 lens prescription is a diverging lens with a virtual focal length of  12  metre. 

A −3 lens will refract light to a greater extent than the −2 lens because it has a shorter focal length of  13  metre.  The strength of the lens is related to both the refractive index of the material from which it is made and the curvature of the surface.  Different prescriptions are manufactured by altering either the lens material or its curvature.

Hyperopia, or far-sightedness, occurs when light is not refracted enough to converge on the retina.  To correct this condition, a converging lens is used to refract the light farther, effectively moving the image formation forward so that the light occurs on the retina tissue.


Far-sightedness is treated with positive diopter prescriptions.  For example, a +3 lens has a real focal length of  13 m.  Reading glasses are a good example of a positive prescription lens that many people use to view objects up close, such as the text in books.

In a general sense, eyeglasses are an extension of the optical system of the human eye and are capable of causing specific amounts of refraction to remedy imperfections in the lens or cornea of the user.  The thin lens equation and ray diagrams are used to understand and predict image formation in such applications.