The Physics
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Focal Length of Eyeglasses

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Cameron, John R.; James G. Skofronick & Roderick M. Grant. Physics of the Body. Second Edition. Madison, WI: Medical Physics Publishing, 1999: 336. "If F is measured in meters, then 1/F is the lens strength in diopters (D) …. A lens with a focal length of +0.1 m has a strength of +10.0 D." 0.1 m
Chapman, Bill. Coping with Vision Loss. Hunter House Inc.: 2001. Page 141. "By definition, a +1 D lens focuses light at one meter. One meter equals about 40 in. The formula for determining the focal length of any lens is: 40/D. = inches of the focal length. The focal length of a +10 D lens = 40/10 = 4 in." 1.0 m
Crick and Khaw. A Textbook of Clinical Ophthalmology, 2nd Edition. World Scientific, 1997: 55. "The power of a lens is measured in diopters (D), the diopter being the reciprocal of the focal length in meters. Thus a 1 D lens has a focal length of 1 meter, while a 2 D lens has a focal length of 0.5 meter." 0.5 m
Cutnell, John D. and Johnson, Kenneth W. Physics: Fourth Edition. Wiley, 1998: 811. "Refractive power of a lens (D) = 1/F (focal length in meters) … Converging lenses have a refractive power of 1 diopter if it focuses parallel light rays to a focal point 1 m beyond the lens." 1. 0 m
Serway, Raymond A. and Beichner, Robert J. Physics: For Scientists and Engineers with Modern Physics. Fifth Edition. Harcourt, 2000: 1170. "The power P of a lens in diopters equals the inverse of the focal length in meters: P = 1/f. For example, a converging lens of focal length +20 cm (+0.2 m) has a power of +5.0 diopters." 0.2 m

As they say, it would be a blurry world without eyeglasses. There are two different types of eyeglasses: those with converging lenses and those with diverging lenses. The distance from the focus to the lens is called the focal length. Converging lenses are convex with a positive focal length and diverging lenses are concave with a negative focal length.

Convex lenses are a series of prisms that get stronger as the outsides of the lens are reached and in this manner, light is refracted to converge on a point called the focus beyond the lens. Concave lenses diverge parallel rays, which appear to originate from a focus in front of the lens. Concave lenses are the ones that make a person&'s eyes look bigger than they are by magnifying them. Convex are the most commonly used lenses for glasses.

Focal length is not stated directly in a prescription for eyeglasses. Instead, the refractive power is used to describe the extent to which a lens refracts light. The formula used to find the refractive power of the lens (in diopters) is the inverse of the focal length (f: given in meters). This relationship shows that the greater the power of a lens, the shorter the focal length. For example a if the refractive power of a lens is 2.0 D then the focal length would be the inverse: 0.5 m.

Refractive Power (diopters) = 1/f (meters)

Ramanjit Kaur -- 2002

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