A magnifying glass is a convex lens that creates a virtual image of the object that appears behind the lens. The image will appear larger than the object when the distance of the magnifying lens to the object is less than the focal length of the magnifying glass. Otherwise, the image will be smaller than the object and inverted.
The highest magnification of a lens may be achieved by bringing the object as close to the eye as possible without it becoming blurry. This distance is known as the “near distance” and it generally increases with the age of the viewer. The near distance may be as short as five centimeters in a young child and up to two meters in an elderly viewer. A near distance of 25 centimeters (cm) is frequently given as a reference standard.
The magnifying glass is then placed very close to the eye, between the eye and the magnifying glass. The distance between the magnifying glass and the object is then adjusted to achieve the best possible focus. The magnification of the lens in this configuration is given as M = n/f + 1 where M is the magnification, n is the near distance and f is the focal length of the lens.
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A magnifying glass may also be used by placing it about one focal length from the object. The object is much more comfortable to view in this configuration because the eye can be further away from the magnifying glass and the focus is not as dependent upon the eye's position. The magnification in this position is given by M = n/f.
Examples and Calculations
The magnification range of a magnifying glass is given as n/f < M < (n/f + 1) where n is the near distance and f is the focal length in meters. The optical power of a magnifying glass is given as d = 1/f where the focal length is measured in meters. A common magnifying glass has an optical power of 4 diopters, giving a magnification range between 4n and 4n + 1. Assuming the near distance average of 25 cm (¼ meter), the magnification range of a 4 diopter magnifying glass would be between 1 and 2 for the average person. However, someone with a near distance of two meters might obtain a magnification of up to 8 or more.