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Concept
Magnifying glass
Summary
A magnifying glass is a convex lens that is used to produce a magnified of an object. The lens is usually mounted in a frame with a handle. A magnifying glass can be used to focus light, such as to concentrate the sun's radiation to create a hot spot at the focus for fire starting.
A sheet magnifier consists of many very narrow concentric ring-shaped lenses, such that the combination acts as a single lens but is much thinner. This arrangement is known as a Fresnel lens.
The magnifying glass is an icon of detective fiction, particularly that of Sherlock Holmes.
"The evidence indicates that the use of lenses was widespread throughout the Middle East and the Mediterranean basin over several millennia". The earliest explicit written evidence of a magnifying device is a joke in Aristophanes's The Clouds from 424 BC, where magnifying lenses to ignite tinder were sold in a pharmacy, and Pliny the Elder's "lens", a glass globe filled with water, used to cauterize wounds. (Seneca wrote that it could be used to read letters "no matter how small or dim".) A convex lens used for forming a magnified image was described in the Book of Optics by Ibn al-Haytham in 1021. After the book was translated during the Latin translations of the 12th century, Roger Bacon described the properties of a magnifying glass in 13th-century England. This was followed by the development of eyeglasses in 13th-century Italy.
The magnification of a magnifying glass depends upon where it is placed between the user's eye and the object being viewed, and the total distance between them. The magnifying power is equivalent to angular magnification (this should not be confused with optical power, which is a different quantity). The magnifying power is the ratio of the sizes of the images formed on the user's retina with and without the lens. For the "without" case, it is typically assumed that the user would bring the object as close to one eye as possible without it becoming blurry. This point, known as the near point of accommodation, varies with age.
Official source
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