Magnifying glass

A magnifying glass (called a hand lens in laboratory contexts) is a convex lens that is used to produce a magnified image of an object. The lens is usually mounted in a frame with a handle (see image).

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.

A magnifying glass can also be used to focus light, such as to concentrate the sun's radiation to create a hot spot at the focus for fire starting.

The magnifying glass is an icon of detective fiction, particularly that of Sherlock Holmes.

The earliest evidence of a magnifying device was a joke in Aristophanes's The Clouds from 424 BC, where magnifying lenses to start kindling 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").Roger Bacon described the properties of a magnifying glass in 13th-century England. Eyeglasses were developed 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, varies with age. In a young child, it can be as close as 5 cm, while, in an elderly person it may be as far as one or two metres. Magnifiers are typically characterized using a "standard" value of 0.25 m.

...
Wikipedia