magnifying glass

The stamp is under the examination of a magnifying glass

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

A magnifying glass works by creating a magnified virtual image of an object behind the lens. The distance between the lens and the object must be shorter than the focal length of the lens for this to occur. Otherwise, the image appears smaller and inverted, and can be used to project images onto surfaces.

The framed lens may be mounted on a stand, keeping the lens at the right distance from the table, and therefore at the right distance from the object on the table. The latter applies if the object is small, and also if the height is adjustable. Some magnifying glasses are foldable (from the handle or stand).

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.

Visually impaired people often benefit from magnifying glasses and similar low vision aids.

History

The earliest evidence of "a magnifying device, a convex lens forming a magnified image," dates back to the Book of Optics published by Alhazen in 1021. After the book was translated into Latin, during the Latin translations of the 12th century, Roger Bacon described the properties of magnifying glass in 13th-century England, followed by the development of eyeglasses in 13th-century Italy.^[1]

Magnification

Magnifying glass on an arm lamp

The magnification of a magnifying glass depends on where it is placed between the user's eye and the object being viewed, and the total distance between the eye and the object. Magnifying glasses are typically described in terms of their magnifying power, which 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.^[2] For the "without" case, it is typically assumed that the user would bring the object as close to the 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.

The highest magnifying power is obtained by putting the lens very close to the eye and moving the eye and the lens together to obtain the best focus. The object will then typically also be close to the lens. The magnifying power obtained in this condition is MP₀=¼Φ+1, where Φ is the optical power in dioptres, and the factor of ¼ comes from the assumed distance to the near point (about 25 cm from the eye). This value of the magnifying power is the one normally used to characterize magnifiers. It is typically denoted "m×", where m=MP₀. This is sometimes called the total power of the magnifier (again, not to be confused with optical power).

Magnifiers are not always used as described above, however. It is much more comfortable to put the magnifier close to the object (one focal length away). The eye can then be a larger distance away, and a good image can be obtained very easily; the focus is not very sensitive to the eye's exact position. The magnifying power in this case is roughly MP=¼Φ.

A typical magnifying glass might have a focal length of 25 cm, corresponding to an optical power of 4 dioptres. Such a magnifier would be sold as a "2×" magnifier. In actual use, an observer with "typical" eyes would obtain a magnifying power between 1 and 2, depending on where lens is held. An older person might obtain an actual magnifying power of 8 or more with this lens, however, due to the eye's longer near point distance.

See also

Wikimedia Commons has media related to: Magnifying glass

• Burning-glass
• Eyeglasses
• Loupe
• Screen magnifier
• Coddington magnifier

References

1. ^ Kriss, Timothy C.; Kriss, Vesna Martich (April 1998), "History of the Operating Microscope: From Magnifying Glass to Micro neurosurgery", Neurosurgery 42 (4): 899–907, doi:10.1097/00006123-199804000-00116 
2. ^ Hecht, Eugene (1987). Optics (2nd ed.). Addison Wesley. pp. 186–188. ISBN 0-201-11609-X.