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A magnifying glass is a
convex lens A lens is a transmissive optics, optical device which focuses or disperses a light beam by means of refraction. A simple lens consists of a single piece of transparent material, while a #Compound lenses, compound lens consists of several simp ...
that is used to produce a magnified
image An image is a visual representation of something. It can be two-dimensional, three-dimensional, or somehow otherwise feed into the visual system to convey information. An image can be an artifact, such as a photograph or other two-dimensiona ...
of an object. The
lens A lens is a transmissive optical device which focuses or disperses a light beam by means of refraction. A simple lens consists of a single piece of transparent material, while a compound lens consists of several simple lenses (''elements''), ...
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 Focus, or its plural form foci may refer to: Arts * Focus or Focus Festival, former name of the Adelaide Fringe arts festival in South Australia Film *''Focus'', a 1962 TV film starring James Whitmore * ''Focus'' (2001 film), a 2001 film based ...
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 A Fresnel lens ( ; ; or ) is a type of composite compact lens developed by the French physicist Augustin-Jean Fresnel (1788–1827) for use in lighthouses. It has been called "the invention that saved a million ships." The design allows the c ...
. The magnifying glass is an icon of
detective fiction Detective fiction is a subgenre of crime fiction and mystery fiction in which an investigator or a detective—whether professional, amateur or retired—investigates a crime, often murder. The detective genre began around the same time as s ...
, particularly that of
Sherlock Holmes Sherlock Holmes () is a fictional detective created by British author Arthur Conan Doyle. Referring to himself as a " consulting detective" in the stories, Holmes is known for his proficiency with observation, deduction, forensic science and ...
.


History

"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 Aristophanes (; grc, Ἀριστοφάνης, ; c. 446 – c. 386 BC), son of Philippus, of the deme In Ancient Greece, a deme or ( grc, δῆμος, plural: demoi, δημοι) was a suburb or a subdivision of Athens and other city-states ...
's ''
The Clouds ''The Clouds'' ( grc, Νεφέλαι ''Nephelai'') is a Greek comedy play written by the playwright Aristophanes. A lampooning of intellectual fashions in classical Athens, it was originally produced at the City Dionysia in 423BC and was not as ...
'' from 424 BC, where magnifying lenses to ignite tinder were sold in a pharmacy, and
Pliny the Elder Gaius Plinius Secundus (AD 23/2479), called Pliny the Elder (), was a Roman author, naturalist and natural philosopher, and naval and army commander of the early Roman Empire, and a friend of the emperor Vespasian. He wrote the encyclopedic '' ...
's "lens", a glass globe filled with water, used to cauterize wounds. (
Seneca Seneca may refer to: People and language * Seneca (name), a list of people with either the given name or surname * Seneca people, one of the six Iroquois tribes of North America ** Seneca language, the language of the Seneca people Places Extrat ...
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 The ''Book of Optics'' ( ar, كتاب المناظر, Kitāb al-Manāẓir; la, De Aspectibus or ''Perspectiva''; it, Deli Aspecti) is a seven-volume treatise on optics and other fields of study composed by the medieval Arab scholar Ibn al- ...
'' by
Ibn al-Haytham Ḥasan Ibn al-Haytham, Latinized as Alhazen (; full name ; ), was a medieval mathematician, astronomer, and physicist of the Islamic Golden Age from present-day Iraq.For the description of his main fields, see e.g. ("He is one of the prin ...
in 1021. After the book was translated during the
Latin translations of the 12th century Latin translations of the 12th century were spurred by a major search by European scholars for new learning unavailable in western Europe at the time; their search led them to areas of southern Europe, particularly in central Spain and Sicily, w ...
,
Roger Bacon Roger Bacon (; la, Rogerus or ', also '' Rogerus''; ), also known by the scholastic accolade ''Doctor Mirabilis'', was a medieval English philosopher and Franciscan friar who placed considerable emphasis on the study of nature through empiri ...
described the properties of a magnifying glass in 13th-century
England England is a country that is part of the United Kingdom. It shares land borders with Wales to its west and Scotland to its north. The Irish Sea lies northwest and the Celtic Sea to the southwest. It is separated from continental Europe b ...
. This was followed by the development of
eyeglasses Glasses, also known as eyeglasses or spectacles, are vision eyewear, with lenses (clear or tinted) mounted in a frame that holds them in front of a person's eyes, typically utilizing a bridge over the nose and hinged arms (known as temples or ...
in 13th-century
Italy Italy ( it, Italia ), officially the Italian Republic, ) or the Republic of Italy, is a country in Southern Europe. It is located in the middle of the Mediterranean Sea, and its territory largely coincides with the homonymous geographical re ...
.


Magnification

The
magnification Magnification is the process of enlarging the apparent size, not physical size, of something. This enlargement is quantified by a calculated number also called "magnification". When this number is less than one, it refers to a reduction in siz ...
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 Magnification is the process of enlarging the apparent size, not physical size, of something. This enlargement is quantified by a calculated number also called "magnification". When this number is less than one, it refers to a reduction in siz ...
(this should not be confused with
optical power In optics, optical power (also referred to as dioptric power, refractive power, focusing power, or convergence power) is the degree to which a lens (optics), lens, mirror, or other optical system converges or diverges light. It is equal to the Mu ...
, 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. 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 one eye, and moving the eye and the lens together to obtain the best
focus Focus, or its plural form foci may refer to: Arts * Focus or Focus Festival, former name of the Adelaide Fringe arts festival in South Australia Film *''Focus'', a 1962 TV film starring James Whitmore * ''Focus'' (2001 film), a 2001 film based ...
. The object will then typically also be close to the lens. The magnifying power obtained in this condition is ''MP''0 = (0.25 m)Φ + 1, where Φ is the optical power in
dioptre A dioptre (British spelling) or diopter (American spelling) is a unit of measurement with dimension of reciprocal length, equivalent to one reciprocal metre, 1 dioptre = 1 m−1. It is normally used to express the optical power of a lens or curv ...
s, and the factor of 0.25 m represents the assumed near point (¼ m from the eye). This value of the magnifying power is the one normally used to characterize magnifiers. It is typically denoted "''m''×", where ''m'' = ''MP''0. This is sometimes called the ''total power'' of the magnifier (again, not to be confused with optical power). However, magnifiers are not always used as described above because it is 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'' = (0.25 m)Φ. A typical magnifying glass might have a
focal length The focal length of an optical system is a measure of how strongly the system converges or diverges light; it is the inverse of the system's optical power. A positive focal length indicates that a system converges light, while a negative foca ...
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.


Alternatives

Magnifying glasses typically have low magnifying power: 2×–6×, with the lower-power types being much more common. At higher magnifications, the image quality of a simple magnifying glass becomes poor due to
optical aberrations In optics, aberration is a property of optical systems, such as lenses, that causes light to be spread out over some region of space rather than focused to a point. Aberrations cause the image formed by a lens to be blurred or distorted, with th ...
, particularly
spherical aberration In optics, spherical aberration (SA) is a type of optical aberration, aberration found in optical systems that have elements with spherical surfaces. Lens (optics), Lenses and curved mirrors are prime examples, because this shape is easier to man ...
. When more magnification or a better image is required, other types of hand magnifier are typically used. A
Coddington magnifier A Coddington magnifier is a magnifying glass consisting of a single very thick lens with a central deep groove diaphragm at the '' equator'', thus limiting the rays to those close to the axis, which minimizes spherical aberration. This allows for ...
provides higher magnification with improved image quality. Even better images can be obtained with a multiple-lens magnifier, such as a
Hastings triplet A triplet lens is a compound lens consisting of three single lenses. The triplet design is the simplest to give the required number of degrees of freedom to allow the lens designer to overcome all Seidel aberrations. The three lenses may be c ...
. High power magnifiers are sometimes mounted in a cylindrical or conical holder with no handle, often designed to be worn on the head; this is called a
loupe A loupe ( ) is a simple, small magnification device used to see small details more closely. They generally have higher magnification than a magnifying glass, and are designed to be held or worn close to the eye. A loupe does not have an attached h ...
. Such magnifiers can reach up to about 30×, and at these magnifications the aperture of the magnifier becomes very small and it must be placed very close to both the object and the eye. For more convenient use or for magnification beyond about 30×, one must instead use a
microscope A microscope () is a laboratory instrument used to examine objects that are too small to be seen by the naked eye. Microscopy is the science of investigating small objects and structures using a microscope. Microscopic means being invisibl ...
.


Use as a symbol

The magnifying glass (, or U+1F50D in
Unicode Unicode, formally The Unicode Standard,The formal version reference is is an information technology Technical standard, standard for the consistent character encoding, encoding, representation, and handling of Character (computing), text expre ...
: 🔍) is commonly used as a symbolic representation for the ability to search or zoom, especially in computer software and websites. U+1F50E is a right-pointing version: 🔎.


See also

*
Aspheric lens An aspheric lens or asphere (often labeled ''ASPH'' on eye pieces) is a lens (optics), lens whose surface profiles are not portions of a sphere or Cylinder (geometry), cylinder. In photography, a camera lens, lens assembly that includes an aspheri ...
*
Binoculars Binoculars or field glasses are two refracting telescopes mounted side-by-side and aligned to point in the same direction, allowing the viewer to use both eyes (binocular vision) when viewing distant objects. Most binoculars are sized to be held ...
*
Burning glass A burning glass or burning lens is a large convex lens that can concentrate the sun's rays onto a small area, heating up the area and thus resulting in ignition of the exposed surface. Burning mirrors achieve a similar effect by using reflecting s ...
*
Dome magnifier A dome magnifier is a dome-shaped magnifying device made of glass or acrylic plastic, used to enlarge words on a page or computer screen. They are plano-convex lenses: the flat (planar) surface is placed on the object to be magnified, and the co ...
*
Glasses Glasses, also known as eyeglasses or spectacles, are vision eyewear, with lenses (clear or tinted) mounted in a frame that holds them in front of a person's eyes, typically utilizing a bridge over the nose and hinged arms (known as temples or ...
* Graphoscope *
Macro lens Macro photography (or photomacrography or macrography, and sometimes macrophotography) is extreme close-up photography, usually of very small subjects and living organisms like insects, in which the size of the subject in the photograph is grea ...
*
Optical microscope The optical microscope, also referred to as a light microscope, is a type of microscope that commonly uses visible light and a system of lenses to generate magnified images of small objects. Optical microscopes are the oldest design of microsco ...
*
Optical telescope An optical telescope is a telescope that gathers and focuses light mainly from the visible part of the electromagnetic spectrum, to create a magnified image for direct visual inspection, to make a photograph, or to collect data through electro ...
*
Reading stone A reading stone is an approximately hemispherical lens that can be placed on top of text to magnify the letters so that people with presbyopia can read it more easily. Reading stones were among the earliest common uses of lenses. The invention o ...
*
Screen magnifier A screen magnifier is software that interfaces with a computer's graphical output to present enlarged screen content. By enlarging part (or all) of a screen, people with visual impairments can better see words and images. This type of assistive te ...
* Stanhope lens


References

{{DEFAULTSORT:Magnifying Glass Magnifiers English inventions