"Hand lens" redirects here. For a magnifying device held close to the eye, seeLoupe.
A pen seen through a magnifying glassJim Hutton as detectiveEllery Queen, posing with a magnifying glass
Amagnifying glass is aconvex lens—usually mounted in a frame with a handle—that is used to produce amagnifiedimage of an object. A magnifying glass can also be used to focus light, such as to concentrate the Sun's radiation to create a hot spot at thefocus for fire starting.
Evidence of magnifying glasses exists from antiquity. The magnifying glass is an icon ofdetective fiction, particularly that ofSherlock Holmes.
An alternative to a magnifying glass is asheet magnifier, which comprises many very narrow concentric ring-shaped lenses, such that the combination acts as a single lens but is much thinner.
Theconvex lens of a magnifying glass can be used to produce amagnifiedimage of an object. A magnifying glass can also be used to focus light, such as to concentrate the Sun's radiation to create a hot spot at thefocus for fire starting.[1]
Themagnification 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. Themagnifying power is equivalent toangular magnification (this should not be confused withoptical 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 one eye as possible without it becoming blurry. This point, known as thenear point ofaccommodation, 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 bestfocus. The object will then typically also be close to the lens. The magnifying power obtained in this condition isMP0 =doΦ + 1, whereΦ is the optical power indioptres anddo is thenear point of the eye, which is typically assumed to be 0.25 m.[2] This value of the magnifying power is the one normally used to characterize magnifiers. It is typically denoted "m×", wherem =MP0. This is sometimes called thetotal power of the magnifier (not to be confused with optical power).
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 roughlyMP =doΦ.[2]
A typical magnifying glass might have afocal 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. The dependence on the value of near point means that an older person obtains more magnification from a magnifying glass than a young person does.
"The evidence indicates that the use of lenses was widespread throughout the Middle East and the Mediterranean basin over several millennia".[3] Archaeological findings from the 1980s in Crete's Idaean Cave unearthed rock crystal lenses dating back to theArchaic Greek period, showcasing exceptional optical quality. These discoveries suggest that the use of lenses for magnification and possibly for starting fires was widespread in the Mediterranean and Middle East, indicating an advanced understanding of optics inantiquity.[4] The earliest explicit written evidence of a magnifying device is a joke inAristophanes'sThe Clouds[5] from 424 BC, where magnifying lenses to ignite tinder were sold in a pharmacy, andPliny the Elder's "lens",[6] 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".[7][8])
Magnifying glasses typically have low magnifying power: 2×–6×, with lower magnification providing a wider lens and larger field of view.[11] At higher magnifications, the image quality of a simple magnifying glass becomes poor due tooptical aberrations, particularlyspherical aberration. When more magnification or a better image is required, other types of hand magnifier are typically used. ACoddington magnifier provides higher magnification with improved image quality. Even better images can be obtained with a multiple-lens magnifier, such as aHastings triplet.
Achromatic triplet loupe
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 aloupe.[12] Such magnifiers can reach up to about 25×, although 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.[13] For more convenient use or for magnification beyond about 25×, amicroscope is necessary.
For the reduction of image distortion and color fringes, a loupe can use multiple lenses, forming acompound lens. These can be designed to correct for both spherical andchromatic aberration effects, with, for example, one lens being biconvex and the other bi-concave. Such are termedapochromatic lens systems. High quality loupes will include three lenses for this purpose, and are termed triplets.[13]
A plasticFresnel lens sold as a TV-screen magnifier
Asheet magnifier comprises 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 aFresnel lens. Fresnel lenses are used as magnifiers, for example for reading printed text.[14]
The magnifying glass (, or U+1F50D inUnicode: 🔍) is commonly used as a symbolic representation for the ability to search or zoom, especially in computer software and websites.[15][16] U+1F50E is a right-pointing version: 🔎.
^The history of the telescope by Henry C. King, Harold Spencer Jones Publisher Courier Dover Publications, 2003 Pg 25ISBN0-486-43265-3,ISBN978-0-486-43265-6
^abKriss, 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.PMID9574655.
^Amsel-Arieli, M. (2014). Magnifying Glass.History Magazine,16(1), 6–7.
