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A magnifying glass is a convex lens that is used to develop a magnified image of an object. The lens is usually mounted in a framework or hand held on the chart table..
Also (known as a hand lens in laboratory contexts) is a convex lens that's used to acquire a magnified image of an object. The lens is commonly mounted in a framework with a handle A sheet magnifier comprises of many very narrow concentric ring-shaped lenses, such that the combination behaves as a single lens but is a great deal thinner.
This placement is called a Fresnel lens. The magnifying glass is an icon of detective fiction, particularly that of Sherlock Holmes.
The earliest evidence of "a magnifying device, a convex lens forming a magnified image" was Aristophanes's "lens", from 424 BC, a glass globe filled up with water. (Seneca wrote that it could be used to read letters "disregarding how small or dim"). Roger Bacon described the properties of a magnifying glass in 13th-century England. Spectacles were developed in 13th-century Italy.
Mounted on an arm with a lamp.
The magnification depends on where it is positioned between the user's eye and the object being looked at, 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.
Since the "without" case, it is typically acquired 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, changes with age. In a young child it can be as close as 5 cm, although in an elderly person it can possibly be as far as one or two meters. Magnifiers are commonly characterized using a "standard" value of 0.25 m.
The most high-level magnifying power is found by positioning the lens very close to the eye and moving the eye and the lens together to find the most acceptable focus. The object will then typically also be close to the lens. The magnifying power found in this condition is MP0 = ¼F + 1, where F is the optical power in diopters, and the factor of ¼ comes from the accepted distance to the near point (¼ m from the eye).
This value of the magnifying power is the one typically used to characterize magnifiers. It is generally denoted "m×", where m = MP0. This is occasionally called the total power of the magnifier (again, not to be mixed up with optical power).
Magnifiers are not always used as depicted above, nevertheless. It is often more comfortable to put the magnifier close to the object (one focal length away). The eye can then be a greater distance away, and a good image can be found really easily; the focus is not very sensitive to the eye's precise position. The magnifying power in this case is approximately MP = ¼F.
A typical glass could have a focal length of 25 cm, proportionate to an optical power of 4 dioptres. Such a magnifier would be traded as a "2×" magnifier. In actual use, an observer with "typical" eyes would find a magnifying power between 1 and 2, depending on where lens is held.
Applying this principle, a magnifying glass can also be used to focus light, such as to concentrate the sun's radiation to produce a hot spot at the focus.
A 30× Hastings triplet magnifier
Magnifying glasses generally 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 gets poor due to optical aberrations, in particular spherical aberration. While more magnification or a improved image is wanted, other types of hand magnifier are commonly applied. A Coddington magnifier allows for higher magnification with improved image quality.
Still better images can be found with a multiple-lens magnifier, such as a Hastings triplet. High power magnifiers are sometimes mounted in a cylindrical or cone-shaped holder with no handle. This is called a loupe.
Such magnifiers can achieve up to about 30×, and at these magnifications the aperture of the magnifier gets very small and it must be situated very close to both the object and the eye. For more convenient use or for magnification beyond about 30×, one must alternatively apply a microscope.
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