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.
A loupe is a small magnification device used by surgeons, dentists, jewelers, watchmakers, and other precision craftsmen. The magnification of jewelers' loupes for studying gemstones is typically on the order of 10×.
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. 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. 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.
- ↑ Wickell, Carly. Jeweler's Loupe. About: Jewelry / Accessories. About.com. Retrieved on 2007-02-20.
- ↑ Hecht, Eugene (1987). Optics, 2nd ed., Addison Wesley, pp. 186-188. ISBN 0-201-11609-X.
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