The Physics
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Focal Length of a Magnifying Glass

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Coleman, Silas Ellsworth. 332: Relative Size and Distance of Object and Image. A Text-Book of Physics, New York: Heath, 1911: 411. "When a lens is used as a simple microscope or a "Magnifying glass" in looking at small objects, the object and the lens are so adjusted that the distance of the magnified virtual image is about 12 or 14 in., or the distance at which a book is held for reading." 30-35 cm
World Encyclopedia. Volume 13M, 2008. "The greater the curve of a lens, the shorter its focal length and the greater it's power. It bends the rays more, and they meet at a smaller distance from the lens. A lens with a focal length 5 inches(13 cm) magnifies an image about 2 times". 13 cm.
Black, Newton Henry. Experiment 48: Magnifying Power of a Simple Lens. A Laboratory Manual in Physics. New York: MacMillan, 1920: 102. "Use a Double convex lens (ƒ 2.5–7.5 cm.)" 2.5–7.5 cm.
Gleichen, Alexander. The Magnifying Glass. The Theory of Modern Optical Instruments. 2nd Ed. Trans. H.H. Emsley & W. Swaine, 1921: 136–137. "In order to view with the unaccommodated eye and in natural sizes photographs which have been taken with a comparatively small objective focal length (say 10–15 cm.) we make use of the magnifying glass." 10–15 cm.
Levine, Judah. The simple Magnifying Glass. Class notes, Fall 2001. University of Colorado at Boulder. Fall 2001. "For example, if the focal length of the additional lens is 5 cm., the magnification is 1+5=6, and the object will be in focus when it is about 4.2 cm. in front of the eye." 5 cm.

All our life we have known a magnifying glass as a tool for discovering clues. We also have known it to be a tool for enlarging and making things bigger. A magnifying glass is not only used for this it is used for many experiments for example to measure the focal length of a convex lens. A magnifying glass is a double convex or converging lens. A lens is a curved transparent material that bends light. These lenses are used to produce a magnified image of an object. In the magnifying glass's lens there is a line that runs through the center of the lens and perpendicular to its surface it is called the principal axis. All light rays that enter parallel to the principal axis are refracted toward the focus, where they come together or converge. The distance from the focal point to the lens is called the focal length (ƒ). The reciprocal of the focal length is a measure of how strongly an optical system focuses or defocuses light.

The mathematical relationship that describes the behavior of all lenses is:

1/focal length = 1/distance from object to lens + 1/distance from image to lens

And that's how you measure the focal length of a magnifying glass.

Mona Zawam -- 2008