File:Image from page 454 of "Practical physics" (1922) (14782824044).jpg

Identifier: practicalphysics00mill Title: Practical physics Year: 1922 (1920s) Authors: Millikan, Robert Andrews Gale, Henry Gordon Subjects: Physics Publisher: Boston : Ginn and Co. Contributing Library: University of British Columbia Library Digitizing Sponsor: University of British Columbia Library

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Text Appearing Before Image: e the light en-ters the eye. If we wish to use a lensas a magnifying glass tothe best advantage, weplace the eye as close toit as we can, so as togather as large a cone ofrays as possible, and thenplace the object at a distance from the lens ecpial to its focallength, so that the waves after passing through it are plane.They are then focused by the eye with the least possibleeffort. The visual angle in such a case is PcQ (Fig. 431,(1)) ;for, since the emergent waves are plane, the rays which passthrough the center of the eye from P and Q are parallel to thelines through Pc and Qc. But if the lens were not present, andif the ol)ject were 25 centimeters from the eye, the visual anglewould be the small angle j^cq (Fig. 431, (2)). The magnify-ing power of a simple lens is due, therefore, to the fact thatby its use an object can be viewed distinctly when held closerto the eye than is otherwise possible. This condition gives avisual angle that increases the size of the imas^e on the retina.

Text Appearing After Image: Fig.431. power of a lens 396 IMAGE FORMATION The less the focal length of the lens, the nearer to it may theobject be placed, and therefore the greater the visual angle,or magnifying power. The ratio of the two angles PcQ and j^cq is approximately25/f, where / is the focal length of the lens expressed incentimeters. Now the magnifying poiver of a lens or microscojyeis defined as the ratio of the angle actually subtended by the imagewhen viewed through the instrument, to the angle subterided by theobject tvhen viewed with the unaided eye at a distance of 25 centi-meters. Therefore the magnifying power of a simple lens is25/f. Thus, if a lens has a focal length of 2.5 centimeters, itproduces a magnification of 10 diameters when the object isplaced at its principal focus. If the lens has a focal lengthof 1 centimeter, its magnifying power is 25, etc. 457. Magnifying power of an astronomical telescope. In the astronom-ical telescope the objective, or forward lens, forms at its principal f

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