. A text book of physics, for the use of students of science and engineering . Fig. 542.—Convex lens. Example.—A double convex lens has radii of curvature 16 and 24 , and the glass has index of refraction 1-5. Find the focallength of the lens, and the position of the image when the object is 32 it. r1= - 24 and ra = +16; In Fig. 542, 1 / 1 1\ l/5\ (from equation . /= -??= -19-2 cm. 1 Again, 96 5 1 1 . v u~f132 I 96 115 2 ~v~32 96 ~ 96 v= -48; that is, the image is 48 cm. from the lens on the side remotefrom the object. EXPT. 128.—Focal length of a converging lens. Place
. A text book of physics, for the use of students of science and engineering . Fig. 542.—Convex lens. Example.—A double convex lens has radii of curvature 16 and 24 , and the glass has index of refraction 1-5. Find the focallength of the lens, and the position of the image when the object is 32 it. r1= - 24 and ra = +16; In Fig. 542, 1 / 1 1\ l/5\ (from equation . /= -??= -19-2 cm. 1 Again, 96 5 1 1 . v u~f132 I 96 115 2 ~v~32 96 ~ 96 v= -48; that is, the image is 48 cm. from the lens on the side remotefrom the object. EXPT. 128.—Focal length of a converging lens. Place a converging lensupright by fixing it in a slit in a cork, as shown at L (Fig. 543). At some XLVI OPTICAL CENTRE OF A LENS 593 distance from L place a blanket pin A. The lens produces an image ofA at A, and if the eye be placed at E to receive the rays, ths invertedimage of the pin will be seen. Place another pin B, so that B and A donot separate when the eye is moved about. B and A are then coincident. Fig. 543.—Measurement of focal length of a converging lens. in position. Measure AL and BL, and calling them u and v, calculate /.Repeat for a number of distances, and tabulate the results as describedon p. 570. Optical centre of a lens.—For every lens there is some point forwhich the rays passing through it are not deviated by the lens. LetA and B be the centres of curvature of the faces of the lens in either(a) or (b) (Fig. 544). Draw AC and BD parallel to each other. These
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