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Combination of Thin Lenses If two thin lenses are used to form an image, the system can be treated in the following virtual object for the second lens (that is, in the thin lens equation, p is negative). The same procedure can be extended to a system Let us consider the special case of a system of two lenses of focal lengths f 1 and f 2 in contact with each other. If p 1 " p is the object distance for the combination, applica- tion of the thin lens equation (Eq. 36.16) to the first lens gives 1 & 1 q
1 " 1 f
1 S E C T I O N 3 6 . 4 • Thin Lenses 1149 this case, the thin lens equation gives $ 3.33 cm q "
1 5.00 cm & 1 q " 1 $ 10.0 cm
and the magnification of the image is This confirms that the image is virtual, smaller than the & 0.667 M " $ $ $ 3.33 cm 5.00 cm % " Example 36.11 A Lens Under Water equation by the second gives Because f air " 40.0 cm, we find that f water " 3.71f air " 3.71(40.0 cm) " The focal length of any lens is increased by a factor 148 cm f
water f
air " n $ 1 n! $ 1 " 1.52 $ 1 " 3.71 A converging glass lens (n " 1.52) has a focal length of Solution We can use the lens makers’ equation (Eq. 36.15) 1 and R 2 remain the same in air and water: where n! is the ratio of the index of refraction of glass
1 f
water " (n! $ 1) $ 1 R 1 $ 1 R
2 %
1 f
air " (n $ 1) $ 1 R 1 $ 1 R
2 % Investigate the image formed for various object positions and lens focal lengths at the Interactive Worked Example link at Light from a distant object is brought into focus by two converging lenses. Henry Leap and Jim Lehman |