|
|
Various lens shapes are shown in Figure 36.27. Note that a converging lens is thicker at the center than at the edge, whereas a diverging lens is thinner at the center Magnification of Images Consider a thin lens through which light rays from an object pass. As with mirrors (Eq. From this expression, it follows that when M is positive, the image is upright and on Ray Diagrams for Thin Lenses Ray diagrams are convenient for locating the images formed by thin lenses or systems To locate the image of a converging lens (Fig. 36.28a and b), the following three rays are drawn from the top of the object: M " h! h " $ q p S E C T I O N 3 6 . 4 • Thin Lenses 1145 (a) (b) Convex– concave Convex– concave Plano– concave Plano– convex Biconvex Biconcave Figure 36.27 Various lens shapes. (a) Converging lenses have a posi- tive focal length and are thickest at the middle. (b) Diverging lenses have a negative focal length and are thickest at the edges. O (a) F 1 Front F 2 Back I 1 2 3 I (b) F 1 Front F 2 Back O 1 2 3 O (c) F 1 Front F 2 Back I 1 2 3 Active Figure 36.28 Ray diagrams for locating the image formed by a thin lens. (a) When the object is in front of and outside the focal point of a converging lens, the image is real, inverted, and on the back side of the lens. (b) When the object is between the focal point and a converging lens, the image is virtual, upright, larger than the object, and on the front side of the lens. (c) When an object is anywhere in front of a diverging lens, the image is virtual, upright, smaller than the object, and on the front side of the lens. At the Active Figures link at http://www.pse6.com, you can move the objects and change the focal length of the lenses to see the effect on the images. • Ray 1 is drawn parallel to the principal axis. After being refracted by the lens, this ray passes through the focal point on the back side of the lens. • Ray 2 is drawn through the center of the lens and continues in a straight line. • Ray 3 is drawn through the focal point on the front side of the lens (or as if coming from the focal point if p * f ) and emerges from the lens parallel to the |