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Chapter 36

Chapter 36. Image Formation. Outline. Forming images with a plane mirror Spherical mirrors Concave mirror and convex mirror Forming images with a concave or convex mirror Ray tracing (ray diagram) Mirror equation Sign conventions. Forming Images with a Plane Mirror.

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Chapter 36

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  1. Chapter 36 Image Formation PHY 1371

  2. Outline • Forming images with a plane mirror • Spherical mirrors • Concave mirror and convex mirror • Forming images with a concave or convex mirror • Ray tracing (ray diagram) • Mirror equation • Sign conventions PHY 1371

  3. Forming Images with a Plane Mirror • Forming a mirror image: • The light from an object reflects from a mirror before it enters our eyes. • To the observer, it appears that the rays are emanating from behind the mirror. • Some properties of a plane mirror image: • It is upright. • It is the same distance behind the mirror as the object is in front of the mirror. • It is the same size as the object. • It is a virtual but NOT a real image. PHY 1371

  4. Conceptual Checkpoint • To save expenses, you would like to buy the shortest mirror that will allow you to see your entire body. Should the mirror be • (a) half your height, • (b) two-thirds your height, or • (c) equal to your height? PHY 1371

  5. Spherical Mirrors • A spherical mirror has the same shape as a section of a sphere. • Concave mirror: The insidesurface is reflecting. • Convex mirror: The outside surface is a reflecting. • Center of curvature C: the center of the sphere with radius R of which the mirror is a section. • Principal axis: a straight line drawn through the center of curvature and the midpoint of the mirror. • Focal point and focal length (see next slide) PHY 1371

  6. Focal Point and Focal Length of Convex and Concave Mirrors • Focal point F • Focal length f: • For a convex mirror: f = - (1/2)R. “-” sign indicates that the focal point F lies behind the mirror. • For a concave mirror: f = (1/2)R. “+” sign indicates that the focal point is in front of the mirror. In this case, the rays of light actually pass through and converge at the focal point F. Convex mirror PHY 1371 Concave mirror

  7. Forming Images with a Convex and Concave Mirror • Two techniques to find the orientation, size, and location of an image formed by a spherical mirror: • (1) Ray tracing (ray diagram): Gives the orientationof the image as well as qualitative information on its location and size. • (2) Mirror equation: Provides preciseand quantitative information without the need for accurate scale drawing. PHY 1371

  8. Raying Tracing • Basic idea behind ray tracing: • Follow the path of representative rays of light as they reflect from a mirror and form an image. • Three representative rays: • (1) Parallel ray (P ray): a ray parallel to the principle axis of the mirror • (2) Focal-point ray (F ray): a ray that passes through (concave mirror) or moves toward (convex mirror) the focal point F • (3) Center-of-curvature ray (C ray): a ray that moves along a straight line extending from the center of curvature C Concave mirror PHY 1371 Convex mirror

  9. Ray Diagram for a Convex Mirror • Image properties: • It is a virtual image:no light actually passes through the image. • Orientation: upright (the same orientation as the object). • Size: smaller than the object. • Location: between the mirror and the focal point F. PHY 1371

  10. Ray Diagram for a Concave Mirror • Consider three situations: (a), (b) and (c). • Question: Is a makeup mirror concave or convex? (a) (c) (b) PHY 1371

  11. Mirror Equation • Mirror equation: • (1/do) + (1/di) = 1/f • do (object distance): distance from the mirror to the object. • di (image distance): distance from the mirror to the image. • f: the focal length of the spherical mirror. • Magnification, m: m = hi/ho= - di/do • hi: height of the image • ho: height of the object PHY 1371

  12. Sign Conventions for the Mirror Equation • Focal length • f >0 for concave mirrors • f<0 for convex mirrors • Magnification • m>0 for upright images • m<0 for inverted images • Image distance • di >0 for images in front of a mirror (real images) • di<0 for images behind a mirror (virtual images) • Object distance • do>0 for objects in front of a mirror (real objects) • do<0 for objects behind a mirror (virtual objects) PHY 1371

  13. Examples • Example 36.4 The image formed by a concave mirror: Assume that a certain spherical mirror has a focal length of +10.0 cm. Locate and describe the image for object distances of (A) 25.0 cm, (B) 10.0 cm, and (c) 5.00 cm. • Example 36.5 The image from a convex mirror: An anti-shopping mirror, as the one shown in the figure, shows an image of a woman who is located 3.0 m from the mirror. The focal length of the mirror is -0.25 m. Find (A) the position of her image and (B) the magnification of the image. PHY 1371

  14. Example: Problem #13 • A certain Christmas tree ornament is a silver sphere having a diameter of 8.50 cm. Determine an object location for which the size of the reflected image is three-fourths the size of the object. Use a principal-ray diagram to arrive at a description of the image. PHY 1371

  15. Homework • Ch. 36, P. 1168, Problems: #2, 7, 13, 14. PHY 1371

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