# ch25 - CHAPTER 25 THE REFLECTION OF LIGHT MIRRORS ANSWERS...

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CHAPTER 25 THE REFLECTION OF LIGHT: MIRRORS ANSWERS TO FOCUS ON CONCEPTS QUESTIONS 1. (e) This is the definition of a wave front (see Section 25.1). 2. (b) Rays are radial lines pointing outward from the source and perpendicular to the wave fronts. They point in the direction of the velocity of the wave. 3. (c) When diffuse reflection occurs, the surface reflects different light rays in different directions. 4. (c) The ray of light strikes the mirror four units down from the top of the mirror with a 45 ° angle of incidence. The ray reflects from the mirror at an angle of 45 ° and passes through point C. 5. (a) The image is as far behind the mirror as the object is in front of the mirror. In addition, the image and the object lie on the horizontal line that is perpendicular to the mirror. 6. (d) The image of your friend is 2 m behind the mirror. The distance between you and the mirror is 5 m. Thus, the distance between you and your friend’s image is 7 m. 7. (b) Letters and words held up to a mirror are reversed left-to-right and right-to-left. 8. (d) As discussed in Section 25.4, rays that are parallel and near the principal axis of a concave mirror converge at the focal point after reflecting from the mirror. 9. (a) Parallel rays that are near the principal axis converge at the focal point after reflecting from a concave mirror. The radius of curvature is twice the focal length (see Equation 25.1), so R = 2 f = 36 cm. 10. (d) This is how real and virtual images are defined. See Sections 25.3 and 25.5. 11. (a) Any ray that leaves the object and reflects from the mirror can be used in the method of ray tracing to locate the image. 12. (c) According to the discussion in Section 25.5, a concave mirror can produce an enlarged image, provided the object distance is less than the radius of curvature. A convex mirror cannot produce an enlarged image, regardless of where the object is located. 13. (b) A convex mirror always produces a virtual, upright image (see Section 25.5).

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3 THE REFLECTION OF LIGHT: MIRRORS 14. (e) A negative image distance means that the image is behind the mirror and, hence, is a virtual image. See the Reasoning Strategy at the end of Section 25.6. 15. (c) A convex lens always produces an upright image that is smaller than the object. 16. f = 4.0 cm 17. (b) The image distance is d i = - md o = - 2(25 cm) = - 50 cm (Equation 25.4). 18. f = 90.0 cm
Chapter 25 Problems 4 CHAPTER 25 THE REFLECTION OF LIGHT: MIRRORS PROBLEMS 1. REASONING AND SOLUTION The drawing at the right shows a ray diagram in which the reflected rays have been projected behind the mirror. We can see by inspection of this drawing that, after the rays reflect from the plane mirror, the angle α between them is still 10 ° . 10 o reflected ray reflected ray M α object 2. REASONING AND SOLUTION Referring to Figure 25.9b and Conceptual Example 2, we find the following locations for the three images: Image 1: 2.0 m, 1.0 m Image 2: 2.0 m, 1.0 m Image 3: 2.0 m, 1.0 m x y x y x y = - = + = + = - = + = + 3. REASONING The drawing shows the image of the bird in the plane mirror, as seen by the camera. Note that the image is as far behind the mirror as the bird is in front of it.

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