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25.7 Image Formation by Mirrors 53. What is the focal length of a makeup mirror that has a power of 1.50 D? 54. Some telephoto cameras use a mirror rather than a lens. What radius of curvature mirror is needed to replace a 800 mm focal length telephoto lens? 55. a Calculate the focal length of the mirror formed by the shiny back of a spoon that has a 3.00 cm radius of curvature. b What is its power in diopters? 56. Find the magnification of the heater element in Example 25.9. Note that its large magnitude helps spread out the reflected energy. What is the focal length of a makeup mirror that produces a magnification of 1.50 when a persons face is 12.0 cm away? Explicitly show how you follow the steps in the Problem-Solving Strategy for Mirrors. A shopper standing 3.00 m from a convex security mirror sees his image with a magnification of 0.250. a Where is his image? b What is the focal length of the mirror? c What is its radius of curvature? Explicitly show how you follow the steps in the Problem-Solving Strategy for Mirrors. An object 1.50 cm high is held 3.00 cm from a persons cornea, and its reflected image is measured to be 0.167 cm high. a What is the magnification? b Where is the image? c Find the radius of curvature of the convex mirror formed by the cornea. Note that this technique is used by optometrists to measure the curvature of the cornea for contact lens fitting. The instrument used is called a keratometer, or curve measurer. 60. Ray tracing for a flat mirror shows that the image is located a distance behind the mirror equal to the distance of the object from the mirror. This is stated d i = d o , since this is a negative image distance it is a virtual image . a What is the focal length of a flat mirror? b What is its power?.