7_Chapter 25 HomeworkCH25 Optical Instruments

# 7_Chapter 25 HomeworkCH25 Optical Instruments - Optical...

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Optical Instruments 343 25.7 Since the exposure time is unchanged, the intensity of the light reaching the film should be doubled so the energy delivered will be doubled. Using the result of Problem 6 (part a), we obtain () 22 2 1 21 2 1 -number -number 11 61 2 I ff I  ==   = , or 2 -number 61 7.8 f Thus, you should use the 8.0 f setting on the camera. 25.8 To focus on a very distant object, the original distance from the lens to the film was . To focus on an object 2.00 m away, the thin lens equation gives 1 65.0 mm qf ( )( ) 3 3 2.00 10 mm 65.0 mm 67.2 mm 2.00 65.0 mm × = −×− 2 2 2 pf q Thus, the lens should be moved qq ∆= − 2.2 mm farther from the film q = 25.9 This patient needs a lens that will form an upright, virtual image at her near point (60.0
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