12_Ch 25 College Physics ProblemCH25 Optical Instruments

# 12_Ch 25 College Physics ProblemCH25 Optical Instruments -...

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348 CHAPTER 25 25.22 (a) The lateral magnification produced by the objective lens of a good compound microscope is closely approximated by 1 O M L f ≈ − , where L is the length of the microscope tube and is the focal length of this lens. Thus, if and O f 20.0 cm L = 1 50.0 M = − (inverted image), the focal length of the objective lens is 1 20.0 0.400 cm 50 = − = + cm .0 O L f M ≈ − (b) When the compound microscope is adjusted for most comfortable viewing (with parallel rays entering the relaxed eye), the angular magnification produced by the eyepiece lens is 25 cm e e m f = . If 20.0 e m = , the focal length of the eyepiece is 25.0 cm 25.0 cm 1.25 20.0 e e f m = = = cm + (c) The overall magnification is ( )( ) 1 50.0 20.0 1000 e m M m = = − = − 25.23 The overall magnification is 1 1 25 cm e e m M m M f = = where 1 M is the magnification produced by the objective lens. Therefore, the required focal length for the eye piece is ( ) ( ) ( ) 1 12 25 cm 25 cm 2.1 cm 140 M
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