348CHAPTER 25 25.22(a) The lateral magnification produced by the objective lens of a good compound microscope is closely approximated by 1OMLf≈ −, where Lis the length of the microscope tube and is the focal length of this lens. Thus, if and Of20.0 cmL=150.0M=−(inverted image), the focal length of the objective lens is 120.00.400 cm50= +−cm.0OLfM≈−(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 cmeemf=. If 20.0em=, the focal length of the eyepiece is 25.0 cm25.0 cm1.2520.0eefm===cm+(c) The overall magnification is ()()150.020.01000emMm==−=−25.23 The overall magnification is 1125 cmeemMm Mfwhere 1Mis the magnification produced by the objective lens. Therefore, the required focal length for the eye piece is ( )( )11225 cm25 cm2.1 cm
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