A compound microscope has a 18-cm-long barrel and an objective lens with a focal length of 8.0 mm.What is the focal length of the eyepiece to give a total magnification of 240? It is adjusted for minimum eyestrain for persons with a near point of 25 cm.
A)1.3 m
B)1.5 cm
C)1.9 cm
D)2.1 cm
E)2.3 cm

Question

Quizplus · Accepted Answer

Using the formula for the total magnification of a compound microscope:

M = -(fo/fe) * (L/f0)

where M = total magnification, fo = focal length of objective lens, fe = focal length of eyepiece, L = length of barrel, and f0 = distance between objective and eyepiece when image is at infinity.

We know M = 240, fo = 8.0 mm, L = 18 cm = 180 mm, and f0 = 25 cm = 250 mm.

Solving for fe:

240 = -(8/fe) * (180/250)

fe = 2.1 cm

A Compound Microscope Has a 18-Cm-Long Barrel and an Objective