MICROSCOPE MAGNIFICATION

Refer to Fig. 1. Suppose that f_o is the focal length (in meters) of the objective lens and f_e is the focal length (in meters) of the eyepiece. Assume that the objective and the eyepiece are placed along a common axis and that the distance between their centers is adjusted for proper focus. Let s represent the distance (in meters) from the objective to the real image it forms of the object under examination. The microscopic magnification (a dimensionless quantity denoted m in this context) is given by
m = [(s - f_o)/f_o)] [(f_e + 0.25)/f_e]
The quantity 0.25 represents the average near point of the human eye, which is the closest distance over which the eye can focus on an object: approximately 0.25 m.
A less formal method of calculating the magnification of a microscope is to multiply the magnification of the objective by the magnification of the eyepiece. These numbers are provided with objectives and eyepieces and

Fig. 1. Calculation of the magnification factor in a compound microscope. See text for details.

are based on the use of an air medium between the objective and the specimen, as well as on the standard distance between the objective and the eyepiece. If m_e is the power of the eyepiece and m_o is the power of the objective, then the power m of the microscope as a whole is
m = m_em_o