Answer
See answers.
Work Step by Step
a. From Figure 25-20, the length of the telescope is the sum of the focal lengths.
$$\mathcal{l}=f_o+f_e=48cm+4.0cm=52cm$$
The magnification is the ratio of the focal lengths (equation 25–3) and the lens with the shorter focal length (4.0 cm) is the eyepiece since we desire the object to be enlarged. Calculate the magnification.
$$M=-\frac{f_o}{f_e}=-\frac{48cm}{4.0cm}=-12\times$$
b. Use equation 25–6b to relate the microscope length to other quantities in the problem.
$$M=\frac{N\mathcal{l}}{f_ef_o}$$
$$\mathcal{l}=\frac{Mf_ef_o}{N}=\frac{(25)(4.0cm)(48cm)}{25cm}=192cm$$
This is much too long to make a practical microscope.