Aptitude Test Problems in Physics Science for Everyone by S Krotov ( PDFDrive.com )

Solutions 295

In the diverging lens approximation, we obtain

R ^.1 rbeam+L(Pmax ^-; rbeam+2kirbeamL

rbeam [I+ ctpa/L iA1)— )^2 1

k RT j'

4.4. The telescope considered in the problem is of
the Kepler type. The angular magnification k =
F/f, and hence the focal length of the eyepiece is
f = F/k = 2.5 cm. As the object being observed
approaches the observer from infinity to the small-
est possible distance a, the image of the object
formed by the objective will be displaced from
the focal plane towards the eyepiece by a distance x
which can be determined from the formula for a
thin lens:
1 _ 1 a— F
a F-fx F-Fx

F

al; F^2
X =
a —F a

since a > F. Thus, we must find x. The eyepiece of
the telescope is a magnifying glass. When an object

Fig. 229

is viewed through a magnifier by the unstrained
eye (accommodated to infinity), the object must
be placed in the focal plane of the magnifier. The
required distance x is equal to the displacement of
the focal plane of the eyepiece during its adjust-
ment. In this case, the eyepiece must obviously be
moved away from the objective.
Figure 229 shows that when an infinitely remote
object is viewed from the shifted eyepiece, the