296 Aptitude Test Problems in Physicslight beam will converge at the exit. When a par-
allel beam of rays is incident on the telescope,
such a position of the eyepiece is required for a
long-sighted person whose eye has an insufficient
focal power for converging the parallel beam on
the retina. The maximum shift of the eyepiece
corresponds to the focal power +10 D. The
spectacles of such a focal power converge a parallel
beam at a distance b = 1/D.,.. This distance b
just determines the displacement x:
1 1 1
f x b fwhence
/2 5The required distance is
F 2 0.25 x 6 m 2a (^) — 30 m.
x — 5 x 10-2 m
4.5. Yes, it can. The answer is illustrated by
Fig. 230. The sought regions are shown hatched.
Fig. 230
4.6. When the pencil enters the water, the water
surface in its vicinity forms a diverging lens: the
rays emerging from the light source are deflected
x =
b — f 6 cm.