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

Fig. 238

Solutions 303

Thus,

nmin=

4.12. By hypothesis, the foci of the two lenses are
made to coincide, i.e. the separation between the
lenses is 3f, where f is the focal length of a lens with
a lower focal power.
In the former case, all the rays entering the tube
will emerge from it and form a circular spot of
radius r/2, where r is the radius of the tube
(Fig. 238). Inithe latter case, only the rays which

enter the tube at a distance smaller than r/2 from

the tube axis will emerge from the tube. Such

rays will form a circular spot of radius r on the

screen (Fig. 239). Thus, if 1 is the luminous in-

Fig. 239

tensity of the light entering the tube, the ratio

of the illuminances of the spots before and after