Week 12: Lenses and Mirrors 411
magnification of a Galilean telescope is still:M=β
α=−fo
fe(1004)
(where nowfe<0 isnegative) but parallel rays from the distant object enter the eye
after passing through the telescope in thesameangular sense that they enter it when
viewed without the telescope. As before, note that we used a ray thatwouldhave passed
through the center of the second lens (and the eye, if the eye were drawn into the figure)
in order to determine the angle all of the parallel rays leave the eyepiece lens before
entering the (normal) eye and being focused on the retina.
Telescopes (in the hands of Galileo and others) were an instrument that ushered in the
Enlightenment in the seventeenth century, putting an end to several thousand years
of human history where mythology and inexact observations prevented the systematic
development of a consistent theory of physics. Let’s look at another instrument that had
a revolutionary impact on human society, the microscope.Microscope
fo fo lαyy’s s’Figure 175: The first magnification stage of a compound microscopebrings asmallobject just
outside of the focal point of the objective lens into focus as areal, magnified imageat the end of
thetube lengthl. By comparing the two dashed similar triangles, one can see that thefirst stage
magnification is−flo.
A compound microscope is used to view a very small, but nearby object. It accomplishes
this in two stages. Two short focal length lenses are situated at ends of a tube much
longer tube. Thetube lengthlof the microscope is by definition the distance between
the focal point of the first, orobjectivelens (which must be converging) and the second,
oreyepiecelens.
The objective stage of the magnification occurs as the the objectis placed on a movable
platform just outside of the focal length of the objective lens of the microscope. The
platform is raised or lowered (alterings, the object distance) until the objective lens
forms amagnified, real imageof the object at the end of the tube length as shown in
figure (175).