Physical Principles of Electron Microscopy: An Introduction to TEM ...

An Introduction to Microscopy 5
Changing the shape of the lens in an adult eye alters its overall focal
length by only about 10%, so the closest object distance for a focused image
on the retina is u � 25 cm. At this distance, an angular resolution of 3 � 10 -4
rad corresponds (see Fig. 1c) to a lateral dimension of:
�R � (�� ) u � 0.075 mm = 75 �m (1.4)
Because u � 25 cm is the smallest object distance for clear vision, �R = 75
�m can be taken as the diameter of the smallest object that can be resolved
(distinguished from neighboring objects) by the unaided eye, known as its
object resolution or the spatial resolution in the object plane.
Because there are many interesting objects below this size, including the
examples in Table 1-1, an optical device with magnification factor M ( > 1)
is needed to see them; in other words, a microscope.
To resolve a small object of diameter D, we need a magnification M* such
that the magnified diameter (M* D) at the eye's object plane is greater or
equal to the object resolution �R (� 75 �m) of the eye. In other words:
M* = (�R)/D (1.5)
Values of this minimum magnification are given in the right-hand column of
Table 1-1, for objects of various diameter D.
1.2 The Light-Optical Microscope
Light microscopes were developed in the early 1600’s, and some of the
best observations were made by Anton van Leeuwenhoek, using tiny glass
lenses placed very close to the object and to the eye; see Fig. 1-3. By the late
1600’s, this Dutch scientist had observed blood cells, bacteria, and structure
within the cells of animal tissue, all revelations at the time. But this simple
one-lens device had to be positioned very accurately, making observation
ver y tiring in practice.
For routine use, it is more convenient to have a compound microscope,
containing at least two lenses: an objective (placed close to the object to be
magnified) and an eyepiece (placed fairly close to the eye). By increasing its
dimensions or by employing a larger number of lenses, the magnification M
of a compound microscope can be increased indefinitely. However, a large
value of M does not guarantee that objects of vanishingly small diameter D
can be visualized; in addition to satisfying Eq. (1-5), we must ensure that
aberrations and diffraction within the microscope are sufficiently low.