Page 60 - Fundamentals of Light Microscopy and Electronic Imaging

CHAPTER
4

LENSES AND
GEOMETRICAL OPTICS

OVERVIEW

In this chapter we discuss some essential principles of geometrical optics, the action of
lenses on light as revealed by ray tracing and explained by principles of refraction and
reflection (Fig. 4-1). With the help of a few simple rules and from studying examples,
we can understand the process of magnification, the properties of real and virtual
images, the aberrations of lenses, and other phenomena. We also examine the designs
and specifications of condenser and objective lenses, review the nomenclature inscribed
on the barrel of an objective lens that specifies its optical properties and conditions for
use, and give some practical advice on the cleaning of optical components.

IMAGE FORMATION BY A SIMPLE LENS

To understand microscope optics, we begin by describing some of the basic properties
of a thin, simple lens. A thin lens has a thickness that is essentially negligible, and by
simple we mean consisting of a single lens element with two refracting surfaces. The
principal plane and focal plane of a lens are defined as those planes, within the lens and
in the focused image, respectively, where rays or extensions of rays intersect and phys-
ically reunite. Thus, for a simple positive lens receiving a collimated beam of light, the
plane in the lens in which extensions of incident and emergent rays intersect is called the
principal plane, and the plane in which rays intersect to form an image is the focal plane.
The focal length of a lens is the distance between the principal plane and the focal plane.
Lenses can be either positive or negative (Fig. 4-2). A positive lens converges parallel
incident rays and forms a real image; such a lens is thicker in the middle than at the
periphery and has at least one convex surface. (See the Note for definitions of real and
virtual images.) Positive lenses magnify when held in front of the eye. A negative lens
causes parallel incident rays to diverge; negative lenses are thinner in the middle than at
the periphery, and have at least one concave surface. Negative lenses do not form a real
image, and when held in front of the eye, they reduce or demagnify.

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