Page 62 - Fundamentals of Light Microscopy and Electronic Imaging

IMAGE FORMATION BY A SIMPLE LENS 45

Note: Real and Virtual Images

Images can be defined as regions where rays, or the extensions of rays, become con-
vergent as the result of refraction by a lens or reflection by a mirror. If the rays inter-
sect and physically reunite, the image is said to be real. A real image can be seen on
a viewing screen or recorded on a piece of film when a screen or film is placed in the
image plane. If rays diverge, but the imaginary backward extensions of the rays
become convergent and intersect, the image is said to be virtual. The plane occupied
by a virtual image cannot be observed on a viewing screen or recorded on film. To
be perceived, a real image must be formed on the retina of the eye. In the case of
viewing an image in a microscope, a real image is formed on the retina but is per-
ceived as a virtual image located some 25 cm in front of the eye. Lens configurations
giving real and virtual images are described in this chapter.

The geometric parameters of a simple thin lens are described in Figure 4-3, where
the vertical line represents the combined principal planes of a thin biconvex lens of focal
length f. The object, an arrow on the left-hand side of the figure, is examined by the lens
and imaged as a magnified real image (magnified inverted arrow) in the image plane on
the right. The focal length is shown as the distance f from the principal plane of the lens
to its focal point F, the front and rear focal lengths having the same value. The optic axis
is shown by a horizontal line passing through the center of the lens and perpendicular to
its principal plane. The object distance a (distance from the object to the principal plane
of the lens) and image distance b (distance from the image to the principal plane of the
lens) are also indicated.
The focal length of any simple lens can be determined by aiming the lens at a bright
“infinitely distant” light source ( 30 times the focal length) such as a lamp across the
room or a scene outdoors; by focusing the image on a sheet of paper held behind the
lens, the focal length is readily determined (Fig. 4-4). We will now examine the basic
rules that determine the action of a simple convex lens.

a b
f
Object
F F
f
Image

Lens
Figure 4-3
Geometrical optics of a simple lens. The focal length f, focal point F, object-lens distance a,
and lens-image distance b are indicated.