Page 63 - Fundamentals of Light Microscopy and Electronic Imaging

46 LENSES AND GEOMETRICAL OPTICS

Parallel light
(sunlight or lamp
bulb not closer than
30 times focal length)

7
6
5
4

3
2

1
Image
0 of sun
Figure 4-4
Determining the focal length of a simple lens. The image of a distant source is projected by
the lens on a viewing surface; the focal length is the distance between the focal plane, and
the lens as measured with a ruler.

RULES OF RAY TRACING FOR A SIMPLE LENS

The three rules governing ray tracing for a simple lens are depicted in Figure 4-5 and are
listed as follows:

1. A light ray passing through the center of a lens is not deviated.
2. A light ray parallel with the optic axis will, after refraction, pass through the rear
focal point.
3. A ray passing through the front focal point will be refracted in a direction paral-
lel to the axis.

Notice that the intersection of any two of the three key rays just described identifies
the location of the image plane.

OBJECT-IMAGE MATH

The well-known lens equation describes the relationship between focal length f and
object and image distances, a and b:

1/f 1/a 1/b,

or

b af/(a f ).