Page 147 - Modern Optical Engineering The Design of Optical Systems

130 Chapter Seven

Figure 7.6 A plane reflecting
surface forms a virtual image of
an object point. Object and
image are equidistant from the
reflecting surface, and both lie on
the same normal to the surface.

In Fig. 7.6, the plane of the page is the plane of incidence. Two rays
from point P are shown reflected from the surface MM′. By extending
the rays backward, it can be seen that after reflection they appear to
be coming from point P′, which is a virtual image of point P. Both P and
P′ lie on the same normal to the surface (POP′), and the distance OP
is exactly the same as the distance OP′.
If we now consider an extended object such as the arrow AB in Fig. 7.7,
we can readily locate the position of its image by using the principles
of the preceding paragraph to locate the images of points A and B. An
observer at E looking directly at the arrow would see the arrowhead
A at the top of the arrow. However, in the reflected image, the arrow-
head (A′) is at the bottom of the arrow. The image of the arrow has
been reoriented (or inverted) by the reflection.
If we add a crosspiece CD to the arrow, the image is formed as shown
in Fig. 7.8, and although the image of the arrow has been inverted,
the image of the crosspiece has the same left-to-right orientation as
the object.

Figure 7.7 The reflected image
A′B′ of the arrow AB appears
inverted to an observer at E.