
40 OPTICS FOR ENGINEERS
X
X´
FS´
F´
Lens
Mirror
s´
s´
s
s
S
f
Figure 2.5 Sign conventions. All distances in these figures are positive. The object distance is
positive if the object is to the left of the interface. The image distance for a refracting interface or
a lens is positive if the image is to the right, and for a mirror, it is positive to the left. In general,
the object distance is positive if light passes the object before reaching the interface, and the
image distance is positive if the light reaches the interface before reaching the image. Complete
discussion of sign conventions will be found in Appendix A.
TABLE 2.1
Imaging Equations
Quantity Definition ...