
INTRODUCTION 31
0
2π
4π
...
Figure 1.12 Optical path length. The OPL between any two points is obtained by integrating
along a path between those two points. The eikonal equation ensures that the integral does not
depend on the path chosen. The phase difference between the two points can be computed from
the OPL.
n
1
n
2
n
3
n
4
n
5
Figure 1.13 Optical path in discrete materials. The integrals become a sum.
Image distance
Physical distance
OPL
Figure 1.14 Optical path in water. The optical path is longer than the physical path, but the
geometric path is shorter.
where
n
m
is the index of layer m and
m
is its thickness.
Such a situation is common in systems of lenses, windows, ...