Page 21 - Modern Optical Engineering The Design of Optical Systems

4 Chapter One

Figure 1.3 Light waves radiating
from a point source in an iso-
tropic medium take a spherical
form; the radius of curvature of
the wave front is equal to the
distance from the point source.
The path of a point on the wave
front is called a light ray, and in
an isotropic medium is a straight
line. Note also that the ray is
normal to the wave front.

Ordinary air has an index of refraction of about 1.000277, and since
almost all optical work (including measurement of the index of refrac-
tion) is carried out in a normal atmosphere, it is a highly convenient con-
vention to express the index of a material relative to that of air (rather
than vacuum), which is then assumed to have an index of exactly 1.0.
The actual index of refraction for air at 15°C is given by

2,406,030 15,996
8
(n 1) 10 8342.1
2
2
(130 ) (38.9 )
where 1/ ( wavelength, in m). At other temperatures the
index may be calculated from
1.0549 (n 1)
15°
(n 1)
t
(1 0.00366t)
The change in index with pressure is 0.0003 per 15 lb/in , or
2
0.00002/psi.
If we trace the path of a hypothetical point on the surface of a wave
front as it moves through space, we see that the point progresses as a
straight line. The path of the point is thus what is called a ray of light.
Such a light ray is an extremely convenient fiction, of great utility in
understanding and analyzing the action of optical systems, and we
shall devote the greater portion of this volume to the study of light
rays. Note well that the ray is normal to the wave front, and vice versa.
The preceding discussion of wave fronts has assumed that the light
waves were in a vacuum, and of course that the vacuum was isotropic,
i.e., of uniform index in all directions. Several optical crystals are
anisotropic; in such media wave fronts as sketched in Fig. 1.3 are not
spherical. The waves travel at different velocities in different directions,
and thus at a given instant a wave in one direction will be further from
the source than will a wave traveling in a direction for which the
media has a larger index of refraction.