Graphics Reference
In-Depth Information
Figure 16.6.
A diagram of a convex lens.
For any lens made of a material with a higher refractive index than air, as
a ray of light from the eye to a point
P
enters the lens, it is bent toward the line
of the normal to the lens at that point. As it then leaves the lens, it is bent away
from the normal to the lens at the point where it leaves. Exactly what happens
to the light depends on the directions of these normals and, of course, on the
exact refractive index of the lens material relative to the air.
For a convex lens, normals point away from the centerline of the lens,
-
z
in Figure 16.6, and so a light ray from the eye is bent back toward the
centerline. This has the effect of focusing light from the eye point on the cen-
terline, which generally magnifies the appearance of any object on that. The
image that is seen can either be seen upright or inverted,
depending on its distance from the lens, as we will see
later.
The focal length
f
of such a lens is given by the lens-
maker's equation,
1
η
η
1
11
1
=
lens
−
−
,
f
R
R
env
2
where the values of η are the refractive indices of the lens
and the environment. The way the light rays and the nor-
mals behave at the points where the rays enter and leave
the lens is shown in more detail in Figure 16.7.
Figure 16.7.
Light rays and inter-
sections with a (convex) lens.