Biomedical Engineering Reference
In-Depth Information
n
′
= 1.52
n
= 1.00
Optical axis
C
A
F
f
Figure 2-5.
The
primary
focal point of this diverging spherical surface,
F
, is located by
sending rays from the secondary medium (crown glass) to the primary medium (air). The
primary focal point is associated with refraction that occurs when light enters the primary
medium. As the rays travel from glass to air, they are refracted away from the normal,
appearing to come from
F
. All material to the right of the surface is assumed to be glass.
length does not equal the secondary focal length.
4
The relationship between
the surface power and primary focal length is as follows:
−
n
f
F
=
The minus sign is necessary because we reverse the direction of the light rays to
locate
F
.
5
The location of the primary focal point for a diverging surface is illustrated in
Figure 2-5. Light rays travel in a reverse direction, from the secondary medium to
the primary medium. Refraction occurs as the rays enter the primary medium; the
focal point associated with this refraction is
F
.
A VERY HANDY FORMULA
A spherical surface is by definition derived from a sphere, allowing us to determine
its dioptric power when we know its radius of curvature and refractive index. The
following is one of the handiest optical formulae that you'll learn
6
:
n
−
n
′
F
=
r
4. As we will learn in Chapter 4, the primary and secondary focal lengths for thin lenses are equal when
the lens has the same medium on both sides.
5. For the surfaces in Figures 2-2 and 2-3, the primary focal lengths are
13.16 cm, respectively.
6. This is the first term of the Lensmaker's formula, which gives the power of a thin lens when its
refractive index and the radii of curvature of both surfaces are known.
−
13.16 and
+
