Biomedical Engineering Reference
In-Depth Information
Plus thin lens
n
= 1.00
n
′
= 1.00
F
F
′
f
f
′
Minus thin lens
n
= 1.00
n
′
= 1.00
F
′
F
f
′
f
Figure 4-3.
Schematic representations of a thin plus lens (top) and a thin minus lens.
The front and back surfaces are collapsed to a single plane—a vertical line—where
all refraction is assumed to occur. When the medium on both sides of a lens is the
same (as for these lenses, which are in air) the secondary and primary focal lengths
are equal.
RAY TRACING
The same principles that we learned for ray tracing with spherical surfaces apply
to thin lenses. The following three key rays, which each emanate from the object,
can be used to locate the image and determine its size (and magnification):
• Ray 1 passes undeviated through the lens's optical center (which is the intersec-
tion of the lens and its optical axis).
• Ray 2 travels parallel to the optical axis and after refraction passes through the
lens's secondary focal point (plus lens) or appears to emerge from the second-
ary focal point (minus lens).
• Ray 3 passes through (plus lens) or is headed toward (minus lens) the primary
focal point and after refraction is parallel to the optical axis.
Figures 4-6 through 4-9 show how these rays can be used to locate the images
formed by converging and diverging lenses.
