Ray Tracing Model for Image Formation by Lenses
The Ray Tracing Model is used to predict the image location and its
orientation for a converging or diversing lens. In the figures
below, f1 or f2 are the focal points
of the lens. The focal length of the lens is the
distance from the center of the lens to f1 or
f2. The optical axis of the lens is
indicated by the dotted line. The three principle light
rays are indicated by the arrows, and are labeled 1,2, and 3.
Note that the three principle rays come from one
point on the object.
Double Convex Lens (converging lens)
Ray 1.
The ray parallel to the optical axis is bent by the lens pass through
f2:
Ray 2.
The ray through the center of the lens continues in a straight line
with no change in direction:
Ray 3.
The ray through f1 is bent by the lens to travel in a path
parallel to the optical axis:
The image of the point from the object will be at
the intersection of the principle rays.
Double Concave Lens (diverging lens)
(Note that f1 and f2 have been reversed,
but that the "rules" for tracing the three principle rays are
similar to those for the converging lens.)
Ray 1.
The ray parallel to the optical axis is bent by the lens, and
appears to come from f2:
Ray 2.
The ray through the center of the lens continues in a straight line
with no change in direction:
Ray 3.
The ray headed toward f1 is bent by the lens to travel in
a path parallel to the optical axis:
The image of the point from the object will be at the intersection of the principle rays. If the rays diverge after passing through the lens, then the point where the extensions of these rays intersect determines the location of the image.
