Introductory Physics Volume Two

168 Geometric Optics 8.6
In the diagram we are able to draw the three rays that are bolded,
because they are, (top ray) parallel to the optical axis, (middle ray)
through the center, and (bottom ray) through the focal point. These
three rays are called the principle rays. From the three principle rays
(we only needed two really) the location of the focus is determined.
After the location of the focus is determined, the other faint rays are
drawn so that they go through the focus.
Notice that the rays of light actually do pass through the focus. If
we place a piece of paper at that point the paper would be illuminated.
This type of image is called a real image.
x o
x i
y o
y i
f
f
The location and size of the image can be computed from the
location and size of the object.
Theorem: Thin Lens Equations
With the dimensions as shown in the figure above,
1
x o
+ 1 x i
= 1 f
Theorem: Magnification Equations
With the dimensions as shown in the figure above,
= − x i
y o y i
where y is negative if below the optical axis.
x o