Introductory Physics Volume Two

8.6 Thin Lens Equation 167
back in an hour the sun will have moved up in the sky, and the rays will
no longer be parallel to the optical axis. The rays will still converge at
a point but it will not be the focal point of the lens. The new focus
will be in the focal plane, a plane parallel to the plane of the lens and
one focal length away from the lens.
Focal Plane
Optical Axis
Notice that the ray that passes through the center of the lens is not
deflected. This is a particularly nice ray for doing constructions of
images, as we will see. In this case it allows us to find the location of
the focus, since the focus is at the intersection of this straight line and
the focal plane. Once we have found the focus, using the central ray,
we can draw the other rays, because they must pass though the focus
also.
We can also reverse these diagrams: Light that comes from a point
in the focal plane and strikes the lens leaves the lens parallel to the ray
that passes through the center of the lens.
We have seen what happens to groups of rays that are parallel to
each other. Let us now see what happens to groups of rays that diverge
from a point that is not in the focal plane. Suppose that you have light
radiating from a point. The point is indicated by the fat arrow on
the left, in the following diagram. We follow the light from this point
source to it’s focus.