Biomedical Engineering Reference
In-Depth Information
Figure 1-4.
The curvature of wavefronts becomes less as the distance from the point
source increases. They are arcs of a circle whose center is the point source. At infinity,
the wavefronts are flat.
way that a pebble dropped into a quiet pond of water generates waves of water. The
peaks of the waves are called wavefronts. Think of them as circles with radii equal
to the distance from the point source.
Let's look at this in more detail. Figure 1-4 shows wavefronts traveling from
left to right. Consider these to be arcs of a circle whose center is the point source.
As you can see, the curvature of these wavefronts decreases as the distance from
the source increases. An arc with a longer radius is flatter than one with a shorter
radius. At infinity (where the radius of the arc is infinity), the wavefronts are flat.
Note that direction of movement of the wavefronts in Figure 1-3 is represented
by arrows—commonly called
light rays
—that are perpendicular to the wavefronts.
A bundle of rays is called a
pencil
. As illustrated in Figure 1-5, the light rays that
form a pencil can be diverging, converging, or parallel. A
diverging pencil
is pro-
duced by a point source of light, such as a star. When light rays are focused at a
point, they create a
converging pencil
. A converging optical system (e.g., a mag-
nifying lens) is required to create converging light. An object located infinitely far
away forms a
parallel pencil
because, as we've seen in Figure 1-4, the wavefronts
are flat (which means that the rays perpendicular to these wavefronts must be paral-
lel to each other).
An
extended source
, such as the arrow in Figure 1-6, is composed of an
infinite number of point sources. Diverging light rays emerge from each of the
point sources.
VERGENCE
When it comes to understanding and solving clinical optical problems, the concept
of vergence goes a long way. At this point, I'll provide some working definitions
that will get you going. Once we start looking at optical problems in subsequent
chapters, vergence will become second nature to you (I hope!).
