http://www.ck12.org Chapter 20. Geometric Optics
FIGURE 20.17
Terminology for converging and diverging lenses
principal axis:A straight line passing through the center of the lens and perpendicular to its surfaces.
optical center:The center of the lens.
focal point:Rays parallel to the principal axis converge at a point called the focal point as long as the rays pass
close to the optical center of the lens. Each lens has two focal points (two foci): one on each side of the lens.
Focal length:The distance from the center of the lens to either focal point.
convex/converging lens:A lens that is thicker in the center than its edges. Converging lenses make rays parallel to
the principal axis(P)converge at the focal point as shown below.
concave/diverging lens:A lens that is thinner at the center than its edges. Diverging lenses make rays parallel to
the principal axis(P)diverge as shown below. In this case, the focal point is the location where the extensions of
the refracted rays converge, to the left of the lens.