Lenses

"The **convex lens** has been shaped so that all light rays that enter it parallel to its central axis cross one another at a single point on the opposite side of the lens. ... Such a lens is called a **converging lens** because of the converging effect it has on light rays." "A **concave lens** is a **diverging lens** because it causes the light rays to bend away (diverge) from its axis. ... Note that the focal length of a diverging lens is defined to be negative." "The power, *P*, of a lens is very easy to calculate. It is simply the reciprocal of the focal length, expressed in meters $P = \frac{1}{f}$. The units of power are diopters, D, which are expressed in reciprocal meters." "The cornea and lens form a system that, to a good approximation, acts as a single thin lens. For clear vision, a real image must be projected onto the light-sensitive retina, which lies at a fixed distance from the lens. The lens of the eye adjusts its power to produce an image on the retina for objects at different distances." "Nearsightedness, or myopia, is the inability to see distant objects clearly while close objects are in focus. The nearsighted eye *overconverges* the nearly parallel rays from a distant object, and the rays cross in front of the retina. ... Farsightedness, or hyperopia, is the inability to see close objects clearly whereas distant objects may be in focus."