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A Closer Look

Optics
istorians tell us there are many
Hearly stories and legends about the
development of ancient optical devices. Light rays
The first glass vessels were made about
1500 b.c., so it is possible that samples of
clear, transparent glass were available soon
after. One legend claimed that the ancient
Chinese invented eyeglasses as early as A
500 b.c. A burning glass (lens) was men-
tioned in an ancient Greek play written
about 424 b.c. Several writers described
how Archimedes saved his hometown of
Syracuse with a burning glass in about
214 b.c. Syracuse was besieged by Roman Light rays
ships when Archimedes supposedly used
the burning glass to focus sunlight on the
ships, setting them on fire. It is not known
if this story is true, but it is known that the
B
Romans indeed did have burning glasses.
Glass spheres, which were probably used BOX FIGURE 7.1 (A) Convex lenses are called converging lenses because they bring
to start fires, have been found in Roman together, or converge, parallel rays of light. (B) Concave lenses are called diverging lenses
ruins, including a convex lens recovered because they spread apart, or diverge, parallel rays of light.
from the ashes of Pompeii.
Today, lenses are no longer needed to
surface of the eye, the retina. The retina is The near point moves outward with age as
start fires, but they are common in cameras,
made up of millions of light-sensitive struc- the lens becomes less pliable. By middle age,
scanners, optical microscopes, eyeglasses,
tures, and nerves carry electrical signals the near point may be twice this distance or
lasers, binoculars, and many other opti-
from the retina to the optic nerve, then to greater, creating the condition known as far-
cal devices. Lenses are no longer just made
the brain. sightedness. The condition of farsightedness,
from glass, and today many are made from
The lens is a convex, pliable material or hyperopia, is a problem associated with
a transparent, hard plastic that is shaped
held in place and changed in shape by the aging (called presbyopia). Hyperopia can be
into a lens.
attached ciliary muscle. When the eye is caused at an early age by an eye that is too
The primary function of a lens is to form
focused on a distant object, the ciliary mus- short or by problems with the cornea or lens
an image of a real object by refracting incom-
cle is completely relaxed. Looking at a closer that focus the image behind the retina. Far-
ing parallel light rays. Lenses have two basic
object requires the contraction of the cili- sightedness can be corrected with a convex
shapes, with the center of a surface either
ary muscles to change the curvature of the lens, as shown in Box Figure 7.4F.
bulging in or bulging out. The outward bulg-
lens. This adjustment of focus by the action Nearsightedness, or myopia, is a prob-
ing shape is thicker at the center than around
of the ciliary muscle is called accommoda- lem caused by an eye that is too long or
the outside edge, and this is called a convex
tion. The closest distance an object can be problems with the cornea or lens that focus
lens (Box Figure 7.1A). The other basic lens
seen without a blurred image is called the the image in front of the retina. Nearsight-
shape is just the opposite, thicker around the
near point, and this is the limit to accom- edness can be corrected with a concave lens,
outside edge than at the center, and is called
modation. as shown in Box Figure 7.4D.
a concave lens (Box Figure 7.1B).
Convex lenses are used to form images
in magnifiers, cameras, eyeglasses, projectors,
telescopes, and microscopes (Box Figure 7.2).

1
Concave lenses are used in some eyeglasses
and in combination with the convex lens to Object 3 C
F F
correct for defects. The convex lens is the
most commonly used lens shape. 2 Image
Your eyes are optical devices with con-
vex lenses. Box Figure 7.3 shows the basic
structure. First, a transparent hole called
the pupil allows light to enter the eye. The d o d i
size of the pupil is controlled by the iris, the BOX FIGURE 7.2 A convex lens forms an inverted image from refracted light rays of an
colored part that is a muscular diaphragm. object outside the focal point. Convex lenses are mostly used to form images in cameras, file
The lens focuses a sharp image on the back or overhead projectors, magnifying glasses, and eyeglasses.
186 CHAPTER 7 Light 7-10