Figure 4-1. Types of simple lenses.

TM 9-258

both spherical. Lenses deceive the eyes by bending rays

class forms real images and lenses in this class are

of light so that, depending on the type of lens, they

termed convex, convergent, positive, or collective lenses.

appear to come from closer and larger objects or from

The other class can form by itself only virtual images and

farther and smaller objects. Lenses are divided into two

lenses in this class are termed concave, divergent,

general classes (fig 4-1). One

negative, or dispersive lenses.

Figure 4-1. Types of simple lenses.

(1) Convex lenses. All convex or converging,

spectacles because they permit undistorted vision

positive lenses are thicker in the center than at the edges

through their margins 'as the eyes are rotated in their

and will converge light from sources and objects. Both

sockers.

(2) Concave Lenses. Concave or diverging,

faces of a convex lens may be convex, one surface may

be convex while the other is flat (termed plano or plane),

negative lenses (fig 4-1) are thinner in the center than at

or one face may convex while the other is concave.

the edges and will diverge light from sources and

(a) A double-convex lens (fig 4-1) is one in

objects. Both surfaces of a divergent lens may have a

which both surfaces have convex curvature. Both

concave curvature (doubleconcave); one surface may be

surfaces contribute to the converging power of the lens.

concave and the other plane (planoconcave); or one face

The greater the convexity of the surfaces, the shorter the

may be concave while the other is convex (concavo-

focal distance.

convex or divergent meniscus), with both centers of

(b) A planoconvex lens (fig 4-1) has a plane

curvature on the same side of lens.

(3) Cylindrical lenses. Cylindrical lenses are

surface and a convex surface. The plane surface does

ground with a cylindrical surface instead of a spherical

not contribute to the converging power of the lens.

or, meniscus

one; they are either positive or converging, or else they

converging lens (fig 4-1) possesses both a convex and a

are negative or diverging. Their use is very limited. They

are used in some of the coincidence range finders.

concave surface with both centers of curvature on the

b. Compound Lenses. As an optically perfect

same side of lens. The more powerful convex curve

single lens cannot be produced, two, three, or more

makes this a positive lens despite the fact that the

lenses ground from different types of optical glass are

concave surface tends to diverge the light, thus

frequently

combined

unit

cancel

subtracting from the converging power of the lens.

Meniscus

lenses

are

mainly

used

for

4-2