Magnification by Convergent Lens

TM 9-258

version and reversion as in figure 2-47. An image

(2) Magnification by reading glass. If the object

formed in this way is real.

is inside one focal length of a convergent lens, the image

will be on the same side of the lens as the object

d. Object Location and Image Size. The image of

magnified, normal, and erect. The image will no longer

an object located at any distance beyond two focal

be real but will be virtual. This is a condition which can

lengths of the lens is smaller than the object. If the

be illustrated easily by the use of a reading glass (figs 2-

object is at two focal lengths, the image is the same size.

48 and 2-69). Eyepieces of all optical instruments

If the object is between one and two focal lengths away,

provide the eye with a virtual image. If the observer

the image is magnified. All of these images are inverted,

looks at the object, for example, a portion of a printed

reverted, and real. If the object is at one focal length, the

page, through a reading glass held close to the type, the

rays are parallel after refraction; the image is said to be

printed letters appear larger than they actually are. As

at infinity.

he moves the glass away from the type, each letter

appears larger and remains clear provided he moves his

e. Magnification by Convergent Lens.

eye backward as he moves the glass away from the

(1) Conjugate foci. These are two points so

printed page. At a certain point the printed letters appear

related that an object at either point will be imaged by a

at their largest and clearest. Beyond this point they

lens at the other point and will be a real image. The

become blurred and then inverted. The time when the

mathematical relationship is as follows

largest and clearest view of the letters is obtained is just

before the focal point of the lens has been reached. The

plus

rays from the object can be plotted for any point in the

Object Distance

Image Distance Focal Length

preceding demonstration (fig 2-48). A ray of light

through the center of the lens passes through in a

(fig 2-47)

straight line unchanged. A ray which is originally parallel

to the axis is directed towards the point of principal focus

(para 2-22). The reason for the magnification of the

Magnification = Size of Image = Image Distance

image is clear considering that the virtual (not real)

Size of Object Object Distance

image is located where the emergent rays would

intersect if they were extended backward as imaginary

If any three of the above quantities are known, it is

lines. These rays form a magnified image of the object

possible to determine the fourth by the application of

which appears to be in back of the object.

simple algebra. To state magnification another way, it is

simply comparing the size of the image with the size of

the object or:

Magnification= Image Size

= Id

Object Size

M = Y1=f

f=x

Y Od-f

2-34