How it Works / Dealing in simple language with steam, electricity, light, heat, sound, hydraulics, optics, etc., and with their applications to apparatus in common use

by Archibald Williams

RELATIVE POSITIONS OF OBJECT AND IMAGE.

Fig. 110.—Showing how the position of the image alters relatively to the position of the object.

From what has been said above we deduce two main conclusions—(1.) The nearer an object is brought to the lens, the further away from the lens will the image be. (2.) If the object approaches within the principal focal distance of the lens, no image will be cast by the lens. To make this plainer we append a diagram (Fig. 110), which shows five positions of an object and the relative positions of the image (in dotted lines). First, we note that the line A B, or A B¹, denotes the principal focal length of the lens, and A C, or A C¹, denotes twice the focal length. We will take the positions in order:—

Position I. Object further away than 2f. Inverted image smaller than object, at distance somewhat exceeding f.

Position II. Object at distance = 2f. Inverted image at distance = 2f, and of size equal to that of object.

Position III Object nearer than 2f. Inverted image further away than 2f; larger than the object.

Position IV. Object at distance = f. As rays are parallel after passing the lens no image is cast.

Position V. Object at distance less than f. No real image—that is, one that can be caught on a focussing screen—is now given by the lens, but a magnified, erect, virtual image exists on the same side of the lens as the object.