Fig. 380.—Wave diagram of light passing through a concave lens.
386. The Formation of Images by Lenses.—If a beam composed of parallel rays of light, as sunlight, is sent in turn through three convex lenses of the same diameter but of different thickness, it is found that the thicker the lens the greater is its converging power, or the shorter is its focal length. (See Fig. 381.) Now if a luminous body, such as a lighted candle, be placed near the convex lens but beyond its focal length, the light will be brought to a focus upon the other side of the lens and an image of the candle may be clearly seen upon the screen placed at this point. (See Fig. 382.) The two points so situated on opposite sides of a lens that an object at one will form an image at the other are called conjugate foci.
Fig. 381.—The thicker the lens, the shorter is its focal length.
Fig. 382.—C and S are at conjugate foci.
It will be helpful to compare the images formed of a candle by an aperture and by a convex lens. Rays of light from each point of the luminous body pass through the aperture in straight lines and produce upon the screen a lighted space of the same shape as the candle. This image is rather hazy in outline. Each cone of rays from luminous points of the flame is brought by the lens to a focus on the screen, producing a sharp image. It is the converging power of convex lenses that enables them to produce clear images.
Fig. 383.—Construction of a real image by a convex lens.
387. The Construction of Diagrams to Represent the Formation of Images by Lenses.—Just as the earth has an axis at right angles to its equator to which are referred positions and distances, so a lens has a principal axis at right angles to its greatest diameter and along this axis are certain definite positions as shown in Fig. 383. Let MN be the principal axis of a convex lens, P and P´ are principal foci on either side of the lens, S and S´ are secondary foci. These are at points on the principal axis that are twice as far from O, the center of the lens, as are the principal foci. In the formation of images by a convex lens, several distinct cases may be noticed: