IMAGE—CONJUGATE FOCI.
Let AB be the section of a double convex lens and C and D ([fig. 21]) be the centers of curvature of the two surfaces. Draw the lines CD′ and DE from C and D parallel to each other, then join D′ and E by a straight line. The point O will be the optical center of the lens. Let us take a point R, on the principal axis as a source of light; the ray RD passes through the optical center and is not deviated. The ray RK on striking will be refracted in the direction KG toward the perpendicular to the surface KD in accordance with the law of refraction, as glass is denser than air. On emerging at G it is refracted away from the perpendicular to the surface CG, since it passes from a denser to a rarer medium, and will intersect the ray RD at the point R′. In a like way the ray RK′ will be found to intersect the ray RD at the same point, R′, which is the focus for all rays coming from R. The point R′ is said to be the image of the object R, and when the two points are considered together they are called conjugate foci. If the incident beam is composed of parallel homogeneous light, the rays will all be brought to a focus at a point on the principal axis, called the principal focus of the lens, and the distance of this point from the optical center is the principal focal length, which is always a fixed quantity for any given lens.
Fig. 21.