Fig. 26.—Double Convex, Plane Convex, and Concavo-Convex Lenses.
Fig. 27.—Convergence of Rays by Convex Lens to Principal Focus.
So far we have brought rays to a focus, and on holding a piece of paper at the focus of the convex lens, as just mentioned, there appears on it a spot of light; and every one knows that if this experiment be performed with the sun, one brings all the rays falling on the lens almost to a point, and the longer waves of light will set fire to the paper; and on this principle burning-glasses are constructed. If, however, the rays are not parallel when falling on the lens, but diverging, they are not brought to a focus so near the lens, and the nearer the luminous source or object is, the further off will the light be brought to a focus on the other side. If matters are reversed, and the luminous source be placed in the focus, the rays of light, when they leave the lens, will converge to the position of the original source; so that there are two points, one on either side of the lens, which are the foci of each other S, S´, Fig. [28], called conjugate foci; as one approaches the lens the other recedes, and vice versâ, and it is obvious that when the one approaches the lens so as to coincide with the principal focus, the other recedes to an infinite distance, and the emergent rays are parallel.
Fig. 28.—Conjugate Foci or Convex Lens.
Fig. 29.—Conjugate Images.
