Ocular refraction and the shadow test . ipal focus and the lens the rays will emerge diver-gent, but not so divergent as they enter; if the light be beyond the LENSES 41 principal focus the rays will emerge convergent. The straight lineK L represents a secondary axis, as it passes through the optical cen-tre O. Figure 39 shows a convex lens whose principal focus is at F,light from the point P beyond the principal focus is converged andfocused at P; these two points, P and P, bear a certain defined rela-tion to each other and are called conjugate foci; either may be takenas the location of the
Ocular refraction and the shadow test . ipal focus and the lens the rays will emerge diver-gent, but not so divergent as they enter; if the light be beyond the LENSES 41 principal focus the rays will emerge convergent. The straight lineK L represents a secondary axis, as it passes through the optical cen-tre O. Figure 39 shows a convex lens whose principal focus is at F,light from the point P beyond the principal focus is converged andfocused at P; these two points, P and P, bear a certain defined rela-tion to each other and are called conjugate foci; either may be takenas the location of the luminous point, at the other the rays will befocused and form a real image of the luminous point. To show the formation of an image of the object by a convexspherical lens, let A, figure ^o, represent the lens, its optical centreat O, its principal focus at F; L and M indicate two points upon the-object situated a greater distance from the lens than its focal path of a ray from the point L parallel to the principal axis will. of imagegreater vi Figure lens. F, Ihe focus; L M. the objectlens than its locall ength; L M, the image,inverted and magnified. 1)6 incident at E and will be refracted through the principal focus F,the path of another ray from L through the optical centre O willmeet E F at L which will locate the image of the point L; in the sameway locate the image of the point M at M. It can thus be shown thatlight frorn every point upon the object is directed to its correspondingposition in the image. j^ convex spherical lens creates a real inverted image of an object by focus-ing upon a screen the divergent rays from every point upon the object, the collection of foci creating the image. Take a convex lens L, figure 41, of say ten inches focal lengthand place a lighted candle at its principal focus A, the light will berefracted as parallel rays on emerging from the lens and the image of OCULAR REFRACTION. the candle will be at infinity (practically n
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