The science of light . ld bedeviated through an angle -527x2A = 1054A, and theF fine would be deviated through -536x2A = l072A,so that the angle between the two rays would be again•018A. Placing these two prisms with their angles inopposite directions, as in Fig. 23, the crown glass prismwill deviate the C ray through 1*054A, and the flintglass prism would deviate it back again through -630A,so that there would be a nett deviation of -424A. TheF ray would be deviated through L072A by the crownglass and through -648A back again by the flint glass,giving a nett deviation of -424A. The C and F ra
The science of light . ld bedeviated through an angle -527x2A = 1054A, and theF fine would be deviated through -536x2A = l072A,so that the angle between the two rays would be again•018A. Placing these two prisms with their angles inopposite directions, as in Fig. 23, the crown glass prismwill deviate the C ray through 1*054A, and the flintglass prism would deviate it back again through -630A,so that there would be a nett deviation of -424A. TheF ray would be deviated through L072A by the crownglass and through -648A back again by the flint glass,giving a nett deviation of -424A. The C and F rayswill therefore emerge parallel, both having suffered adeviation of -424A. DISPERSION 37 The deviation of the D ray would be 1-060A--635A, *425A. This is very nearly equal to the deviationof the C and F rays, but is not exactly, and so althoughthere is no dispersion between the C and F rays thereis a very small amount of dispersion between the otherrays. For most practical purposes this dispersion is «.--->.. FLINT Fig. 23. small enough to be negligible, but it is possible to arrangethree prisms of different dispersive power so that threecolours have exactly the same deviation. Even thenthe deviation of the remaining colours would not beexactly the same, but the dispersion would be verymuch smaller than with the two prisms only. Dispersion in Lenses.—Achromatic prisms are ofno practical use, but the principle is of enormous im-portance in the making of achromatic lenses. Owingto the difference in the refractive indices of the different i : , i i Fig. 24. colours the images formed by single lenses are alwaysimperfect and coloured. This can very readily beshown. Let S, in Fig. 24, be a point from which raysof light are diverging to a lens, and let us fix our atten-tion on two of the rays. The two regions of the lensthrough which the rays pass are shaded in the figure 38 THE SCIENCE OF LIGHT in order to bring out the fact that each portion of thelens may be considered
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectlight, bookyear1910
