Page:Elementary Text-book of Physics (Anthony, 1897).djvu/453



357. Dispersion.—When white light falls upon a prism of any refracting medium, it is not only deyiated from its course but separated into a number of colored lights, constituting an image called a spectrum. These merge imperceptibly from one into another, but there are six markedly different colors: red, orange, yellow, green, blue, and violet. Red is the least and violet the most deviated from the original course of the light. Newton showed by the recomposition of these colors by means of another prism, by a converging lens, and by causing a disk formed of colored sectors to revolve rapidly, that these colors are constituents of white light, and are separated by the prism because of their different refrangibilities. To arrive at a clear understanding of the formation of this spectrum, let us suppose first a small source of homogeneous light, $$L$$ (Fig. 132). If this light fall on a converging lens from a point at a distance from it a little greater than that of the principal focus, a distinct image of the source will be formed at the distant conjugate focus $$l.$$ If now a prism be placed in the path of the light, it will, if placed so as to give the minimum deviation, merely deviate the light without interfering with the sharpness of the image, which will now be formed at $$l'$$ instead of at $$l.$$ If the source $$L$$ give two or three kinds of