DISPERSION OF LIGHT 



43 



Fig. 17. Dispersion of light by a 

 plate with parallel sides. 



bent out of their course least, the violet rays most, and the other rays will fall in their 

 proper order between the two extreme*. In Fig. 16 the ray of white light AB falls upon 

 the surface MN of a refracting substance. Owing to the different refrangibilities of the 

 constituents of the ray, these latter are separated and we get the original single ray of white 

 light split up into red {R), yellow (G), green {Gr), blue {Bl), and violet (F), rays, deviating 

 from each other slightly in direction. Between the rays of the colours just mentioned lie 

 rays of intermediate tints. The decomposition of white light into its coloured con- 

 stituents, or the dispersion of light, varies according to the 

 dispersive power of the refracting substance. It is the 

 more distinct the greater the angle between the extreme 

 red ray and the extreme violet ray. 



We have now to consider the dispersion of the light 

 which passes through a precious stone. We will take first 

 the case in which the stone has the form of a plate with 

 parallel sides, as in Fig. 17, and afterv\ards the case in 

 which these bounding surfaces are inclined to each other 

 and so form a prism. 



The ray of white light, AB, falling obliquely on the 

 surface MN of the precious stone, is split up into the 

 differently coloured rays lettered BR, BG, BGr, BBl, BV. 

 These rays pass out of the precious stone at the surface 



PQ in the directions RR', VV, &c., all being parallel to the original white ray AB, as 

 was explained before in connection with Fig. 14. The eye placed at R'V will receive all 

 these differently coloured rays at the same time and in the same direction ; the effect of 

 this will be to produce in the eye the sensation of white 

 light just as if the ray of light from A had not passed 

 through the plate. With such a parallel-sided plate, then, 

 a decomposition of white light into its coloured constituents 

 takes place, but is not observable, since the effect produced 

 by the first surface is neutralised by the parallelism im- 

 parted to the rays at the second surface. 



The dispersion of light produced by a prism, on the 

 other hand, is very noticeable, and is illustrated in Fig. 18. 

 A ray of white light, AB, falls upon the surface MN of the 

 prism, and is separated into its variously coloured con- 

 stituents. Between the extreme red ray, BR, and the 

 extreme violet ray, BV, lie the yellow, green, blue, and 

 rays of intermediate colours. On passing again into air 

 at the second sui-face, NP, of the prism, these rays are 

 again refracted, and emerge still more widely separated. 



The angle between the extreme red ray RR.^ and the extreme violet VV^ is Rj^CV^ ; and, as 

 before, this measures the amount of the dispersion, and varies with the substance of which 

 the prism is made. An eye placed at -ffiF^ will receive this bundle of coloured rays, 

 diverging apparently from C, the various colours being pei'fectly distinct and brilliant. 

 The ray of white light thus gives rise to an elongated band of colour which is known as a 

 spectrum. The red end of the spectrum lies nearest to the refracting edge, N, of the 

 prism, and the violet end furthest away from it ; the other colours lying between these 

 two, and following each other with no break or interruption in the same order as the 



Fig. 18. Dispersion of light by a 

 prism. Formation of the spectrum of 

 white light. 



