PRISM. In solid geometry a prism is a solid described by the motion of a straight line which in passing round the boundary of a plane rectilineal figure always preserves its parallelism, the solid being terminated at the other extremity by a plain figure parallel to the first. In optics, however, the term prism is confined to the case of a prism with a triangular base, and its sides rectangles perpendicular to the base.
In optical experiments with the prism the edge of the prism is in general very sharp, the two adjacent planes which form it being in clined at a very small angle, called the " refracting angle of the prism." When the prism is so placed with respect to a ray of light refracted through it as that the emergent and incident rays make equal angles with the sides of the prism, the deviation of the refracted ray is a minimum.
Suppose a prism to be placed in its position of minimum devia tion with respect to a ray refracted tluough it near its edge, and let D be the deviation of the refracted ray, it the index of refraction of the material of which the prism is made, and a the refracting angle of the prism ; then, if the angle made by the incident ray be small; D=01-1) a.
A ray of light refracted through a prism is decomposed into rays of different refrangibility and colour, because the deviation of a ray depends upon the refractive index of the prism for that my, and since white light is not homogeneous but composed of light of different degrees of refrangibility, the refractive index will vary with the dif ferent rays of which white light is composed, being greatest for the violet and least for the red rays, therefore the deviation will be different for rays of different colours.
If a second prism, precisely similar to the first, be placed against it in such a way as that its edge is next to the base of the first prism, the two prisms will form a plate, and a ray refracted through them wiU neither suffer deviation nor decomposition ; that is to say the effects produced. upon it by refraction through the first prism will be exactly counteracted by refraction through the second, so that the second prism will re-compose into whit,e light the rays which were dispersed by the first. The second prism therefore achromatizes the first; but the refracted ray does not suffer deviation, there is therefore no'optical utility in such an arrangement. But if the second prism be made of a different material from the first, having different refractive and dispersive powers, and a suitable refracting angle be given to it, the first prism will be achromatized by the se,cond, *id the ray will suffer deviation. This important result depends on 1e fact that the dispersive power of a medium is not proportional to le deviation produced by it.
This being the case, opticians are fortunately able to achromatize uses and prisms. To be very exact however, it must be remem led that in consequence of the irrationality of dispersion 2 prisms in aitact can only unite 2 of the coloured rays or lines of the spectrum.