respectively and the ratio BM/CN is the index of refraction
when the incident ray is passing through air. All substances which transmit light have a definite index of refraction.
All
the components of white light are not equally refracted by a given
substance, the rays at the violet end of the spectrum being most bent
and those at the red end being least bent. This is not well seen in the
case of a beam of white light passing through a parallel-sided piece of
glass because the various component rays become parallel again ; but if
the beam pass through a prism the components are further separated on
passing again into the Fig. 2.—Diagram
of Dispersion. air (Fig. 2). This separation is known as dispersion ;
the power of dispersion varies in different substances, but is high in
many of the gem stones, particularly in Diamond ; to this property
Diamond owes much of its beauty, as the light being much dispersed in
the stone emerges again in marked rays of coloured light.
Since
all colours are not equally bent in one given substance it is
necessary to refer the index of refraction to light of one particular
colour. The middle part of the spectrum is usually taken for this
purpose ; the difference in index for different rays is not great, for
even in Diamond, which has such a great dispersive power, the index for
the red
