rays is 2-407, while for the violet rays it is 2*465 ; for the middle of the spectrum it may be taken as 2-44.
Since
the sine of the angle of refraction increases with the sine of the
angle of incidence, the former will have its maximum value when the
latter is unity; thus if r be the angle of refraction, this angle will be at its greatest when
sin r = 1/n, where n is the index of refraction; the angle
having
the value given by this equation is called the critical angle.
Supposing light were proceeding from within the optically denser medium
and impinged on the surface at this critical angle, the ray on passing
into the rarer medium would just skim the surface; but if the angle in
the denser body were greater than the critical angle, no light would
pass out of the denser medium, but all would be reflected again within
it. Such a phenomenon is known as total internal reflection. Applying
this principle to a cut gem stone it is obvious that light impinging
on one of the facets from air at any angle may be at least partly
refracted and pass within the stone, but that on meeting the surface of
a facet from within it may be wholly turned back. In Fig. 3 this is
shown in the case of the Diamond, where a ray is shown undergoing first
refraction, then totai Internal reflection three times, and finally
refraction a second time. When the cutting of precious stones is
described, it will be found that in the Diamond at least there are
definite proportions at which the cutter normally aims; these
proportions ensure the facets being at
