INDEX OF REFRACTION

115

Light of short waves is refracted more than that of longer
waves. Therefore in expressing the index of refraction the
character of light must be indicated. Ordinarily the refraction
for the DI sodium line of the spectrum is understood unless some
other light is specified.

Apparatus. — For the determination of
refractivity an instrument must provide
(a) a prism of known index of refraction,
a plane surface of this lying against the
liquid or solid under investigation and
(6) an optical system of lenses for exam-
ination of the effect of refraction. All
other parts may be regarded as acces-
sories, for adding convenience of manipu-
lation or for increasing the accuracy of
observations.

Abbe Refractometer. — This instru-
ment (Fig. 31) serves very well for
measuring refractive indices of fairly
viscous and non-volatile liquids. The
optical system is represented in section
in Fig. 32. The lower half of the prism
(not shown in the figure) serves merely
as a means for holding a layer of liquid
in contact with the upper half. A
mirror, below, reflects light into the
system and this passes through the lower
prism and the layer of liquid, emerging
from the latter into the upper prism at
all possible angles. The ray o, a' a",

. ,, , „ » ,,

grazing the lower surface of the upper
prism, represents the limiting angle of incidence (90° to the
normal) and the angle of refraction of this ray (or of one whose
angle of incidence is infinitesimally less than 90°) forms the
bounding line between a region of light and one of darkness.
This will be seen as the line G in Fig. 32.

If the liquid from which light emerges is exchanged for one of
different index of refraction the angle of refraction of the grazing
ray will be changed. In other words the boundary between

. 32.— Path of rays in tho
Abb6 refractometer.