  Light-absorbing materials are widely used, and their optical properties are
an important factor. Snell's law does not hold in materials that partially
absorb light. Hence, the optical path in refraction is calculated from
Maxwell's law. We used it to obtain the deviation angle when a light passes
through a prism made of light absorbing material. As a result, the deviation
angle has a local maximum point. The deviation angle near the local maximum is
sensitive to the real and imaginary parts of the refractive index. The local
maximum deviation angle and its incident angle are used to determine the
complex index of refraction. This measurement has the same advantages as
measuring the minimum deviation angle of a transparent prism. That is, when the
optical bench is slightly rotated and the measurement light is observed, the
moving direction of the light-spot is reversed at the extreme value. The
detection is easy. Then, it is necessary to determine the complex refractive
index from the measured local maximum deviation angle and its incident angle.
These two angles are plotted parametrically by varying the real part of the
refractive index under a fixed imaginary part. The similar curves are drawn
under the fixed real parts. Each curve has a fixed value for the real or
imaginary parts of the refractive index. Drawings of these many curves are made
prior to measurement. Then we select the four curves that are closest to the
local maximum deviation angle and its incident angle. Four fixed values
attached to each of the four curves determine the complex index of refraction.
Various liquids can be put in a prism container made of parallel plate glass
and their local maximum deviation angles are measured. The method in this paper
can easily determine the complex refractive index and can be used for material
identification
