The photo above is known as a Refractometer.
It is used by brewers who produce wine or other food products to measure the % sugar in their products.
concentration of a sugar solution.
The refracting prism has a refractive index of 1.75.
If distilled water is used, which has a refractive index of 1.00,
then the ray AB will bend alot towards the normal.
Through the eyepiece, it will appear bright throughout.
If sugared water is used, which has a refractive index of 1.50,
then the ray AB will not bend as much. If the refracted angle is
higher than the critical angle of the refracting prism, then there
is total internal reflection.
This will cause part of the eyepiece to appear as dark.
The greater the sugar concentration, the less is the bending of light,
more total internal reflection takes place and more regions will appear dark.
If calibration is done with existing liquids, then the sugar refractometer
can be used to measure sugar concentration.