Refractometers respond to refractive index (how strongly something bends light passing through it at an angle.) It's possible to calibrate the concentration of lots of things in
binary solutions with water against the refractive index of the solution, and then measure the concentration with a refractometer. Sugar concentration is the one we care about, and is what a Brix refractometer reads, but you can also buy refracts that measure salinity of salt water, and you
could calibrate one to measure alcohol in a
binary solution containing only ethanol and water.
The problem is beer contains more than ethanol and water (dextrins, proteins, etc.) Measuring beer (not wort) with a refractometer is analogous to trying to solve for two variables with one equation. Adding in the OG measurement to the final Brix measurement, gives us the second equation that we need when solving for two variables.
Mixing sugar with water increases the index of refraction, the more sugar the higher the index of refraction. Mixing ethanol with water also increases the index of refraction, and again the more ethanol the higher the index of refraction. In either case you can correlate the index of refraction to the concentration of solute. If you mix both sugar and ethanol with water, the index of refraction also increases, but there is no way to tell from the refractive index measurement how much of the increase was due to sugar, and how much was due to ethanol - unless you have some more information.
Brew on