Warning: overly detailed nitpick ahead...
However, in normal beer brewing, it's easier to cope with the confounding due to alcohol on a hydrometer, as alcohol reduces the density while sugar raises it, which means you can use apparent reduction in density as a measure of the progress of fermentation (although apparent attenuation overstates the amount of sugars in wort turned to alcohol), which is what's being done with the traditional use of OG and FG, and why people convert refractometer readings to gravities. For a refractometer, both alcohol and sugar increase the refractive index, and so you have to separate the effects of alcohol and sugar using a slightly more complex calculation, and the equivalent of apparent attenuation measured with a refractometer is less intuitively related to how far the fermentation has progressed.