An optical method for measuring carrier concentration and mobility in n-type semiconductors is presented. The method depends on an accurate knowledge of the free electron effective mass,(M), as a function of temperature and dopant concentration. These values are not generally available in the literature. Measured values for M in HgCdTe, for example, are rare and, according to several authors, significantly higher than calculated values in the literature. M for n-type HgCdTe, InSb, GaAs, and Si were measured using Faraday rotation and van der Pauw testing over the range 77 K < T < 296 K. Good agreement was obtained with available published values of M in InSb and GaAs. M in HgCdTe was found to be on the order of 90 percent higher than calculated values at 296 K. The specific results have ben sent to the open literature for publication and consequently cannot be included for republication here. Faraday rotation and absorption can be combined with a constant, C, to yield carrier mobility. C was measured in the above materials at selected wavelengths. With M and C for a material known, carrier concentration and mobility can be determined from Faraday rotation and absorption measurements alone.