[¦mō·lər mag¦ned·ik rō′tā·shən] (optics) A measure of the strength of the Faraday effect in a substance, equal to M αρ′/(M ′α′ρ),where α is the angle of rotation, M is the molecular weight of the substance, ρ is its density, and α′, M ′, and ρ′ are corresponding quantities for water.