i'm really confused! Is it (1) Abs = Ɛbc or (2) Abs = log10 [Io/I] ??
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It's both! Ɛbc = log10 [Io/I]. It's two ways of phrasing the same phenomenon. The left hand side is a way to predict absorbance from physical variables (molar absorptivity Ɛ, path length b, concentration c), while the right side is a way to calculate it from observed measurements (intensity at the source Io and intensity at the detector I).