= Solution
The decadic <Beer-Lambert law> can be written
$$
\boxed{I=I_0\,10^{-\varepsilon lc},\qquad \log_{10}(I_0/I)=\varepsilon lc.}
$$
Here $l$ is the path length in metres, $c$ is concentration in $\mathrm{mol\,L^{-1}}$ or M, and $\varepsilon$ is the decadic <molar attenuation coefficient> in $\mathrm{M^{-1}\,m^{-1}}$. The intensity ratio and the exponent are dimensionless. Equivalently $I=I_0e^{-\kappa lc}$ with $\kappa=(\ln10)\varepsilon$ in the same units. Defining the logarithm convention avoids a missing factor of $\ln10$.
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