In my opinion, Beer-Lambert law is no use. I would rather write $$\frac{dI}{I \times dl} = 0.8 \ \mathrm{m^{-1}}$$ which leads to the integrated formula $$ \ln \left( \dfrac {I_o}I \right) = 0.8· l $$ And, if $\dfrac I{I_o} = 0.5$, the corresponding depth is : $l = \dfrac { \ln 2 }{0.8} = 0.846$ m

In my opinion, Beer–Lambert law is no use. I would rather write

$$\frac{\mathrm dI}{I\,\mathrm dl} = \pu{0.8 m^-1},\tag{1}$$

which leads to the integrated formula

$$\ln\left(\frac{I_0}{I}\right) = 0.8 l.\tag{2}$$

And, if $\displaystyle\frac{I}{I_0} = 0.5,$ the corresponding depth is

$$l = \frac{\ln 2}{0.8} = \pu{0.846 m}.\tag{3}$$

In my opinion, Beer-Lambert law is no use. I would rather write $$\frac{dI}{I \times dl} = 0.8 \ \mathrm{m^{-1}}$$ which leads to the integrated formula $$ ln (I_o/I) = 0.8· l $$ And, if $I/I_o = 0.5$, the corresponding depth is : $l = ln2 /0.8 = 0.846$ m

In my opinion, Beer-Lambert law is no use. I would rather write $$\frac{dI}{I \times dl} = 0.8 \ \mathrm{m^{-1}}$$ which leads to the integrated formula $$ \ln \left( \dfrac {I_o}I \right) = 0.8· l $$ And, if $\dfrac I{I_o} = 0.5$, the corresponding depth is : $l = \dfrac { \ln 2 }{0.8} = 0.846$ m