Light attenuation through water at an angle I know that light intensity decreases exponentially governed by 
\begin{equation*}
\frac{dy}{dx} = -ky 
\end{equation*}
where $y$ is the intensity and $x$ is the distance.
Now what happens when light strikes down at an angle ? Suppose at $45^\circ$ through water. 

Beer-Lambert's formula as expressed above holds if you consider $x$ to be the distance measured along the path of the ray.
If your ray is moving along different straight segments (i.e. in various homogeneous mediums), the easiest is to set your axes so that the ray moves along the $Ox$ axis for each straight segment.
