I know this is an older course, my curiosity on the Beer Lambert Law brought me here...
I cannot understand however, why are we using the Backfacing value?
In the blender manual, Backfacing is described as follows: "1.0 if the face is being viewed from the back side, 0.0 for the front side."
How does multiplying it with the ray length gives us the thickness of the geometry?
HI Ali AAliserhan ,
That's a great question!
The definition of the Ray Length is what's important here; the Blender Manual says: "Distance traveled by the light ray from the last bounce or camera."
So we are not talking about one single Ray and it's Length, but the Length of every (relevant) Ray.
By Multiplying the Ray Length by Backfacing, the Length of the yellow Ray in the image below, becomes 0 and the green Ray gets to keep its Length:
Does that help?
Oh okay, now it makes a lot of sense!! Thanks for the explanation :)