1. The problem statement, all variables and given/known data I am given a distance at which a particular object is just discernible and am asked to find the opacity of the air. 2. Relevant equations 3. The attempt at a solutionI know that: I(x)=I0e-kρx but I don't know at what point you have total extinction. Obviously you would only have zero intensity with the density, opacity, or distance at infinity. I'm using Foundations of Astrophysics by Ryden and I can't find a reference to what intensity of light is not visible.