With an index, the signifier can not exist without the presence of the signified. Your shadow is an index. And in some sense, so is the photograph (as shadow).
Been a long time since I “thought semiotically”. With my newfound awareness of light as the be-all and end-all of photography, I suppose I could take your comment here and add that every photograph and every shape and shadow in every photograph is always and beforehand an index of “light”.
Smoke and Mirrors 
With an index, the signifier can not exist without the presence of the signified. Your shadow is an index. And in some sense, so is the photograph (as shadow).
Been a long time since I “thought semiotically”. With my newfound awareness of light as the be-all and end-all of photography, I suppose I could take your comment here and add that every photograph and every shape and shadow in every photograph is always and beforehand an index of “light”.