Answer:
Step-by-step explanation:
You want optical density in terms of intensity of transmitted (Jt) and incident (Ji) light, written as a single term and as the difference of logarithms.
Optical transparency is the ratio of transmitted to incident light intensities:
T = Jt/Ji
Opacity is the inverse of transparency:
O = 1/T = Ji/Jt
Optical density is the common log of opacity:
D = log(O)
D = log(Ji/Jt)
As the difference of logarithms, this is ...
D = log(Ji) -log(Jt)
__
Additional comment
The relevant rule of logarithms is ...
log(a/b) = log(a) -log(b)