Question 876989
The first 1-cm of glass thickness allows {{{80/100=8/5=0.8}}} of the light to filter through.
The next 1-cm of glass thickness allows {{{80/100=8/5=0.8}}} of the remaining light to filter through.
The fraction that filter through the 2-cm thickness is
{{{(4/5)(4/5)=(4/5)^2=16/25=64/100=0.64}}} .
So 64% of the light will filter through.
 
If it is a solution of an unchanging chemical species, the concentration of that substance plays the same role as {{{d}}}
Analytical chemists (like me) use that for chemical analysis.