b
(b)\(I' = I\,{e^{ - \mu x}}\) ==> \(x = \frac{1}{\mu }{\log _e}\frac{I}{{I\,'}}\) (where \(I \) = original intensity, \(I'\) = changed intensity)
\(36 = \frac{1}{\mu }{\log _e}\frac{I}{{I/8}}\) = \(\frac{3}{\mu }{\log _e}2\) ....\((i)\)
\(x = \frac{1}{\mu }{\log _e}\frac{I}{{I/2}}\) \( = \frac{1}{\mu }{\log _e}2\) .....\((ii)\)
From equation \((i)\) and \((ii),\) \(x = 12\,mm\).