Problem 5.214

Two plates, one of thickness $d_{1}=3.8 \mathrm{~mm}$ and the other of thickness $d_{2}=9.0 \mathrm{~mm},$ are manufactured from a certain substance. When placed alternately in the way of monochromatic light, the first transmits $\tau_{1}=0.84$ fraction of luminous flux and the second, $\tau_{2}=0.70 .$ Find the coefficient of linear absorption of that substance. Light falls at right angles to the plates. The secondary reflections are to be neglected.