Two plates, one of thickness dl = 3.8 mm and the other of thickness d2 = 9.0 ram, are manufactured from a certain substance. When placed alternately in the way of monochromatic light, the first transmits τ1 = 0.84 fraction of luminous flux and the second, τ2 = 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.