Two plates, one of thickness `d_(1) = 3.8 mm` and the other oof thickness `d_(2) = 9.0 mm`, are manufactured from a certain substance. When placed alternately in the way of monichromatic light, the first trasmits `tau_(1) = 0.84` fraction of luminous flux and that substance. Light falls at right angles to the plates. The secondary reflections are to be neglected.

Explanation

We have
`tau_(1) = e^(-chi d_(1)) (1- rho)^(2)`
`tau_(2) = e^(-chi d_(2)) (1 - rho)^(2)`
where `rho` is the reflectivity, see previous problem, multiple reflection have been ignored
Thus `(tau_(1))/(tau_(2)) = e^(chi(d_(2) - d_(1)))`
or `chi = (In((tau_(1))/(tau_(2))))/(d_(2)-d_(1)) = 0.35 cm^(-1)`