A thin transparent film with refractive index 1.4, is held on circular ring of radius 1.8 cm. The fluid in the film evaporates such that transmission…

A thin transparent film with refractive index $\displaystyle 1.4$, is held on circular ring of radius $\displaystyle 1.8$ cm. The fluid in the film evaporates such that transmission through the film at wavelength $\displaystyle 560$ nm goes to a minimum every $\displaystyle 12$ seconds. Assuming that the film is flat on its two sides, the rate of evaporation is $\displaystyle \_\_\_\_$ $\displaystyle \pi \times 10^{-13} \mathrm{~m}^3 / \mathrm{s}$.