If $2.5 \times 10^{-6} \mathrm{~N}$ average force is exerted by a light wave on a non-reflecting surface of $30 \mathrm{~cm}^2$ area during 40 minutes of time span, the energy flux of light just before it falls on the surface is $\_\_\_\_$ $\mathrm{W} / \mathrm{cm}^2$.
