A drop of water is falling through air. The terminal velocity of the drop is $1.2 \times 10^{2} \mathrm{~ms}^{-1}$ and co-efficient of visocosity of air, $\eta= 1.8 \times 10^{-5} \mathrm{~N} \mathrm{sm}^{-2}$ What is the dimeter of the drop?