The mean free path for a gas, with molecular
diameter d and number density n can be expressed
as :
(1)
``\frac{1}{\sqrt{2} . n \pi d}``
(2)
``\frac{1}{\sqrt{2} .n \pi d^2}``
(3) ``\frac{1}{\sqrt{2} .n^2 \pi d^2}``
(4) ``\frac{1}{2 .n^2 d^2 π^2}``