where \(n\) is the number density and \(d\) is the diameter of the molecule.
As \(d=2 r\)
\(\therefore \quad \lambda=\frac{1}{4 \sqrt{2} n \pi r^{2}} \quad\) or \(\quad \lambda \propto \frac{1}{r^{2}}\)
$[k_B\, = 1 .4\times10^{-23}\,J/K;\, m_{He}\, = 7\times10^{-27}\,kg]$