<Dl> <Dd> l = 2 d 3 Φ Q s, (\ displaystyle l = (\ frac (2d) (3 \ Phi Q_ (\ text (s)))),) </Dd> </Dl> <Dd> l = 2 d 3 Φ Q s, (\ displaystyle l = (\ frac (2d) (3 \ Phi Q_ (\ text (s)))),) </Dd> <P> where Q is the scattering efficiency factor . Q can be evaluated numerically for spherical particles using Mie theory . </P> <P> In an otherwise empty cavity, the mean free path of a single particle bouncing off the walls is: </P>

The mean free path of a molecule of a gas is inversely proportional to