Mean free path is a statistical measurement of the average path length a particle travels before suffering a collision and being absorbed or scattered.

The mean free path (having units of length) is defined as

l = 1/(n σ)

where l is the mean free path, n is the number density (with units of number per cubic centimeter) and σ is the collision cross section (with units of square centimeters). One should note that the mean free path decreases with increasing density and/or cross section. This makes intuitive sense -- if you encounter more particles or particles with larger sizes, chances are greater that you will bump into one sooner, meaning that the mean distance you travel before hitting one is shorter.

The concept of mean free path is used in many fields of physics, and is particularly important in the fields of gaseous diffusion, radiation physics and radiative transfer of photons through gases and plasmas, and nuclear reactions. The equation above is quite simple, but the physics of cross sections is more complicated, particularly when dealing with charged particles. It is also a statistical quantity since individual particles may travel more or less than the mean free path in a given event.

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