mean free path

The term mean free path is used in the physics of moving particles (atoms, electrons, photons, etc.) to indicate the average length they travel between interactions. When performing calculations in a substance where some type of particle moves around, estimates of the consequences can be calculated by assuming all the particles always move the same distance between interactions, using the mean free path as that distance. The mean free path can be determined from the number density and the cross section of interactions, as might be calculated from the minimum intervening distance between the particles' centers at which the particles interact. Some situations where it is used:

• Distance a molecule travels in a gas before encountering another molecule, say, for some specific gas at some specific temperature and pressure.
• Distance a photon travels in space until absorption or scattering. Optical depth is distance divided by the mean free path (in the case where the latter is constant over the distance).
• More specifically, the distance a photon travels within a star before encountering an atom or electron (radiative transfer).
• Rate of nuclear reactions in the central region of a star: a factor in the fusion rate is how often nuclei hit each other, which is related to the mean free path of the nuclei.