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I was introduced to mean free path in my astro physics class, and the professor posts a question for us to ponder: "Why do we need the "mean" in mean free path?"

This further makes me to wonder what is the mean free path for, and how is it used?

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In every context we talk about "mean free path", we talk about particles. Particles usually bounce around a lot, from collision to collision. In between two bounces a particle moves in a straight line. That is the expected behaviour from Newton's laws of motion.

Between one collision and the next, a particle therefore moves a distance. That is the "free path". So why do we need to include "mean"? The thing is, you can never be sure about how far the particle moves after a collision. It might as well immediately bounce into another particle, as well as move a considerable distance. However, if you measure really many particles, their average is going to be pretty stable and predictable. That gives a nice, usable number. The "mean" here is for average. (In case you wonder, yes, this applies to optics too, but the concept requires a little more abstraction).

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  • $\begingroup$ Photons have a mean free path, and don't bounce! $\endgroup$ – Rob Jeffries Mar 7 '16 at 21:36
  • $\begingroup$ @RobJeffries bouncing particles is the best way to grok the concept at first. Optics are tricky, but the concept remains basically the same. $\endgroup$ – Hohmannfan Mar 7 '16 at 21:41
  • $\begingroup$ Seems straightforward - the mean free path is the average distance a photon travels before interacting. $\endgroup$ – Rob Jeffries Mar 7 '16 at 21:50

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