<p>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.</p>

<p>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 <a href="https://en.wikipedia.org/wiki/Mean_free_path#Mean_free_path_in_optics" rel="nofollow">applies to optics too</a>, but the concept requires a little more abstraction).</p>

CC BY-SA 3.0
2016-03-07T21:36:36.787Z
Photons have a mean free path, and don't bounce!
ProfRob
CC BY-SA 3.0
2016-03-07T21:41:35.947Z
@RobJeffries bouncing particles is the best way to grok the concept at first. Optics are tricky, but the concept remains basically the same.
SE - stop firing the good guys
CC BY-SA 3.0
2016-03-07T21:50:02.553Z
Seems straightforward - the mean free path is the average distance a photon travels before interacting.
ProfRob
CC BY-SA 3.0
2016-03-07T16:58:49.983Z
2016-03-07T16:58:49.983Z
SE - stop firing the good guys
<p>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?"</p>

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

<fundamental-astronomy>
Why do we need the "mean" in mean free path?
J.S. M
# Why do we need the "mean" in mean free path?

(Asked by: username_0 on 2016-03-07)

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?

**Tags:** fundamental-astronomy

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## 1 answers

### Answer by username_1 on 2016-03-07. Score: 2 | Accepted answer

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](https://en.wikipedia.org/wiki/Mean_free_path#Mean_free_path_in_optics), but the concept requires a little more abstraction).

> **Comments:**
> **username_2**: Photons have a mean free path, and don't bounce!  
> **username_1**: @RobJeffries bouncing particles is the best way to grok the concept at first. Optics are tricky, but the concept remains basically the same.  
> **username_2**: Seems straightforward - the mean free path is the average distance a photon travels before interacting.  

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