What is free path and mean free path?

(N/A) Example: Gas leaking from a cylinder in a kitchen takes considerable time to diffuse to the other corners of the room.
This is due to the fact that gas molecules act like rigid spheres with small volume; hence,they collide with one another during random motion,causing their speed and direction to change.
Between two successive collisions,molecules travel in a straight line with constant speed.
Free Path: The linear distance travelled by a molecule of gas with constant speed between two successive collisions is called the free path.
Mean Free Path: The average of such free paths travelled by a molecule is called the mean free path.
Let the free paths of a molecule in successive collisions $1, 2, 3, \ldots$ be $l_{1}, l_{2}, l_{3}, \ldots$
$\therefore \text{Mean free path } \bar{l} = \frac{\text{sum of free paths}}{\text{number of collisions}} = \frac{l_{1} + l_{2} + l_{3} + \ldots}{n}$