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mean free path

n. (context physics English) the average distance travelled between two similar events; especially by molecules in a gas or liquid, electrons or phonons in a crystal or neutrons in a moderator

Wikipedia
Mean free path

In physics, the mean free path is the average distance traveled by a moving particle (such as an atom, a molecule, a photon) between successive impacts (collisions), which modify its direction or energy or other particle properties.

The following table lists some typical values for air at different pressures and at room temperature.

Vacuum range

Pressure in hPa ( mbar)

Pressure in mmHg ( Torr)

Molecules / cm

Molecules / m

Mean free path

Ambient pressure

1013

759.8

2.7 × 10

2.7 × 10

68 nm

Low vacuum

300 – 1

225 – 7.501×10

10 – 10

10 – 10

0.1 – 100 μm

Medium vacuum

1 – 10

7.501×10 – 7.501×10

10 – 10

10 – 10

0.1 – 100 mm

High vacuum

10 – 10

7.501×10 – 7.501×10

10 – 10

10 – 10

10 cm – 1 km

Ultra high vacuum

10 – 10

7.501×10 – 7.501×10

10 – 10

10 – 10

1 km – 10 km

Extremely high vacuum

<10

<7.501×10

<10

<10

>10 km